Table of contents

We have investigated the magnetic and dielectric properties of A₂CoSi₂O₇ (A=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO₄ and SiO₄ tetrahedra connected with each other through the corners. In Ca₂CoSi₂O₇, a weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant parallel to the c axis shows a concomitant anomaly. The large magnetocapacitance effect is observed below 5.7 K; Δ(H)/(0) = [(H) – (0)]/(0) reaches 13 % at 5.1 K. These results indicate a strong coupling between the magnetism and dielectricity in Ca₂CoSi₂O₇. Sr₂CoSi₂O₇ shows a similar magnetoelectric behavior to that of Ca₂CoSi₂O₇. In contrast, in Ba₂CoSi₂O₇, which has the different arrangement of SiO₄ and CoO₄ tetrahedra from that of Ca₂CoSi₂O₇, the magnetocapacitance is hardly observed. The key for the magnetocapacitance effect of A₂CoSi₂O₇ lies in the quasi-two-dimensional crystal structure.

We have performed ultrasonic measurements at low temperatures in order to investigate vacancy in single crystal silicon. The longitudinal elastic constants of non-doped and boron-doped silicon grown by a floating zone method exhibit appreciable softening with decreasing temperature down to 20 mK. The softening of boron-doped silicon is easily suppressed in applied magnetic field up to 2 T, while the softening of non-doped silicon is robust in fields even up to 16 T. The softening of elastic constants in high-purity crystalline silicon is certainly caused by the coupling of elastic strains of the ultrasonic waves to electric quadrupoles of the vacancy orbital.

We performed electron spin and nuclear magnetic resonance and inelastic neutron scattering measurements of a single crystal of lightly hole-doped La₁-_xSr_xCoO₃, x ∼ 0.002 in order to establish the origin of a surprisingly strong magnetization due to a very small Sr doping. The data provide experimental evidence for the creation at low temperatures of extended spin clusters with a large spin multiplicity. We argue that the doped hole couples ferromagnetically seven magnetic Co ions yielding a spin-state polaron with a huge local magnetic moment with a strong orbital contribution.

We have analyzed the magnetic susceptibility on sputtered structure-disordered (a-) Ce_xRu₁₀₀-x alloys for 15 ≤ x ≤ 80 from 1.8 to 280 K published previously. The magnetic susceptibility χ for x ≥ 39 shows a typical Curie-Weiss paramagnetic behavior in the measurement temperature region. The effective magnetic moment p_eff is about 1.8 μ_B for x = 80 and decreases rapidly with decreasing the Ce-concentration (p_eff ≈ 0.16 μ_B/Ce-atom, for x = 39). The Weiss temperature θ for the Ce-rich side shows a negative value and the absolute value decreases with decreasing the Ce-concentration. The χ for x < 23 has a very small Curie-Weiss term above 4 K and a large diamagnetic susceptibility has been observed below 4 K, suggesting the superconductivity (T_c = 3.4 K for x = 15). The superconducting transition temperature T_c increases linearly with decreasing the Ce-concentration as shown in the previous resistivity measurement.

Low temperature magnetoresistance (MR) has been studied on RB₆ (R=Pr, Nd) single crystals at temperatures in the range 2–20 K in magnetic fields H≤8 T. The small and negative MR is observed in paramagnetic (PM) phase and it changes to a large positive MR effect in antiferromagnetic (AFM) states of PrB₆ and NdB₆. The analysis of the experimental data allowed us to deduce three contributions to MR in PrB₆ and NdB₆. In addition to the main Brillouin type negative component -Δ∼M_loc²∼H² which is interpreted in terms of K. Yosida theory both the linear and nonlinear magnetic contributions were also established. The detailed analysis permited us to interpret the last component in terms of the MR response from ferromagnetic (FM) nanodomains embedded in the metallic RB₆ matrix. The results of the study allow to conclude in favour of the concurence between AFM and FM interactions as the reason of incommensurate (IC) magnetic structure formation in RB₆ (R=Pr, Nd).

We report the de Haas-van Alphen (dHvA) effect studies across the first order metamagnetic transitions in samples of Ce_xLa_1-xRu₂Si₂ with x = 0.16, 0.51 and 0.84. It is found that the dHvA frequency and effective mass do not change across the first order metamagnetic transition. These behaviors are in striking contrast to those observed in magnetic fields across and above the metamagnetic crossover in CeRu₂Si₂.

We present the results of a generalized mean-field theory of all the conventional electron-hole condensates CDW,SDW and FM taken on the same footing using an eight dimensional spinor formalism. We study in detail the influence of temperature, particle-hole symmetry and external magnetic fields on the interplay of these three order parameters. We show that particle-hole asymmetry implies the presence of either one or else all three order parameters. Considering all order parameters on the same footing we reveal novel phenomena in the presence of a magnetic field like field-induced density waves, single and double first order transitions to itinerant metamagnetism as well as negative colossal magnetoresistance. We argue that the coexistence of CDW, SDW and FM is a quite generic phenomenon that may be involved in a number of open problems of great interest like for example the CMR phenomenon in manganites [4], the metamagnetic transitions in bilayer ruthenites and in heavy fermion systems etc.

Doping the heavy-fermion compound CeCu₂Si₂ with Ge allows for studying the unconventional superconducting state in the presence of a more stabilized antiferromagnetic phase. We performed heat capacity and elastic neutron scattering measurements on single crystals of 2% and 10% Ge doped CeCu₂Si₂ at different temperatures and magnetic fields. The zero field superconducting and magnetic transition temperatures in 2% Ge doped CeCu₂Si₂ are still close to those in pure CeCu₂Si₂, whereas T_c and T_n differ by an order of magnitude in 10% Ge doped CeCu₂Si₂. Our neutron scattering results show that the magnetic Bragg peak intensity decreases with decreasing temperature below T_c in CeCu₂(Si_0.98 Ge_0.02)₂. However, in CeCu₂(Si_0.9Ge_0.1)₂, it stays constant in the superconducting regime down to lowest temperatures. We conclude that, while a phase separation between magnetic and superconducting volumes takes place in 2% Ge doped CeCu₂Si₂, antiferromagnetism and superconductivity coexist on the microscopic scale in 10% doped CeCu₂Si₂.

The antiferromagnetic order in the cubic heavy-fermion compound CeIn_3-xSn_x can be suppressed by Sn doping until a quantum critical point is reached at x_c = 0.67. Thermodynamic measurements indicate changes to the simple magnetic order for x > 0.2, which is corroborated by the fact that neutron scattering experiments failed in observing signatures of the magnetic order for x > 0. We performed muon spin relaxation measurements in zero magnetic field (ZF-μSR) on single crystals of CeIn_3-xSn_x with x = 0.2; 0.4; 0.55. The spectra for x = 0.2 can be fitted by a Gaussian Kubo-Toyabe function with exponential depolarization, whereas the damping in the case of x = 0.55 has Lorentzian shape. The data for x = 0.4 are represented best by a superposition of Gaussian and Lorentzian damping. For all investigated Sn concentrations the muon spin depolarization rate is high at low temperatures and decreases towards T_N. Our measurements demonstrate that magnetic order is present in CeIn_3-xSnx also for higher Sn content (x ≤ 0.2) and that the distribution of internal fields broadens with increasing x.

The orbital ordered states of V 3d in RVO₃ (R=Y, Tb) have been investigated by a resonant x-ray scattering (RXS) technique. The RXS signals at (0 1 1) were measured near the V K-edge energy in the G-type orbital ordered (G-OO) phase. We found two kinds of the RXS signals, which reflect the orbital state and the local crystal symmetry. The former signal disappears in the orbital disordered phase, while the latter still survives. Moreover, the signals have different azimuthal angle dependence. Using the dependence of the RXS signal reflecting the orbital state, we evaluated the orbital ordered structure in the G-OO phase. Consequently, we succeeded in determining the G-type orbital arrangement of V orbitals by the RXS technique.

High precision measurements of magnetoresistance Δp/p=f(T,H) and magnetization M(T,H) have been carried out on single crystals of rare earth dodecaborides RB₁₂ (R – Ho, Er, Tm) at temperatures 1.8–30 K in magnetic fields up to 70 kOe. The high accuracy measurements allowed us to analyze quantitatively a behavior of derivative d(Δp/p)/dH=f(T,H) and magnetic susceptibility χ(T,H)=dM/dH in paramagnetic and magnetically ordered phases of dodecaborides under investigation. It was shown that negative magnetoresistance anomalies observed in present study in paramagnetic state of RB₁₂ (R – Ho, Er, Tm) can be consistently interpreted in frameworks of a simple relation between resistivity and magnetization -Δ1 /p~M2 proposed by K. Yosida. A local magnetic susceptibility χ_loc(T,H) = (/H(d(Δp/p)/dH))^1/2 was deduced directly from the magnetoresistance measurements and compared with bulk susceptibility χ(T,H)=dM/dH results of the present study. Moreover, the susceptibility dependences χ_loc(T,H) and χ(T,H) have been applied to analyze in detail the H-T magnetic phase diagram of RB₁₂ (R – Ho, Er, Tm).

We study the Ising model of a ferromagnetic nanopyramid deposited on a ferromagnetic substrate. The interaction between the pyramid and the substrate is calculated in terms of the reduced-state (density) operator. The spatial distribution of fluctuations of the molecular field and magnetization is obtained within the Gaussian approximation.

We show that the application of external hydrostatic pressure can reverse the ferroelectric polarization of multiferroic YMn₂O₅ at low temperatures. This unique effect is explained by the peculiarities of the highly frustrated magnetic order that is typical for all RMn₂O₅ (R=rare earth) in the commensurate and incommensurate phases. Our results indicate that the pressure changes the relative phase of adjacent antiferromagnetic spin chains propagating along the orthorhombic a-axis. With further compression the incommensurate phase transforms into the commensurate phase with the highest ferroelectric polarization.

We have investigated the thermal expansivities of an ErFeO₃ single crystal along the three crystallographic orientations using a high-resolution capacitance dilatometer and found clear signatures of lattice distortions in the spin rotation region as evidence for strong spin-lattice coupling. The observed lattice strain is consistent with the smooth rotation of the magnetization and it reveals the importance of the magnetoelastic interaction. Heat capacity measurements show a well-defined plateau-like enhancement in the spin re-orientation regime as predicted by the Landau theory of second order phase transitions. The data are used to estimate microscopic anisotropy parameters. Heat capacity is also measured near the low-temperature erbium magnetic ordering transition.

We have performed a high resolution angle-resolved photoemission study on edge-sharing one-dimensional (1D) chain cuprate LiCu₂O₂ single crystals at room temperature. The crystals were grown by the floating zone technique in air and cleaved in-situ at a pressure better than 9x10^-11 Torr. The low energy electron diffraction (LEED) of cleaved (001) surfaces show a well ordered (2x1) pattern with twin domains. The normal emission spectra were collected at various photon energies. While bands of deeper binding energies display a clear dispersion with photon energies disclosing a coupling along the c-axis, the bands nearest the Fermi energy shows no such dependence revealing a localized character within the ab-plannar layers. Off-normal emission measurements were carried out along the high symmetry directions. Several dispersive bands near the Fermi energy are observed. The highest energy band can be identified as copper 3d_xy states strongly hybridized with oxygen 2p states.

We report results of ultrasonic investigations of the quantum S = 1 spin-chain magnet NiCl₂-4SC(NH₂)₂, also known as DTN, in magnetic fields up to 18 T and temperatures down to 0.3 K. A field H along the [001] direction induces a transition into an antiferromagnetic phase with T_N^max ≈ 1.2 K. Accordingly, at T = 0 there are two quantum critical points at ∼2.1 T and at ∼12.6 T. The acoustic c₃₃ mode, propagating along the spin chains, shows a pronounced softening close to the phase transition, accompanied by energy dissipation of the sound wave. The H-T phase diagram obtained from our measurements is compared with results from other experimental investigations and the low-temperature acoustic anomalies are traced up to T > T_N. We also report frequency-dependent effects, which open the possibility to investigate the spin fluctuations in the critical regions. Our observations show an important role of the spin-phonon coupling in DTN.

PACS numbers: 72.55.+s, 75.45.+j

Experimental studies of the spin-1 Haldane-chain material [Ni(C₂H₈N₂)₂NO₂](BF₄) (NENB) doped with diamagnetic Zn(II) ions in a range up to nominally 5% by means of the magnetic-susceptibility and the electron-spin resonance (ESR) techniques are reported. The presence of fractional S = 1/2 chain-end states, revealed by ESR and susceptibility measurements is found to be responsible for spin-glass freezing effects. It is suggested that a higher doping with Zn ions suppresses the spin-glass behaviour by creating shorter and isolated chain fragments.

³He adsorbed on Graphite enables to create model 2D ferromagnetic Heisenberg systems. The exchange energies are of the order of 2 mK, typical sizes on the order of a thousand spins.

By adding ⁴He (which is non magnetic) to the system, one can tune the effective size of one ferromagnetic domain. Up to now, the theoretical tools available did not allow a quantitative understanding of the magnetism of these clusters. For the first time, "engineered" ferromagnetic nano-clusters are compared to accurate theoretical models in order to understand the finite size effects. The experimental magnetization of a cluster of about 16 spins is compared to exact diagonalization and Monte-Carlo simulations based on the Heisenberg Hamiltonian.

Superconducting a-Nb_xSi_1-x thin films experience a lowering of the T_c until the superconductivity disappears through a Superconductor-Insulator Transition (SIT). We here present transport measurements on 2D a-Nb_xSi_1-x films, close to the SIT, for different compositions and thicknesses. We investigate the lowering of the T_c in light of existing theories, especially the one developed by Finkel'stein.

The two-orbital degenerate Hubbard model proves to be a powerful tool in the investigation of several 3d and 4d transition metal oxides where orbital degeneracy is known to play a crucial role. We present here a finite-size cluster study of this model where an exact numerical diagonalization procedure is used, based on the implementation of the symmetries generated by the spin, the pairing and the orbital pseudospin operators. The technique is then applied to the solution of the model on a four-site ring, and an explicit comparison is presented between the behavior of the spin, charge and orbital gaps as the on-site Coulomb repulsion is varied.

Up to now the only the physical properties of polycrystals of the series RECu_x (RE = Rare Earth, x ∼ 4.0) have been investigated. Here, we present first results on a single crystal of YbCu_4.4. The magnetization of YbCu_4.4 has been measured with B ∥ c-axis and B ⊥ c-axis. For both orientations, the susceptibility follows a Curie-Weiss law down to about 30 K revealing a Weiss temperature of Θ = -24 K (μ_eff = 4.39μ_B) and Θ = -54 K (μ_eff = 4.39μ_b) for _B ⊥ c and B ∥ c, respectively. The resistivity of YbCu_4.4 increases with decreasing temperature from 300K and exhibits a Kondo maximum around 15K. No magnetic ordering is observed. In addition, we succeeded to produce a single phase ErCu_4.13 polycrystal. ErCu_4.13 is of similar structure as the Yb-phase. Magnetization and specific heats measurements clearly show multiple phase transitions at T ≈ 4.3 K and T ≈ 3.3 K. The later transition might be attributed to Er₂O₃.

Double expansion renormalization group treatments are still employed in literature to describe the effects of symmetry-conserving quenched disorder on quantum phase transitions as an alternative to more or less phenomenological models. If one assumes = 4 – d and the dimensionality _τ of the imaginary-time space involved in a generalized quantum action as simultaneously small parameters, a stable random fixed point is found which governs a new disorder-induced quantum criticality usually considered meaningful also after extrapolation to the physical time-like dimension _τ = 1. In contrast, by considering _τ = 1 at the beginning, a runaway takes place in the parameter space for dimensionalities d < 4. To give some insight into these contrasting results, we employ here a simple self-consistent approach for case of short-range correlated quenched disorder which allows us to analyze analytically what happens by continuous variation of the fictitious time-like dimension _τ We find that a _τ*(n) < 1 exists, depending on the symmetry index n, above which the disorder inhibits the occurrence of a conventional quantum phase transition. This suggests that the usual procedure to extrapolate the small _τ-predictions to the value of interest _τ = 1 may have no real physical meaning.

Rare-earth tritelluride CeTe₃, which belongs to the family of quasi-two dimensional compounds RTe₃ (where R = Y, La-Sm, Gd-Tm), has highly two-dimensional crystal structure. Fermi surfaces consist of inner and outer square sheets, large regions of which are nested by a single incommensurate wave-vector. Because of the characteristic quasi-two-dimensional nature of square Fermi surfaces, the charge density wave of RTe₃ is formed with an extremely large gap and extensively investigated. Despite the extensive studies on the charge density wave in recent years, remarkably little is known about magnetism and low-temperature properties of CeTe₃. We have investigated the low-temperature magnetic-ordered-phases of the rare-earth tritelluride CeTe₃ with single crystals. We measured specific heat, electrical resistivity and differential magnetic susceptibility using a ³He cryostat down to about 0.45 K and a ³He/⁴He dilution refrigerator down to 0.1 K. We have found that heavy quasi-particles form spin density wave at low temperatures, reflecting the square Fermi surfaces with the quasi-one-dimensional nature.

We discuss the effect of multisite interactions on the spin-Peierls instability for a spin-1/2 XX chain with three-site interactions of (XZX + Y ZY)-type. By a Jordan-Wigner transformation we arrive at a model of spinless fermions on a chain with nearest-neighbor and next-nearest-neighbor hoppings. We compute the ground-state energy of the model for static lattice distortion patterns corresponding to dimerization and trimerization in order to identify the ground-state configuration of the lattice. The additional three-site interactions cause a number of new phases to appear in the ground-state phase diagram, including trimerized phases.

Magnetoelastic phenomena of irreversible character were investigated on the rare-earth germanides Nd₅Ge₃ and Gd₅Ge₃ which are prominent members of the hexagonal R₅Ge₃ series. Both compounds order antiferromagnetically at 52 K and 76 K, respectively. A strong magnetostructural irreversibility (i.e. a relative length change of about 10^-3 which can be induced by a magnetic field and stays stable after ramping down the field) was detected for both samples by measurements of magnetostriction and thermal expansion using capacitive dilatometry. This transition can be reversed by heating the sample near the ordering temperature. Additional experiments by X-ray and neutron scattering at Gd₅Ge₃ in order to analyze the effect itself and the structural reversal on an atomistic scale indicate the polymorphic (or metastable) magnetic character (e.g. several propagation vectors (0 0 0.4) and (0.3 0.3 0) were found) which allow to induce strong lattice distortions by an external magnetic field via the magnetoelastic coupling.

The optical conductivity σ(ω) is calculated for CuC₂ chain clusters at finite temperature T within a pd-Hubbard model. Data at T = 300 K for Li₂CuO₂ are reanalyzed within this approach. The relative weights of ZhangRice singlet (ZRS) and triplet charge excitations near 2.5 and 4 eV, respectively, depend strongly on T at low temperature, and a dramatic dependenc of σ(ω) on the ratio of the 1st to 2_nd neighbor exchange integrals is predicted. Infomation about exchange interactions for edge-shared cuprates can be btained from Tdependent optical pectra at higher T. A reduced intensity of the ZRStansition with increasing T is also relevant for unfustated cuprates in general.

Rare earth sesquisulfides α-Tb₂S₃ and α-Dy₂S₃ possess orthorhombic crystal structure having two independent rare earth sites. High field magnetization processes for single crystals of these compounds have been investigated up to μ_oH = 18 T. Some step-like transitions have been observed in both compounds. The magnetization of α-Tb₂S₃ single crystal under the field along the 6-axis at T = 1.4 K shows a steep increase of about 4.5 μ_B, which corresponds to a half of full saturation moment of free Tb³⁺, at μ_oH = 2.2 T. It is considered that the reorientation of Tb³⁺ moments on only one Tb site into a ferromagnetic arrangement occurs. When the magnetic field is perpendicular to the b-axis, one-step transition at μ_oH = 0.7 T for H∥c and two-step transitions at 0.7 and 1.2 T for H∥a are observed. As for α-Dy₂S₃, the magnetization curve at T = 1.5 K shows two-step transitions at 1.3 and 1.6 T (H∥b), three or four dull steps (H∥a or c). The total height of two steps in the M-H curve for H∥b corresponds to a half of full Dy³⁺ moment.

We have studied the effect of nuclear quadrupolar moments on the energy level structure of tunneling systems in KCl:Li. The resulting fine structure splitting leads to a quantum beating in echo decay measurements and to a non-monotonic dependence of the echo amplitude on magnetic fields, which was both observed in dielectric polarization echoes experiments. Since the microscopic nature of Li tunneling centres in KCl is well known, this system can be used to compare experimental results with calculations based on a detailed microscopic model. We present the experimental data and discuss the role of nuclear quadrupole moments of the KCl host material in the vicinity of tunneling Li ions on the observed effects.

A direct evidence of X-ray scattering by electron modulation in a charge-density wave metal dominating over the periodic lattice distortion (PLD) is presented for hexagonal modification of niobium diselenide (2H-NbSe₂). Namely, new pronounced features appear in X-ray diffraction below T_CDW indexed in the space group D_3h¹ with a doubled in-plane parameter a and the inversion symmetry axis instead of screw one. Above T_CDW transition, the space group D_6h⁴ with lattice parameters a₀=3.44Å, c=12.8Å is revealed in agreement with published data. The observed difference is due in particular to different mechanisms of neutron and X-rays scattering by spatial distribution of nuclei or electron density, respectively. A CDW formation in 2H-NbSe₂ is then accompanied by spatial modulation of electron density with periodicity 2a, manifested in X-ray scattering with a nuclei shift insufficient for neutron scattering. Indeed, numerous studies show enigmatic absence of Fermi-surface nesting in 2H-NbSe2 and, eventually, long coherence length. Within McMillan's consideration it means that in this case, electron modulation should dominate the distortion of periodic crystal potential rather than commonly observed periodic lattice distortion, e.g. in TaSe₂.

Recently the natural mineral Azurite has been proposed as model substance for the distorted S = ½ diamond chain in the spin fluid state. Azurite has alternating doublet monomers and singlet dimers along the chains yielding plateau-like features in the magnetization curves. Although Azurite was also reported to order antiferromagnetically at 1.86 K, the detailed phase diagram and its relationship to the 1/3 plateau is largely unknown. In the present paper, we report preliminary results from a dilatometry study on Azurite carried out in the 0.05 – 2.30 K temperature range at magnetic fields up to 31 T. It is shown that sizable structural distortions accompany the magnetic ordering and that at 100 mK the long range order between monomers is suppressed precisely at the transition field where the 1/3 plateau sets in.

Effect of Y-doping on interactions in the spin system of the colossal magnetoresistance compound Nd_2/3Ca_1/3MnO₃ has been studied by the thermal relaxation heat capacity and neutron diffraction techniques in the 2 — 30 K temperature range. In 5 K temperature region a broadened Schottky-like specific heat anomaly C_sch(T) has been foDnd both in the parent Nd_2/3Ca_1/3MnO₃ and in the doped (Nd_0.9Y_0.1)_2/3Ca_1/3MnO₃ compounds. The experimental data were successfully fitted by a set of three two-level Schottky contributions, implying a broadening of the exited level in the system with randomly distributed Y ions. Replacement of 10 % Nd ions by nonmagnetic Y ions leads to the about 10 % enlargement of the splitting of the ground state doublet of Nd³⁺ ion.

A few years ago, surprising magnetic field effects were found in the low-temperature dielectric properties of glasses. They are attributed to the interaction of nuclear electric quadrupole moments with local electric field gradients. In our dielectric polarisation echo measurements on partially deuterated glycerol we found a similar effect on a smaller energy scale which is caused by the interaction of the magnetic dipoles of the hydrogen nuclei. Detailed numerical calculations were performed which allow to create a model for the microscopic nature of tunnelling systems in glycerol. Moreover, an unexpected temperature dependence of the nuclear effects may shed some light on the particularities of relaxation in glasses.

We investigate the ternary Ce-Ni-Ga system with samples prepared by the flux-growth method in Ga-flux. The series CeNi_xGa_1-x crystallizes in the tetragonal BaAl4 structure. The Ga-rich compounds of this series have previously been reported to exhibit ferromagnetism. Our systematic study shows that with increasing Ni content the lattice parameters shrink. The magnetic transition temperature, as identified from the sharp maximum in the magnetic susceptibility, increases slightly with increasing Ni content. On the other hand the absolute value of the magnetization at the transition drops by around one order of magnitude. We did not find any difference between field-cooled and zero-field cooled magnetization measurements nor a hysteresis in the magnetization versus field curves, suggesting that our samples order antiferromagnetically.

Organic magnet [Ph(NH₃)](18-crown-6)[Ni(dmit)₂] is a candidate for an S = 1/2 two leg spin ladder system [S. Nishihara, T. Akutagawa, T. Hasegawa, and T. Nakamura, Chem. Commun. (Cambridge) 2002, 408.]. In order to investigate the non magnetic impurity effect for this compound, we performed X-band ESR measurement of [Ph(NH₃)](18-crown-6)[Ni(dmit)₂]_1-x[Au(dmit)₂]_x. Non magnetic impurity concentration were estimated to be x = 0.029. ESR measurements have been performed from T = 4.5 K to 273 K using the Bruker ESR spectrometer. At T = 4.5 K, the fine structure ESR signals were observed. These signals suggest the existence of ferromagnetically coupled S = 1/2 dimers in the system.

Filled skutterudite TbRu₄P₁₂ exhibits antiferromagnetic ordering below 20 K. Additional magnetic ordering below 10 K exists under zero field and the origin of this successive magnetic ordering might be related to multipole ordering. In order to reveal the detailed properties of the magnetic transition in TbRu₄P₁₂, we performed zero-field ³¹P nuclear magnetic resonance(NMR), ¹⁰¹Ru nuclear quadrupole resonance(NQR), muon spin rotation(μSR). The zero-field ³¹P-NMR spectra indicate the magnetic transition only at 20 K, while the time spectra of μSR of TbRu₄P₁₂ indicate the additional increase of the internal field below 10 K. Moreover, we observed an anomalous temperature dependence of the spin-lattice relaxation rate T₁^-1 just below 20 K and small anomaly at around 10 K in ¹⁰¹Ru-NQR.

Static properties of Cu²⁺(3d⁹; S=1/2) spins in a new class of quantum spin system, a twisted Heisenberg triangular spin tube, [(CuCl₂tachH)₃Cl]Cl₂, have been investigated by using nuclear magnetic resonance (NMR) in a temperature region of T=0.05–300 K. An observation of magnetic broadenings of ¹H-NMR spectrum at low temperatures directly revealed a gapless ground state of the system.

The positive hyperfine field at Co nucleus in a Heusler alloy Co₂TiGa is discussed to be not dominated by the orbital hyperfine field from a quite different point of view from former studies. Field-induced-moment nuclear coupling for ⁵⁹Co via spin-orbit interaction on Co atom in Co₂TiGa is discussed to be small from the observed high-field shift of the NMR of ⁵⁹Co in the ferromagnetic state of only +0.83 % in contrast to the case of CoCl₂.2H₂O with +29%.

Systematic AC and DC magnetic measurements were carried out to investigate the magnetic properties of La_1-xCa_xMn_0.90Cu_0.10O₃ (x=0.05–0.30) system. A striking paramagnetism (PM) to ferromagnetism (FM) transition is observed at the FM ordering temperature T_C. Below T_C, the samples with x≤.0.20 show a visible unexpected drop in zero-field cooled (ZFC) DC magnetization curves near 100 K, which is believed to be associated with the domain wall pinning effects. As for the sample with x=0.30, there is no anomalous behavior near 100 K, while another different magnetic phase transition is shown up below 50 K in both FC and ZFC curves. Detailed experimental results show that the phenomenon is resulted from the phase competition rather than solely magnetic interactions.

We present a detailed study on the memory effect results in Na_0.5§5CoO₂ single crystals. We analyze the temperature dependence of the nonvolatile current-pulse-induced resistance memory state. These results allow us to have more insight in the mobility of Na⁺ ions induced by current and their effect on the memory effect. We also developed X-ray diffraction studies under pressure at ambient temperature in the N_a0.5CoO₂ powder compound. An orthorhombic to hexagonal phase transition was observed at 9GPa. This transition can be explained taking into account the Na ions displacement between two allowed positions. These structural results allow us to confirm that the non-volatile resistive commutation can be interpreted by the displacement of the Na ions induced by the current pulses.

A comparative magnetic resonance study on correlated semiconductors is presented. Nuclear spin-lattice relaxation measurements provide an excellent method to gain local information about the gap formation in this new class of materials. In contrast to U2Ru2Sn (Δ/k_B ≈ 230 K) in CeRu₄Sn₆ the gap is slightly reduced (Δ/k_B ≈ 200 K) and correlations form out of a residual density of states in the gap. For FeSb₂ there is revived interest after classifying this system as the second Fe containing Kondo insulator beside FeSi. Surprisingly, FeSb₂ shows a colossal Seebeck coefficient at low temperatures. Using ^121,123Sb nuclei as a local probe, our NMR/NQR investigations strongly support the gap scenario. The spin-lattice relaxation rate ¹²³(1/T₁) in the entire investigated temperature range 2–200 K is perfectly fitted with rectangle DOS model with narrow impurity in-gap band. These in-gap states might originate the high thermopower observed. The obtained gap value is (Δ/k_B ≈ 473 K).

For interface-roughness scattering and for zero temperature we compare theoretical results for the mobility of the two-dimensional electron gas present in thin AlAs quantum wells with experimental results for a well of width L=45Å. The importance of many-body effects (exchange and correlation) on the mobility is discussed. For the mobility reasonable agreement between theory and experiment is obtained by taking into account a density dependent effective mass and multiple-scattering effects, which lead to a metal-insulator transition.

Quantum phase transitions are transitions between the ground states of physical systems traversed by varying a parameter of the Hamiltonian. We have been examining the properties of ultrathin quench-condensed films of amorphous Bi, which exhibit a number of quantum phase transitions tuned by disorder, perpendicular and parallel magnetic fields and charge density. We have developed a procedure in which the substrate for film growth serves as the gate insulator in a field effect transistor configuration. This permits electrostatic tuning of a transition between insulating and superconducting behaviour. Finite size scaling of the conductivity has been carried out yielding critical exponents that suggest that the insulator-superconductor transition belongs to the universality class of the 3D XY model. This electric field-induced superconductivity can in turn be destroyed by magnetic fields and the data can also be scaled in with essentially the same results. These findings are quite different from those obtained in the study of substantially thicker films of indium oxide and titanium nitride.

The visual evidence of existence of space inhomogeneous superconductor-insulator state (SISIS) in high-T_c system Ba_0,6K_0,4BiO₃ (T_c≈30 K) was obtained with the help of magnetooptic visualization technique. The appearance of SISIS was observed near 20 K in accordance with the results of our earlier halvanomagnetic investigations of Ba_0,6K_0,4BiO₃, where numerous anomalous phenomena were found out at temperatures below T*≈17 K. All anomalous properties were managed to be explained by the phase transition from homogeneous superconductive state into SISIS. The properties of new phase were investigated by magnetooptic method that allows detecting variations of the superconductive properties of material by patterning the distortions of magnetic field caused by variations of the local magnetic permeability. It is found that the crystal behaves as a uniform superconductor at 21 K< T < 30 K. The material heterogeneity appears at T < 21 K as a magnetic pattern fragmentation, which increases progressively with decreasing T down to 13,5 K – the lowest temperature in our investigations. The fragmentation is dependent on the cooling conditions. The characteristic scale of the found inhomogeneities is about tenths of microns.

We are using μSR to probe the evolution of magnetism from single-ion to spin glass behaviour in LiY_1-xHo_xF₄ with increasing x. For x = 0.002 the Ho spin dynamics exhibit single-ion behaviour similar to that observed in molecular magnets. Near T≈ 10 K, the energy gap between the Ho ground state electronic doublet and first excited singlet energy levels, we find that the longitudinal depolarization rate exhibits a peak that depends on the Larmor frequency. At temperatures T << 10 K we find sharp reductions in the transverse field depolarization rates in the vicinity of magnetic field induced (avoided) level crossings (ALCs) in the hyperfine-split ground state energy manifold. This is attributed to a reduction in the quasistatic magnetic disorder due to resonant Ho spin fluctuations at frequencies much larger than the muon Larmor frequency. As the Ho concentration is increased toward x = 0.05, magnetic correlations are observed to become significant, changing the 10 K peak into a plateau, and significantly broadening the rate changes observed at ALCs.

The result of dc and ac magnetization, heat capacity and magneto-resistance measurements in the polycrystalline and single crystal samples at the stoichiometry, Nd_0.75Ho_0.25Al₂, are presented. Magnetic compensation between the antiferromagnetically coupled Nd and Ho moments in nominal zero field, and the field induced reorientation of the Nd/Ho moments in H = 5 kOe are observed to happen at the same temperature for H ∥ [100], [110] and [111] directions. The ac magnetization data also appear to imprint the field-induced moment reversal process in polycrystalline as well as in a single crystal sample (for H ∥ [100]). Intriguingly, the magneto-resistivity data in H = 10 kOe show three sign reversals below T_c.

Using a hydrothermal method, single crystals of Co₄(BO₂)₆O were synthesized. The DC susceptibility revealed a Curie-Weiss behavior at high temperature. The Weiss temperature was estimated at -190 K, showing very strong antiferromagnetic interactions. However, the susceptibility does not show any phase transitions down to 2 K, although there is a broad maximum in the vicinity of 14 K. This demonstrates an effect of the geometric frustration arising from the crystal structure based on isolated Co₄ tetrahedral clusters. The magnetization curve exhibits an onset of a transition to higher magnetization state at around 5 T. The result suggests that the Co₄ cluster has a nonmagnetic ground state with a spin gap and a magnetic field induces a transition into a magnetic state above the spin gap.

We report the magnetization curve under magnetic fields H up to 30 T at low temperatures in a tetrahedral spin-chain system Cu₃Mo₂O₉ single crystal. This material shows successive phase transitions to an antiferromagnetic phase at T_N = 7.9 K and a weak ferromagnetic (WF) phase at T_c ≊ 2.5 K in low magnetic fields. As the magnetic field increases below T_N two rapid increases in magnetization occur at 0 – 0.1 and 19.3 T for H//a and 0.3 – 0.8 and 17.5 T for H//c. However, these increases do not occur for H//b. These facts refute the conventional spin flop transition. The rapid increases disappear above T_N. The magnetic moments are canted toward the ac plane by the Dzyaloshinskii-Moriya interaction in the WF phase. Consequently, these rapid increases in magnetization at high magnetic fields are interpreted by sudden rotations of the WF moments around the b axis.

⁶⁹Ga and ⁷¹Ga NMR signals in GdGa_2-xAl_x (x = 0.1, 0.25, 0.5, 0.75,1) have been observed at a low temperature in a zero magnetic field. Gd NMR signals were observed only for the samples with x ≤ 0.5 in the frequency range less than 110MHz, but Ga NMR signals have been observed for all samples in the frequency range higher than Gd NMR frequencies. The features of the observed Ga NMR spectra of the samples with low Al concentration are quite different from those of highly Al concentrated samples. This difference is attributed to the change of the magnetic structure of these compounds. NMR parameters calculated from the Ga NMR results are reported, too. The magnetic structures of these compounds are discussed on the basis of the Ga NMR results.

The heavy-fermion compound YbRh₂Si₂ exhibits an antiferromagnetic (AFM) phase transition at an extremely low temperature of T_n = 70 mK. Upon applying a tiny magnetic field of B_c = 60 mT the AFM ordering is suppressed and the system is driven toward a field-induced quantum critical point (QCP). Here, we present low-temperature thermopower S(T) measurements of high-quality YbRh₂Si₂ single crystals down to 30 mK. S(T) is found negative with comparably large values in the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like behavior is observed within the magnetically ordered phase. However, a sign change from negative to positive appears at lowest temperatures on the magnetic side of the QCP. For higher fields B > B_c a linear extrapolation of S to zero clearly evidences the recovery of LFL regime. The crossover temperature T_lFl is sharply determined and coincides perfectly with the one derived from resistivity ρ(T) and specific heat c_p(T) investigations.

The cuprate Cu₂CdB₂O₆ has two Cu sites (Cu1 and Cu2). Cu1 spins are in a nearly non-magnetic state because of formation of AF dimers, while Cu2 spins form antiferromagnetic (AF) long-range order. We investigated effects of substitution of Zn for Cu on magnetism of this cuprate. The reduction in AF transition temperature per Zn is small in comparison with other quasi-one-dimensional antiferromagnets. We consider that the system of the Cu2 spins is a two-leg ladder.

The cuprates Cu₆Ge₆O_18-xD₂O (x = 0, 6) have spin-1/2 chains with first-and second-nearest-neighbor (1NN and 2NN) antiferromagnetic (AF) exchange interactions. Magnetic long-range order (LRO) appears below 74 and 39 K for x = 0 and 6, respectively. We observed two magnetic Bragg peaks in each sample in neutron diffraction measurements on powder samples. These peaks indicate appearance of collinear AF LRO. We confirmed that both the 1NN interaction and nearest-neighbor interchain interaction were AF.

A few spinel compounds exist with the occupation of Ag atom on tetrahedral (or A) sites and have been less extensively studied. It is hard to synthesize high purity specimens of Ag-based thiospinels. The substituted quaternary spinel compounds Ag(Cr_1-xSn_x)2S₄ (x = 0.50, 0.60, and 0.70) have been successfully prepared. These compounds exhibit the spin-glass nature with the freezing temperature T_g = 18.0, 15.4, and 11.4 K, respectively, where the irreversibility arises between ZFC (zero-field-cooled) and FC (field-cooled) magnetization with decreasing temperature. The asymptotic Weiss temperature θ has small value, which indicates a spin-glass magnetic property. The effective moment peff per Cr-atom is close to the spin only value for Cr³⁺ ion. These values are θ = – 0.90 K, peff = 3.92 Cr-atom^-1 for x = 0.50; θ = 1.96 K, peff = 3.90 Cr-atom^-1 for x = 0.60; and 8 = – 4.07 K, peff = 3.87 Cr-atom^-1 for x = 0.70. The random distribution of Cr and Sn atoms on octahedral (or B) sites can introduce magnetic frustration. The spin frustration is due to the competition between ferromagnetic and antiferromagnetic interactions. A collinear spin alignment is inhibited in Ag(Cr_1-xSn_x)2S₄.

An anomalous non-magnetic insulator phase called "gapless spin liquid state" has been found in various materials with geometrically frustrated lattices. Organic conductor κ-(ET) ₂Cu₂(CN)₃, which has quasi-two-dimensional triangular lattice, is one of them. Mott insulator phase of this material is known by several experiments to show neither sign of magnetic ordering nor spin excitation gap in low temperatures. To clarify the physical origin of this peculiar state, we perform resonating-valence-bond mean-field analysis of the spin-1/2 Heisenberg antiferromagnet on a triangular lattice with one-dimensional (1D) anisotropy. We take an approach from 1D spin chains assuming it is likely that the gapless behavior observed in experiments descends from 1D spin excitations. As a result of calculation at zero temperature, almost 1D gapless excitation spectra are obtained in the wide range from the 1D limit. We further extend our analysis to finite temperatures and find that the anisotropy parameter range expands in which the "one-dimensionalization" occurs. The gapless features characteristic of 1D spin systems are also found in the temperature dependences of several thermodynamical quantities.

Dynamic dielectric properties were studied on the newly discovered multiferroic spinel, CdCr₂S₄. Two single crystals, grown under the same condition but from different batches were used, which showed identical behavior. Frequency-dependent dielectric constant as a function of temperature (^'(ω)) exhibits relaxation behavior only below ferromagnetic transition temperature, T_C ∼ 84 K, which indicated that the charge dipoles were associated with ordered magnetic spins. By fitting with extended Debye model (Cole-Cole equation), the relaxation characteristic time (τ) at different temperatures was obtained. When 55 < T < 84 K, the τ value decreases with decreasing temperature. It is somehow unusual as it doesn't obey the thermal activation model (Arrhenius law). These results suggest that the relaxation behavior is strongly affected by spin not by thermal fluctuation. Different ac voltages were also selected to study the field effect on the relaxation behavior. For V_ac = 20 V, ^'(ω) dramatically changes and τ value becomes difficult to obtain as T < 55 K, suggesting that some unknown phases could exist below 55 K. Complex relaxation behavior below T_C could be originated from the coupling among spin, lattice and dipole via exchangestriction.

We have carried out the measurement for a pyrochlore-type Mo oxide (Sm₂Mo₂O₇) by means of hard-x-ray and soft-x-ray photoemission spectroscopy (PES) with high energy resolution. The PES spectrum obtained with the hard x-ray clearly shows a coherent part and a incoherent part. And its boundary is relatively distinguishable. The ratio of the coherent part to the incoherent part is more enhanced in the hard-x-ray PES compared to the soft-x-ray PES. The results indicate that hard-x-ray photoemission spectroscopy (HAXPES) is powerful method for investigating the bulk electronic state.

We propose a practical scheme for calculating the ground-state pair density (PD) by utilizing the correlated wave function. As the correlated wave function, we adopt a linear combination of the single Slater determinants that are constructed from the solutions of the initial scheme[Higuchi M and Higuchi K 2007 Physica B387, 117]. The single-particle equation is derived by performing the variational principle within the set of PDs that are constructed from such correlated wave functions. Since the search region of the PD is substantially extended as compared with the initial scheme, it is expected that the present scheme can cover more correlation effects. The single-particle equation is practical, and may be easily applied to actual calculations.

We investigate the kinetic energy contribution to the exchange-correlation (xc) energy functional of the extended constrained-search (ECS) theory. First, we derive the difference between expectation values of an arbitrary quantity with respect to the ground-state wave function and that with respect to the Kohn-Sham determinant of the ECS theory. This is regarded as the generalization of Bauer's relation that has been derived in the conventional density functional theory. Using this relation, we obtain the kinetic energy contribution to the xc energy functional of the ECS theory. As an example, these results are applied to the current density functional theory (CDFT), which leads to useful exact relations in developing and/or evaluating the approximate form of the xc energy functional of the CDFT.

We have been trying to find Heusler compounds with high spin polarization which are robust against chemical disorders, because it is considered that disorders often degrade high spin polarization. The Heusler compound Ru_2-x Fe_:cCrSi is shown to be such a material from first principles band structure calculations, and it is also shown that for the Ru-rich compound antiferromagnetic states are energetically preferable to the ferromagnetic state. Experimentally the Fe-rich compound is shown to be ferromagnetic, whereas for x ≤ 0.2 ferromagnetism is found to disappear and a cusp in the temperature dependence in magnetization is found, which appears to indicate an antiferromagnetic transition. These results seem to agree with the theory on the whole. The experimental results are examined in the light of calculated band structures.

The stability of the multimode Peierls (MMP) state, which has been predicted for the 2D square-lattice electron system with the Su-Schrieffer-Heeger (SSH) type electron-lattice (e-l) coupling and with a half-filled electronic band, is discussed in the presence of a dilation-type e-l interaction. To understand the role of the latter interaction, the ground state for the model with only the latter interaction is numerically investigated for the 1D chain and the 2D square lattice. The results indicate that this interaction favors a phase separation into two regions, one with sites doubly occupied by electrons and the other with sites having no electron. Detailed numerical analyses show that the MMP state in the 2D system can survive even in the presence of the dilation-type interaction as long as its coupling strength is weak compared to that of the SSH-type interaction.

The variational cluster approach based on the self-energy functional theory is applied to consider the two-dimensional symmetric periodic Anderson model at half filling We calculate a variety of physical quantities including the single-particle spectra in the thermodynamic limit at zero temperature and show that the symmetry breaking due to antiferromagnetic order of localized spins occurs in the strong coupling region, whereas in the weak coupling region, the Kondo insulating state without symmetry breaking is observed. We thus discuss the competition between antifomagnetim and Kondo seening in heavy fmion ystems.

In order to promote our understanding on magnetically robust heavy-fermion phenomenon observed in Sm-based filled skutterudites, we focus on Kondo-like behavior due to multipole degree of freedom. By employing a numerical renormalization group method, we evaluate the Sommerfeld coefficient γ of electronic specific heat of a seven-orbital Anderson model with five f electrons corresponding to Sm³⁺ ion under a magnetic field H. From the numerical results, we discuss the dependence of γ on H for (0,0,1), (1,1,0), and (1,1,1) directions. It is observed in common that γ is eventually independent of H for large magnitude of H. Note, however, that for (1,1,1) direction, we find a sharp increase of γ around at 11 Tesla, which may be a signal for the relevance of xyz octupole moment.

Thermodynamically YMnO₃ (YMO) can only exist with hexagonal structure at ambient conditions. In this study, we demonstrate that the stain between film and substrate can play an important role in forming orthorhombic YMO (o-YMO). o-YMO films with nearly perfect crystallographic orientation alignment were obtained on LaAlO₃(110) substrates. These films allow us to unambiguously disclose the intrinsic magnetic property and electronic structure anisotropies along different crystallographic orientations. The antiferromagnetic (AFM) phase transition was around 42 K. Moreover, we observed a signature of spin reordering at lower temperatures when the applied field was parallel to the a- or c-axis. On the other hand, only the AFM transition was observed when the field is parallel to the b-axis. By comparing the X-ray absorption spectroscopy (XAS) results to standard manganese oxides, YMnO₃ exhibits the dominant Mn⁺³ characteristics as that obtained from the standard Mn₂O₃ powder. The polarization dependent XAS also revealed anisotropic bonding behavior along each crystalline axis.

We have performed low temperature magnetoresistance (MR) and magnetization measurements for a series of thin Co films with different degrees of oxidation to investigate the role of surface oxidation on magnetic properties. For films with less oxidation, magneto-transport exhibits the typical anisotropic magnetoresistance behavior. However, magneto-transport flips completely in films with severe oxidation. The inverse MR is replaced by the normal MR when the applied magnetic field is along the current. The normal MR is changed to the inverse MR when the applied magnetic field is perpendicular to the current. Since the disorder will enhance the spin-orbital interaction and electron-electron interaction in thin films, we suggest that both mechanisms may be responsible for the flipping in anisotropic MR induced by surface oxidation.

Transition metal oxides with spinel crystal structure exhibit intriguing and non trivial magnetic phenomena owing to magnetic frustration between spins having antiferromagnetic coupling interaction on triangle or kagome lattice. GeCo₂O₄(GCO) and GeNi₂O₄(GNO), which belong to above category, are very rare normal spinels containing Ge ion. Both reveal antiferromagnetic-like phase transitions at 20 K and 12 K, respectively. According to previous neutron and x-ray diffraction measurements, GNO keeps its cubic structural symmetry down to 2 K which is not natural because such a magnetic transition tends to associate with symmetry breaking structural transitions. In order to know whether the structural transition or symmetry change occur or not at the magnetic transition in detail, convergent beam electron diffraction measurements is employed for the compounds.

Based on the recently proposed effective Hamiltonian for typical heavy fermion compound YbAl₃, the correlation effect is systematically investigated in the physical properties. The band part of the Hamiltonian consists of conduction bands described by nearly free electron method, hybridization between the conduction and 4f-electrons, and the localized 4f-states of Yb ions on the lattice site. The correlation effect is considered by using the self-consistent perturbation theory with local approximation. The temperature dependence of the specific heat coefficient is calculated, and the anomalous behaviors are found in the low temperature region in accord with the experiments. We show that the anomalies may originate from the correlation effect and the structure of the non-interacting density of states.

We investigate the two-dimensional Hubbard model on the triangular lattice with anisotropic hopping integrals at half filling. By means of a self-energy functional approach, we discuss how stable the non-magnetic state is against magnetically ordered states in the system. We present the zero-temperature phase diagram, where the normal metallic state competes with magnetically ordered states with (π, π) and (2π /3, 2π /3) structures. It is shown that a nonmagnetic Mott insulating state is not realized as the ground state, in the present framework, but as a meta-stable state near the magnetically ordered phase with (2π /3, 2π/3) structure.

Result of specific heat measurements on the single crystal of DMACuCl₃ is presented and compared with previous one obtained for the powder crystal. Possible reason for the marked difference between them is discussed on the basis of competition between critical fluctuation in the magnon BEC process and short correlation length in the powder crystal.

We report a synchrotron X-ray study of charge density waves (CDW) in an o-TaS₃ crystal. CDW of o-TaS₃ has been known to undergo a commensurate-incommensurate transition at 100 K, below which the wavevector locks in with the pristine lattice. We exploited the beamline BL02B1 of SPring-8. Temperature dependence of the Bragg peak (002) and satellite peak (1 -1 2)+ was measured from 7 K to 180 K. We found that a new phase in a temperature range of 130--50 K, where two independent CDWs coexist. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors q_c=0.252c* and 0.250c*, respectively. By lowering the temperature, intensity of the incommensurate CDW was decreased, while that of the commensurate CDW was increased. At 50 K, the incommensurate CDW was completely diminished. Based on the concept of discommensuration, we determined the dislocation configuration from the intensity of the two CDWs.

Recently, resonant x-ray diffraction has been used for detecting charge, magnetic and multipole orders. In practice, however, this technique surveys only a small portion of the reciprocal space. In this study, we applied oscillation photography to resonant x-ray diffraction. For a test sample, we explored nearly a whole Brillouin zone and successfully observed resonant peaks. This feasibility study indicates that oscillation photography is useful for expanding observable areas and enhances the capability of resonant x-ray diffraction.

We have been investigated the electronic states of LiV₂O₄ under high pressures and low temperatures analyzed by optical study using an infrared microscope. The optical conductivities derived from reflectivities clearly exhibit the change from a metallic state at ambient pressure to an insulating state at 13 GPa with the pseudo gap of Δ ∼ 0.5 eV at the low temperature of 40 K. The Drude response observed at ambient pressure declines at high pressures even at 300 K. A softening phenomenon of a residual ray band structure is verified through the temperature change from 300 K to 40 K at 13 GPa.

The rattling motion in Ce-based filled skutterudite CeFe₄Sb₁₂ has been investigated by means of ultrasonic measurements. We found ultrasonic dispersion obviously in elastic constant C₄₄ between 35 and 70 K, suggesting the rattling motion of Ce atoms, and obtained a relaxation time τ₀ = 1.5 x 10^-12 s and an excitation energy E = 345 K of the rattling motion. τ₀, E and the temperature range of ultrasonic dispersion in CeFe₄Sb₁₂ are a little lower than that in LaFe₄Sb₁₂ and PrFe₄Sb₁₂. Elastic softening in C₄₄ is not observed down to 4.2 K.

We have measured the susceptibility χ and resistivity ρ on single-crystalline Ce_0.8La_0.2(Ru_1-xRh_x)2Si₂ in directions parallel ∥ and perpendicular (⊥) to the c-axis. The χ∥ for each Rh concentration increases with decreasing temperature, and then rapidly decreases for x ≤ 0.2 and x ≥ 0.8, indicating the AF transition at the Néel temperature T_N. The ρ∥ for x = 0 shows a cusp at around T_N = 5.5 K, however, ρ⊥ indicates a broad maximum at higher temperature than T_N and a rapid decrease at much lower temperature than T_N, without evidence of a gap-type AF ordering. The ρ⊥ for x = 0.2 indicates a clear cusp at about T_N. Such sharp anomaly in ρ⊥ once disappears for x = 0.6 and is again observed at T_N for x ≥0.8 probably due to another type of the AF ordering.

A Y-type hexaferrite Ba₂Mg₂Fe₁₂O₂₂ undergoes a transition to a proper-screw type helimagnetic structure with a propagation vector k₀ ∥ [001] at 195 K, below which the system shows field-induced successive transitions under magnetic fields up to 1 T. Magnetization measurements also indicate the presence of the conical spin structure at low magnetic fields (∼ 10 mT) below about 50 K. We report on the magnetic control of the electric polarization in Ba₂Mg₂Fe₁₂O₂₂ with using the linearly oscillating fields up to ± 1 T at 5 K. The polarization vector can be cyclically reversed by weak magnetic fields of ± 30 mT. In addition, the polarization vector is repeatedly reversed without significant decay even by applying fields of ± 1 T, which suggests that the sense of the spin helix is somehow conserved in the process of the field-induced phase transitions. We propose that the conical spin structure carries the polarization vector upon the reversal of magnetic field.

The specific heat C(T) of polycrystalline PrPd₃ has been measured down to 0.37 K in magnetic fields H_ext up to 8 T. Two clear peaks, T₁ and T₂, were observed in zero field at 0.88 and 0.77 K, respectively. T₁ increases very slightly with increasing H_ext initially up to 2 T, and decreases rapidly over 5 T. On the other hand, the peak at T₂ weakens with the increase of H_ext and disappears around 3 T. The value of C(T)/T in 0 T is almost constant below 0.7 K with an extremely large one. By applying H_ext, C(T)/T at 0.5 K gradually decreases to 5.5 J/mol K² in 8 T. The nuclear contribution to C(T) is not apparent in the present C(T) even in the fields. The giant C(T)/T at low temperatures indicates presumably that PrPd₃ is a Kondo system similar to the diluted system of Pr in Pd. The effect of quadrupole interaction as one possibility might be responsible to the characteristic behaviors of the giant C (T)/T and the H_ext-dependence of T₁.

We report the first observation of the de Haas-van Alphen (dHvA) oscillations in U-based alloys U_1-xTh_xPd₃ up to x = 0.5. In U_0.5Th_0.5Pd₃, the dHvA oscillations β₂ can be observed in the (1100) plane and the frequencies of the oscillations as well as their angular dependences are found to be nearly the same as those in UPd₃. On the other hand, the effective mass of the β₂ oscillation is found to decrease with x from 1.78m₀ of UPd₃ to 1.32m₀ of U_0.5Th_0.5Pd₃. From these observations we argue that the f electrons in UPd₃ are nearly localized and the interactions between the f electrons and the conduction electrons are weak.

The magnetic properties of the spin S (=3/2 and 2) Ising systems with the bilinear exchange interaction J₁S_izS_jz and the four-site four-spin interaction J₄S_izS_jzS_kzS_lz are discussed by making use of the Monte Carlo (MC) simulation for the magnetization <S_z> and the magnetic specific heat C_M and spin structures. The ferromagnetic spin arrangement ordered by the positive interaction J₁ is destroyed and changed into lots of complicated spin structures with low energy by introducing negative four-spin interaction J₄. Near the phase transitions of J₄/J₁ = -2/9 and -1/8 for S = 3/2 and 2, respectively, the thermal averages <S_z>, <S_izS_jzS_kzS_lz> and the specific heat C_M show characteristic temperature dependences in the low temperature region. The change of spin structures of the system with the variation of the values of J₁ and J₄ are also investigated by using the result of the MC simulation and by the comparison of energies of the possible ground state spin structures.

We report the magnetization data of single crystalline Sr_3-xCa_xRu₂O₇ (x = 1.5, 2.0, 3.0) grown by a floating-zone method. In the magnetic phase diagram of this system, a phase with a ferromagnetic correlation (0.5 ≤x ≤ 1.2) adjoins a two-dimensional (2D) metallic and antiferromagnetic phase (1.2 ≤ x ≤ 3). In the antiferromagnetic and 2D-metallic phase, the magnetic susceptibilities indicate that the magnetic easy axis changes continuously from the ab-plane to c-axis with increasing Sr content. Our results exhibit that the metamagnetic transition of Ca₃Ru₂O₇, which is on the basis of a tunneling magnetoresistance, shifts to lower field and the transition becomes broad with increasing Sr content. Moreover, the magnetization data at T = 5 K for x = 1.5, 2.0 show a hysteresis, which suggests that the magnetic ground state is a canted antiferromagnetic one in this phase.

We observed phonon echoes in P-doped Si (Si:P) at very low temperatures. We applied two radio-frequency pulses separated by a time delay of τ on Si:P and observed echo signal at t = 2τ in both insulating and metallic samples with varying dopant concentrations and of different sample forms of powders and bulk plates at temperature between 45 mK and 4 K. The echoes were much more pronounced in insulating powder samples than in metallic ones and in bulk ones. The echo intensity for a fixed τ increased very strongly as temperature was lowered but the echoes disappeared toward the superfluid-to-normal transition temperature of helium mixture in which the samples were immersed. We observed no appreciable change in the echo intensities as external magnetic field was varied up to 8 T. The echoes are interpreted to be dynamical polarization phonon echoes in piezoelectric powders of insulating Si:P with a dopant concentration n = 6 x 10¹⁷ cm^-3.

A series of Mn-site doping samples La_2/3Ca_1/3Mn_1-xCo_xO₃(0 ≤x ≤ 0.15) with an excellent single phase structure have been prepared. The magnetic and transport properties of this system have been systematically investigated. The Curie temperature T_C decreases gradually and the temperature range of transition becomes wider with the increase of x. The abnormal transport properties induced by Co were doping characterized by the resistivity behavior of double peaks and low temperature minimum.

In spintronics, the spin-Hall effect and its reciprocal, the inverse spin-Hall effect (ISHE), is of crucial importance, since these effects allow the mutual conversion between a charge current and a spin current. We have investigated ISHE in metallic films. The temperature dependence of the ISHE signal critically depends on material species, implying the coexistence of different electronic mechanisms of ISHE in metallic systems.

A correlation of phase diagrams with sharp jumps of the magnetization, which were previously observed upon magnetic-field-induced AFM-FM transition, has been studied for a number of low-bandwidth manganites. This transition was shown to proceed in milliseconds and is connected to a spontaneous heat avalanche, but mechanisms responsible for an origin and a development of the avalanche are still unclear. Different manganites (Pr_0.67Ca_0.33MnO₃, Sm_0.55Sr_0.45Mn¹⁸O₃, Eu_0.58Sr_0.42MnO₃) are shown to characterize by a qualitatively identical phase diagram, with the slope of the AFM-FM boundary in the H-T-plane being negative, that is probably guides the avalanche-like character of the transition.

Single crystal neutron diffraction experiments on non-centrosymmetric pressure-induced superconductor CeCoGe₃ were carried out in order to reveal a magnetic structure in the ground state. This compound shows multi-step magnetic transitions at T_N1=20 K, T_N2=11.5 K and T_N3=7.5 K. We have observed the appearance of the magnetic reflection at (1 01/2) together with the weaker one at (10 1/4) and its higher-order harmonics at 2.9K. These results indicate that the ground state magnetic structure consists of two components with dominant q₁=(0 01/2) and q₂=(0 03/4). No magnetic peak was observed along (0 0 l), indicating that the directions of corresponding antiferromagnetic moments are parallel to the c-axis. Consequently, CeCoGe₃ has at least two Ce sites in the ground state with respect to the magnitude of the magnetic moment.

The S=1 quasi-one dimensional Heisenberg antiferromagnet Ni(C₅H₁₄N₂₎₂N₃(PF₆), abbreviated as NDMAP, has been studied by electron spin resonance (ESR) in a magnetic field exceeding the critical field (H_c) above which this compound shows a long range order (LRO) at sufficiently low temperatures. Our recently developed ESR apparatus with a dilution refrigerator is utilized to study the spin excitations of NDMAP above H_c for H∥c, because this compound exhibits the LRO below about 1 K. The spin excitation modes above H_c observed below 1 K do not show any difference from those at 1.5 K and the lowest excitation mode observed at 30 mK in the inelastic neutron experiments above H_c was not observed at about 200 mK in this study. Accordingly, the result suggests that the lowest excitation mode is very sensitive to temperature.

Measurements of magnetization, electrical resistivity and nuclear- and electron spin resonance in In₂VO₅ reveal a remarkable interplay of different degrees of freedom in the zigzag V-0 chains (V⁴⁺, 3d¹, S = 1/2) which are realized in this material. At high temperatures the d electrons are itinerant and magnetic interactions are ferromagnetic. Below a crossover temperature T* ∼120 K the d states get localized and interact predominantly antiferromagnetically. Though magnetic resonance data clearly indicate the slowing down of the spin-spin correlations by approaching a characteristic temperature T_sro ∼ 20 K the system does not long-range order magnetically. We attribute these peculiar crossover phenomena to the unusual anisotropic thermal contraction of the lattice which controls the localization of the d-states, magnetic exchange and frustration.

A multi-frequency electron spin resonance (ESR) study of the heavy fermion compound YbRh₂Si₂ has been performed in fields up to ∼ 8T. In this regime the system is close to the breakdown of the heavy-fermion behavior which is confined to temperatures and fields T <T₀ ≈25K and B < B* ≈10T, respectively. A well-defined strongly anisotropic ESR mode is observed. We find striking similarities between the properties of this signal and the behavior of the electronic specific heat and the ²⁹Si NMR data which reflect the crossover between different electronic regimes in the Kondo state. These similarities suggest that the observed ESR mode corresponds to a "heavy electron spin resonance", i.e., a joint resonance excitation of the strongly hybridized f- and conduction electron states.

A Gd³⁺ electron spin resonance study in a frequency range 10 – 350 GHz on a single crystal of the spin-only antiferromagnet GdNi₂B₂C is reported. The Korringa relaxation broadening of the Gd-ESR signal is surprisingly anisotropic implying anisotropic interactions between the localized Gd³⁺ spins and the conduction electrons. In the antiferromagnetic state at T <T_N = 19.4 K we observe an out-of-plane anisotropy gap of ∼76 GHz. We also observe an in-plane gap with the same order of magnitude. Numerical analysis of the spin excitation modes in the ordered state assuming classical dipole-dipole interactions agrees with the experiment qualitatively well. However, quantitatively it yields appreciably higher gap values.

We have succeeded in growing large-size single crystals of the quasi-one-dimensional S = 1/2 spin-dimer system Pb₂V₃O₉ by the floating-zone method, and also investigated the thermal conductivity, specific heat and magnetization in magnetic fields. In high magnetic fields, it has been found that the suppression of the thermal conductivity by the application of magnetic field is relaxed. This behavior is caused by the enhancement of the thermal conductivity due to triplons and/or phonons owing to the extension of the mean free path of triplons and/or phonons in the Bose-Einstein condensed (BEC) state of field-induced triplons. We have estimated the critical field H_c(T) between the BEC state of triplons and the gapped normal state from the thermal conductivity, specific heat and magnetization measurements and obtained the value of the critical exponent oslash; ∼ 2.0 in [H_c(T) – H_c(0)] ∝ T^ø using H_c(T)'s at low temperatures.

We have measured ac susceptibility X_ac and electric resistivity ρ of polycrystalline CeCuAl₃ under pressure. At ambient pressure, X_ac(T) shows a peak and ρ(T) also a kink, the temperature of which increases gradually with pressure. These anomalies are attributed to a localized antiferromagnetic transition. The ρ(T) shows another broad peak at around 10 K at ambient pressure, which increases monotonically with pressure. We suggest that this peak comes from the small crystalline electric field splitting combined with Kondo effect and the increase is due to enhancement of the splitting by applying pressure.

The charge and spin properties of spin chains decorated with dimers and closed trimers (equilateral triangles) with commensurate partial filling (1/4 and 1/3, respectively) are considered. It is shown that due to the charge separation both systems prefer the ground state with even occupation per elementary cell, where the spin spectrum possesses the Haldane gap for negative spin exchange and magnon-like for positive coupling. The charge spectrum is always gapped.

We report the results of thermodynamic measurements on the quasi one-dimensional (1D) Ising-like antiferromagnet BaCo₂ V₂O₈ in the magnetic fields for H//c down to 200 mK. An appearance of a long range magnetic ordered phase is observed in the field induced region above H_c ≊ 3.9 T at temperature below 1.8 K. A peculiar incommensurate ordering, which reflects a quantum critical nature of S = 1/2 1D Ising-like antiferromagnet, realizes in this ordered phase.

The non-equilibrium phase transition in a ferromagnetic Ising model is investigated by use of a new type of effective field theory (EFT) which correctly accounts for all the single-site kinematic relations by differential operator technique. In the presence of a time dependent oscillating external field, with decrease of the temperature the system undergoes a dynamic phase transition, which is characterized by the period averaged magnetization Q, from a dynamically disordered state Q = 0 to the dynamically ordered state Q ≠ 0. The results of the dynamic phase transition point T_c determined from the behavior of the dynamic magnetization and the Liapunov exponent provided by EFT are improved than that of the standard mean field theory (MFT), especially for the one dimensional lattice where the standard MFT gives incorrect result of T_c = 0 even in the case of zero external field.

The charge and spin properties of spin chains decorated with dimers and closed trimers (equilateral triangles) with commensurate partial filling (1/4 and 1/3, respectively) are considered. It is shown that due to the charge separation both systems prefer the ground state with even occupation per elementary cell, where the spin spectrum possesses the Haldane gap for negative spin exchange and magnon-like for positive coupling. The charge spectrum is always gapped.

The magnetic susceptibility of the quasi-one-dimensional system with the periodic potential such as slow cooled (TMTSF)₂ClO₄ is studied numerically. The magnitude of the periodic potential is given by V. The imperfect nesting on the Fermi surface originates from higher harmonics (t'_bt₃, t₄) of the tight-binding model. We have found that q_x of Q = (2k_F +q_x, π/b+q_y) which gives a maximum of χ₀(Q) has a negative value at t₃/t_b ≥ 0.007, t₄/t_b ≥ 0.0001, and V < 2t'_b — 2t₄, where t'_b/t_b is fixed to be 0.1. It means that the negative quantum Hall coefficient is possible in these parameters. Since the sign reversal of the quantum Hall coefficient is observed in (TMTSF)₂ClO₄, t₃, t₄ and V should satisfy the above conditions. We also discuss the periodic oscillation of Hall resistance with sign reversal which has been observed by Uji et al.

We investigate the single-particle dynamics and magnetic properties in the two-orbital Hubbard model on the square lattice at quarter filling by using a cluster extension of dynamical mean field theory. We find that the Fermi-liquid state is stabilized up to the large intra- and inter-orbital interactions in the symmetric case without Hund's coupling. It is clarified that the Hund's coupling enhances the antiferro-orbital fluctuations, which gives rise to the pseudo gap behavior in the single-particle excitations. We also find that the Hund's coupling with intermediate strength causes the competition between the Fermi-liquid formation and the antiferro-orbital fluctuations, resulting in the nonmonotonous temperature dependence in the single-particle dynamics.

R₃Al₁₁(R=La, Ce, Pr, Nd, Sm) crystallizes in the orthorhombic La3Al11-type structure with two inequivalent R sites. We have succeeded in growing these single crystals by using Al self flux method, and have performed the magnetization. All of the compounds except for nonmagnetic La₃Al₁₁ show successive phase transition and R₃Al₁₁(R=Ce, Pr, Nd) exhibits metamagnetism at low temperature. As reported previously, Ce₃Al₁₁ shows a ferromagnetic transition at 6.2 K and an antiferromagnetic one at 3.2 K for the b axis and, in addition, we have found another anomalies inferred to be antiferromagnetic transition. Pr₃Al₁₁ shows an antiferromagnetic transition at 12.7 K and a ferromagnetic one at 3.5 K. Nd₃Al₁₁ shows three antiferromagnetic transitions at 13.5 K, 9.5 K and 2.9 K. Sm₃Al₁₁ does not follow the Curie-Weiss low and shows two antiferromagnetic transitions at 50 K and 26 K. These complex phase transitions and metamagnetism can be explained by two inequivalent R ions with different crystal symmetry.

Previously measured thermopower data of CeNiSn exhibit a significant sample dependence and non-monotonous behavior in magnetic fields. In this paper we demonstrate that the measured thermopower S(T) may contain a contribution from the huge Nernst coefficient of the compound, even in moderate fields of 2 T. A correction for this effect allows to determine the intrinsic field dependence of S(T). The observed thermopower behavior can be understood from Zeeman splitting of a V-shaped pseudogap in magnetic fields.

Dynamical properties of frustrated XXZ models in the Ising limit on spatially anisotropic two-dimensional lattices are investigated by a weak-coupling approach from the one-dimensional limit. Restricting the Hilbert space to that spanned by exact eigenstates of XXZ chains in the Ising limit, we calculate dynamical structure factor 5^-+(k,ω) in spatially anisotropic two-dimensional systems. As in the case of frustrated Heisenberg models [M.Kohno, et al., Nat. Phys. 3, 790 (2007)], spinons form bound states through hopping process between chains. We calculate the density-density correlation function of spinons in the Ising limit, and show that spinons in the bound state hop together as a pair, although those in a continuous spectrum behave almost independently. We discuss possible implications of the present results on fractionalization of bosonic and fermionic particles in spatially anisotropic frustrated systems with strong repulsion.

The effects of pressure on iron oxide hydroxide ferrihydrite (FeOOH∣nH₂O) nanoparticles with particle size of 4.7 ± 0.2 nm were investigated through DC magnetic measurements in the pressure region up to P = 13.5 kbar. The magnetization curve up to 50 kOe exhibits the coexistence of saturation and linear contributions. The value of high field susceptibility, the latter contribution, increases with increasing pressure for P ≤ 10.0 kbar. In contrast, the former contribution, associated to the uncompensated moment, decreases for P ≤ 10.0 kbar. At further pressure, there is no distinct pressure response. These effects of pressure are probably related to changes in the anisotropy energy due to the volume shrinkage.

From first-principle band structure calculations the Heusler alloys Fe₂MnSi and Co₂MnSi are predicted to be half-metals, if the ferromagnetic state is assumed. In reality, however, Fe₂MnSi exhibits a complex magnetic state with antiferromagnetic components, while Co₂MnSi is ferromagnetic with high Curie temperature and probably a half metal. In order to search better half-metals the alloys Fe_2-xCo_x MnSi are worth investigating. We have studied the magnetic properties of Fe_2-xCo_:cMnSi experimentally. With increasing Co concentration from x = 0, the antiferromagnetic phase rapidly disappears. The saturation magnetization increases with increasing x. For x≥ 1.0 the saturation magnetizations agree well with those calculated theoretically. The dependence of the saturation magnetization on x is well described by the Slater-Pauling rule. This result suggests that Fe_2-xCo_xMnSi is probably a half-metal for a wide range of x.

Thermal expansion of two-dimensional itinerant nearly ferromagnetic metal is investigated according to the recent theoretical development of magneto-volume effect for the three-dimensional weak ferromagnets. We particularly focus on the T²-linear thermal expansion of magnetic origin at low temperatures, so far disregarded by conventional theories. As the effect of thermal spin fluctuations we have found that the T-linear thermal expansion coefficient shows strong enhancement by assuming the double Lorentzian form of the non-interacting dynamical susceptibility justified in the small wave-number and low frequency region. It grows faster in proportional to y^-1/2 as we approach the magnetic instability point than two-dimensional nearly antiferromagnetic metals with ln(1/y_s) dependence, where y and y_s are the inverses of the reduced uniform and staggered magnetic susceptibilities, respectively. Our result is consistent with the Grüneisen's relation between the thermal expansion coefficient and the specific heat at low temperatures. In 2-dimensional electron gas we find that the thermal expansion coefficient is divergent with a finite y when the higher order term of non-interacting dynamical susceptibility is taken into account.

We investigate spin and orbital states of the two-orbital Hubbard model on a square lattice by using a variational Monte Carlo method at quarter-filling, i.e., the electron number per site is one. As a variational wave function, we consider a Gutzwiller projected wave function of a mean-field type wave function for a staggered spin and/or orbital ordered state. Then, we evaluate expectation value of energy for the variational wave functions by using the Monte Carlo method and determine the ground state. In the strong Coulomb interaction region, the ground state is the perfect ferromagnetic state with antiferro-orbital (AF-orbital) order. By decreasing the interaction, we find that the disordered state becomes the ground state. Although we have also considered the paramagnetic state with AF-orbital order, i.e., purely orbital ordered state, and partial ferromagnetic states with and without AF-orbital order, they do not become the ground state.

We report here on the discovery of novel features in a near-zero magnetization alloy, Nd_0.75Gd_0.25Rh₃B₂, which comprises two quasi-antiferromagnetic (or ferrimagnetic) transitions, and concomitantly repeated magnetic compensation feature, one after the other. The half-widths of the magnetization hysteresis loops associated with the (residual) remanent magnetization also has an oscillatory characteristic, with two local minima close to the two crossovers of the M = 0 axis in nominal zero field. We conjecture that the unusual observations arise from anisotropic hexagonal structure of the admixed compound, resulting in a quasi-one dimensional electronic structure, and due to the accentuated role of conduction electron polarization near the 'zero-magnetization' limit.

The phase separated state that is associated with a broad first order electronic phase transition exhibit various dynamic phenomena such as relaxation of resistivity and magnetization, cooling rate dependence, rejuvenation after ageing etc. In the case of magnetic systems, where the coexisting phases are ferro and anti-ferromagnetic, the presence of slow glass-like relaxation has led many authors to treat the phase separated state as a cluster-glass or spin-glass and it has been suggested that the observed time dependent effects are the result of inter-cluster interaction. In this work we report the time dependence resistivity data on the non-magnetic phase separated system, NdNiO₃. Our experimental results rule out the possibility of the formation of glassy state. We propose that the dynamical behavior in the phase separated systems has its origin in the presence of high temperature phases in their supercooled state persisting below the first order transition temperature. These supercooled phases are metastable and they switch to the ground state stochastically. We claim that this switching gives rise to time dependent effects in the phase separated regime. We further propose that all the systems which undergo a broad first order transition will exhibit time dependence in their physical properties in the phase separated regime owing at least partially to supercooled to normal switching of the different single phase regions of the material.

Raman spectra in TlCuCl₃ under the magnetic field up to 12 T are reported. Above the critical magnetic field of the quantum phase transition to the magnon Bose-Einstein condensation phase, the one-magnon Raman peak is observed. Above 10 T, the one-magnon energy approaches the energy of the A_g phonon (3.1 meV) and the anticrossing effect is observed. By using the Green-function theory with three adjustable parameters, we estimate the amplitude of the attractive magnon-phonon interaction as 0.043 meV. The consistency between the present results and those of previous works is discussed.

An effect of antiferromagnetic fluctuations on single-particle excitations is examined on the basis of an extended dynamical mean-field theory that takes account of long-range spin fluctuations onto the local self energy. We demonstrate that the single-particle spectral intensity in the two-dimensional Hubbard model at half-filling exhibits the pseudo-gap behavior near the Fermi energy due to the critical antiferromagnetic fluctuations. We also show that for the symmetric case in the Anderson lattice model, the hybridized-band gap is enlarged by the antiferromagnetic fluctuations. In both models the antiferromagnetic fluctuations suppress the double occupancy of the correlated electrons.

We have investigated temperature and magnetic-field dependence of dielectric properties in the orthorhombic GdMnO₃ single crystal which is located near the phase boundary between the ferroelectric/spiral-antiferromagnetic phase and the paraelectric/A-type-antiferromagnetic one. In this compound, strong phase competition between these two phases results in a unique phase diagram with large temperature and magnetic-field hystereses. Based on the phase diagram, we have successfully demonstrated the persistent and reversible phase switching between them by application of magnetic fields.

An enriched ¹⁹⁵Pt system was investigated by susceptibility measurements. Among isotopes of Pt, only ¹⁹⁵Pt has the nuclear spin I = 1/2 with the nuclear magnetic moment 0.597μN and its natural abundance of ¹⁹⁵Pt is 33.8%. Large magnetic moments are distinctly observed in enriched ¹⁹⁵Pt wire samples which are isotopically 97.3, 80.0 and 33.8%-enriched ¹⁹⁵Pt and in 97.3%-enriched thin film samples. These large magnetic moments were proportional to the ¹⁹⁵Pt concentration. Observed magnetic moments for the powder samples with 97.3, 86.6% and natural abundance of ¹⁹⁵Pt, and for the wire sample with the natural abundance was smaller than that for enriched wire samples. These behaviors can be described in part by Fe impurities, but moments were still larger than the pure magnetic moment of Fe for wire samples.

The spin reorientation (SR) phase transition in dysprosium iron garnet (Dy₃Fe₅O₁₂ or DyIG) have been studied by specific heat C_p(T) and high field magnetisation measurements M_T(H) and M_H(T) on single crystals at low temperature. A first order SR is observed with a sharp jump at T_SR = 14.5±0.5 K in the C_p(T) curve which corresponds to a spontaneous change from the high temperature (HT) easy direction ⟨111⟩ to an ⟨uuw⟩ angular low temperature (LT) phases. Above T_SR, the magnetic structure is described by the irreducible representation (IR) A_2g of the rhombohedral space group R 3 c. Below T_SR, the magnetic structure changes in the monoclinic the space group C2/c with the IR A_g. When the field H is kept aligned along the hard symmetry directions ⟨100⟩ and ⟨110⟩, we obtain respectively the variation of the angular positions θ(T) and θ'(T) from the total spontaneous magnetisation down to 1.5 K (θ = 39.23° and θ' = 30.14°) and the results are in good agreement with the previous observations in low fields. When the sample is allowed to rotate freely on itself, the critical field H_c(T) between the HT⟨111⟩ and the LT⟨uuw⟩ angular phases permits us to precise the transition line up to 15 T and 40 K between the so called canted field induced (FI) and the associated collinear magnetic phases. The experimental magnetic phase diagram (MPD) is precisely determined in the (H_c-T) plane and the domains of the existence and the stability of the two magnetic phases are specified.

Angle-dependent magneto-resistance measurements in the vicinity of the pre-martensite phase (T_PM∼219 K) in Ni₂MnGa are reported. Magneto-resistance measurements detect a change in slope, from positive to negative near T_PM that can be suppressed with a modest magnetic field. Mirroring this change in slope, published ARPES data [Opeil et al., Phys. Rev. Lett. 100, 165703 (2008).] show a significant depletion of states (pseudo-gap) forming at 0.3 eV below the Fermi energy.

We study the electronic correlation effects in armchair graphene nanoribbons that have been recently proposed to be the building blocks of spin qubits. The armchair edges give rise to peculiar quantum interferences and lead to quenched kinetic energy of the itinerant carriers at appropriate doping level. This is a beautiful one-dimensional analogy of the Landau-level formation in two dimensions except the magnetic field is not needed here. Combining the techniques of effective field theory and variational wave function approach, we found that the ground state exhibits a new type of flat-band ferromagnetism that hasn't been found before. At the end, we address practical issues about realization of this novel magnetic state in experiments.

We have shown that resonant quantum tunnelling of the magnetisation (QTM), until now observed only in 0D cluster systems (SMMs), occurs in the molecular Ising spin chain, CoTAC ([(CH₃)₃NH]CoCl₃ – 2H₂O) which orders as a canted 3D-antiferromagnet at T_C = 4.15 K. This effect was observed around a resonant like field value of 1025 Oe. We present here measurements of the relaxation of the magnetisation as a function of time, from the zero field cooled (ZFC) antiferromagnet state and from the saturated ferromagnet state. We show that, at the resonant field, the relaxation from the saturated state occurs in a complicated process, whereas, surprisingly, in the case of the ZFC state, the relaxation is exponential.

Magnetotransport and magnetocaloric properties of polycrystalline La_2/3Ca_1/3Mn_1-xSi_xO₃ (x = 0.05, 0.10, 0.15 and 0.20) have been studied. The colossal magnetoresistance (CMR) effect and large magnetic entropy change (ΔS_M) have been found in the vicinity of Curie temperature (T_C). With the increases of Si content x, the MR and ΔS_M peak positions as well as TC gradually shift to low temperatures, while MR peak value gradually increases from 84% to 140%, and the maximum value of magnetic entropy change remains nearly the same high value, in the range of 8.4 − 9.6 J Kg⁻¹ K⁻¹ for a magnetic field variation of 6 T.

The magnetocaloric effect (MCE) in a TbNiAl₄ single crystal was firstly investigated by magnetization and heat capacity measurements. Along a-axis the sample exhibits a large inverse MCE, where the material cools when a field is applied. The maximum value of magnetic entropy change (ΔS_M^max) and adiabatic temperature change (ΔT_ad^max), for a field change of 4 T and at around 20 K, are found to be 5.1 J/Kg K and -3.3 K, respectively. It is proposed that the large inverse entropy change is associated with the sensitive field dependent magnetic phase transition.

The orthorhombic phase HoMnO₃ (o-HMO in Pbnm symmetry setting) thin films were prepared on LaAlO₃(110) (LAO(110)) substrates by pulsed laser deposition. While for films grown on LAO(110) substrates, the compressive strain along the b-axis was resulted from the tensile strain in the a-c plane. The films provide the opportunity of investigating the effects of strain, hence lattice elasticity, on the physical properties of this material. For o-HMO films an antiferromagnetic ordering with T_N∼42 K, irrespective to the direction of applied field was clearly observed. However, an additional magnetic ordering occurring around 26.4 K was observed when the field was applied along the c-axis of o-HMO. This transition, however, was absent when the field was applied along a- and b-axis. These results indicate that the second magnetic ordering observed along the c-axis could be more relevant to the Mn moments lying along the partially strained b direction of the o-HMO which has been theoretically expected to result in incommensurate-commensurate lock-in transition.

Electron spin resonance (ESR) was performed on the mixtures of two spin-gap compounds IPA-Cu(Cl_xBr_1−x)₃ for 0.87 ⩽ x < 1. Although the ESR line for x = 1 split into two lines below 10 K, no splitting ESR lines appeared down to 1.4 K for x = 0.99 and 0.98. This difference is due to bond randomness, i.e., a change from the pure spin-gap state to bond-randomness-induced disordered states.

Quantized Berry phases proposed as local order parameters for gapped quantum liquids at zero temperature are studied to detect the Kondo singlet phase which is known as a theoretical prototype of the Kondo insulators. We deal with the Kondo lattice model and the periodic Anderson model as two basic models for heavy fermions. We show that the Berry phase is actually quantized as trivial or non-trivial value, i.e., 0 or π, and analytically obtained both in the strong coupling limit of the Kondo lattice model and in the weak coupling limit of the periodic Anderson model. In addition, the Heisenberg Kondo lattice model is studied numerically by the quantized Berry phase.

We have measured the electrical resistivity of YbT₂Zn₂₀ (T = Ir, Rh) single crystals under pressure up to 2.4 GPa. We found that the resistivity of both compounds shows a pronounced maximum T_max under pressure as seen in YbCo₂Zn₂₀ at ambient pressure. The temperature T_max decreases with increasing pressure, reflecting the lowering of the Kondo temperature T_K. Low temperature resistivity behaves as expected for a conventional Landau Fermi liquid ρ = ρ₀ + AT². The A coefficient is strongly enhanced upon applying pressure and can be related to the T_max by A^−1/2 ∝ T_max. This relation holds for YbT₂Zn₂₀ (T = Ir, Rh and Co) compounds in the pressure range investigated here, suggesting the validity of the Fermi liquid description in this system.

Spinless fermion t-V model on a triangular lattice has been studied. In the model, the presence of a novel metallic charge-ordered ground state named a pinball liquid state has been proposed. To clarify the metallic features in the pinball state, we calculate the momentum distribution function of the model. For comparison, we also analyze a corresponding model where charge ordering is strongly pinned. The results are consistent with the picture expected for the pinball liquid, where an insulating ordered state (pin) and a metallic state on the honeycomb lattice (ball) coexist.

We measured the electrical resistivity in the current J||[100] and [001] and the Hall coefficient for the magnetic field H||[010] and [001]. The resistivity is anisotropic for the current direction. The Hall coefficient R_h is also extremely anisotropic, revealing that R_H for H||[010] and [001] possesses a positive maximum at 55 K for H||[010], and a negative minimum at 25 K for H||[001], respectively. The temperature dependence of R_H is well explained by the skew scattering processes at high temperatures.

DCNQI-Cu system exhibits a mysterious metal-insulator-metal (M-I-M) phase transition. Deuteration of methyl basis can control critical temperatures of these peculiar phase transitions. We have been studying a mechanism of the transitions by measuring thermal properties. To quantitatively discuss the phase transitions, we have developed specific heat measurement system in a wide temperature range between 5 K and 200 K for the sample with the weight of less than 0.3mg. We have succeeded in an observation of the specific heat jump and latent heat at this first order phase transition in the cooling and heating processes.

We report the pressure effect on a steplike magnetostriction of single crystalline (La_0.4 Pr_0.6)_1.2Sr_1.8Mn₂O₇ bilayered manganite, for our understandings of the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase. Application of pressure suppresses a steplike transformation and causes a broad change in the magnetostriction. A delocalization of carriers is promoted with the applied pressure, resulting in the suppressed steplike behavior. The steplike behavior observed is closely related to the existence of localized carriers.

We have firstly observed ²⁷Al-NQR signal in CeCuAl₃, which is assigned to the Al(2) site, one of the two crystallographic sites of Al (Al(1) and Al(2)). The NQR spectrum in the paramagnetic region is composed of two kinds of Al(2) signals associated with the different environments. One comes from Al(2) in the regular atomic configuration and the other is attributed to Al(2) affected by the neighboring site-disorder between Cu and Al(1) sites. NQR measurement were performed on sample containing less than 5% site-disorder which is the best one we obtained. The start of the spectral broadening as well as the critical divergence of the nuclear spin-lattice relaxation rate 1/T₁ indicate that T_N ≃ 3.3 K. The broad spectrum suggests an incommensurate modulation in the antiferromagnetic state. 1/T₁ in the paramagnetic state is well explained in terms of the dynamics of Ce 4f localized moment through RKKY and the cf exchange interactions.

The low-temperature critical properties and crossovers of a spin-½ planar ferromagnet in a longitudinal magnetic field are explored in terms of an anisotropic bosonic action, suitable to describe the spin model in the low-temperature regime. This is performed adopting a procedure which combines an averaging over dynamic degrees of freedom and the classical Wilson renormalization group transformation. Within this framework we get the phase boundary, ending in a quantum critical point, and general expressions for the correlation length and susceptibility as functions of the temperature and the applied magnetic field within the disordered phase. In particular, two crossovers occur decreasing the temperature with the magnetic field fixed at its quantum critical point value, which might be actually observable in complex magnetic compounds, as suggested by recent experiments.

⁷⁷Se NMR signals were observed in the metallic and insulating states of the quasi-one-dimensional organic compound (DIETSe)₂GaCl₄. The ⁷⁷Se NMR spectra at low temperatures are broadened, indicating the presence of magnetic ordering. The shape of the spectra in the magnetically ordered state is characteristic of the spin density wave (SDW) ordering. The divergent behavior of the temperature dependence of the nuclear spin-lattice relaxation rate suggests that the SDW ordering occurs at about 7 K which is lower than the metal-insulator transition temperature of 12 K.

We derive a formula for the differential conductance measured in Scanning Tunneling Spectroscopy for two magnetic atoms absorbed on metal surface. We find that the shape of the differential conductance spectra varies with the distance between the two atoms. This change indicates the variation in effective range of the Kondo effect and the RKKY interaction effect.

Dielectric polarization P of a geometrically frustrated spin system MnWO₄ has been examined for B || x and E || b* in pulsed magnetic fields up to 52 T. We have observed two new phased IV (38 ⩽ B ⩽ 48 T) and V (B ⩾ 48 T). Moreover, we found that the IV-phase also exhibits ferroelectricity, in addition to the AF2-phase already known. Interestingly, the sign of P of the IV-phase is found to be always opposite to that of AF2 irrespective of the direction aof a poling electric field. The results implied that the ferroelectric domain is preserved between the AF2 and IV phases.

We have measured the Hall effect of the single crystalline CeAl₂ under high pressure up to 3 GPa. An anomalous Hall effect was observed at high pressures. The Hall coefficient R_H at 0.1 GPa is positive (R_H ∼ 1.7×10⁻⁹ m³/As). It decreases smoothly with increasing pressure below 2 GPa, but rapidly above 2 GPa. The sign of the Hall coefficient is found to change from positive to negative as pressure increases. These results suggest a pressure-induced crossover in the electronic state around 2.7 GPa.

We investigate the ground-state properties of the one-dimensional two-band Hubbard model with different bandwidths. The density-matrix renormalization group method is applied to calculate the averaged electron occupancies n as a function of the chemical potential μ. Both at quarter and half fillings, "charge plateaux" appear in the n-μ plot, where dμ/dn diverges and the Mott insulating states are realized. To see how the orbital polarization in the one-quarter charge plateau develops, we apply the second-order perturbation theory from the strong-coupling limit at quarter filling. The resultant Kugel-Khomskii spin-orbital model includes a magnetic field coupled to orbital pseudo-spins. This field originates from the discrepancy between the two bandwidths and leads to a finite orbital pseudo-spin magnetization.

Electrical resistivity (ρ) and DC magnetization (M) have been measured as a function of temperature (T) for layered Li_xCoO₂ (x = 0.92, 0.47 and 0.42) single crystals, which are obtained by deintercalating Li from single crystals grown in a floating-zone furnace through immersion in NO₂BF₄/CH₃CN solution. The ρ—T curve for x = 0.92 is found to be insulating but a metallic behavior is observed for x = 0.47 and 0.42. The M—T curve both for x = 0.47 and 0.42 exhibits a sudden decrease below T_s∼170 K. Also, the ρ—T curve both for x = 0.47 and 0.42 shows a jump-like anomaly at T_s∼170 K with thermal hysteresis, indicating a first-order phase transition. These behaviors of the M—T and ρ—T curve suggest an occurrence of spin state transition of Co⁺⁴ at ∼170 K. Furthermore, the M—T curve after rapid cooling becomes different from that after slow cooling below T_F. T_F depends on x and appears to correspond to the temperature at which the motional narrowing of the ⁷Li NMR line width is observed, suggesting that the Li ions are released from the regular site of the rigid lattice and begin to diffuse above T_F.

We report the temperature dependence of electrical resistivity and thermoelectric power of the manganese perovskites Bi_1−xCa_xMnO₃ (0.7 ⩽ x ⩽ 0.95). The resistivity for x = 0.75 and 0.7 is well fitted with ρ ∝ exp(l/T)^(1/4) below the charge-ordering temperatures (T_co). The temperature dependence of thermoelectric power for samples with x = 0.8, 0.75 and 0.7 shows the rapid decrease below T_co. In the lower temperatures than T_co, it goes through minimum and absolute thermoelectric power decreases and tends to change the sign. The activation energy was estimated as 57, 64, 148 meV for x = 0.8, 0.75, 0.7 from the linear fits in the form S − 1/T plots. The activation energy increases with decreasing the Ca content in the Bi_1−xCa_xMnO₃ system.

We investigate the reduced dynamics of two qubits coupled to an interacting quantum spin bath modeled by a XXZ spin chain. By using the method of time-dependent density matrix renormalization group (t-DMRG), it is shown that when the spin bath is in an ordering state, the reduced dynamics of qubits strongly depends on the range of the spin chain that qubits coupled to. Only for ordering spin baths, both decoherence and entanglement show qualitatively different behaviors when the coupling range changes. In particular, In particular, we find that in the parametic regime where there is no ordering in the bath, the coupling range is irrelevant. On the other hand, for ordering spin baths, when the coupling range increases from local coupling to infinite range coupling, the reduced dynamics of qubits exhibits a transition in which initially entangled states no longer suffers from the entanglement sudden death when the coupling range is large.

We report a comparative study of magnetotransport in high quality epitaxial and polycrystalline thin films of Sr₂FeMoO₆ deposited on single crystal substrates of SrTiO₃ (STO) and Yttria-stabilized zirconia (YSZ) respectively. The polycrystallinity of the films on YSZ affects their magnetotransport significantly. The resistivity ρ(T) of such films is thermally activated in contrast to metallic behavior of ρ(T) of the thin films on STO. A large (≈ 13 %) low-field magnetoresistance (MR) at 10 K is seen in the polycrystalline samples compared to the negligibly small MR (≈ 2.5 %)of their epitaxial counterparts. A detail analysis of the temperature and magnetic-field-dependent resistivity and magnetization provide strong evidence for tunneling of spin-polarized carriers through a magnetic intergranular material of insulating ground state.

We present measurements of the low temperature Hall effect and magnetoresistance in the heavy fermion superconductor CeCo(In_0.925Cd_0.075)₅ as a function of applied magnetic field. The Hall resistivity ρ_xy is observed to be negative indicating electron dominated transport. Features corresponding to the possible destabilization of the antiferromagnetic ground state are observed. Magnetic field sweeps show that the resistivity ρ_xx(H) is hysteretic in this regime, indicating that these transitions could be first order in nature.

We report the real-space observation of magnetic domains in the ac-plane at the metamagnetic transition in a triple-layer ruthenate Sr₄Ru₃O₁₀ by magneto-optical imaging and scanning Hall probe measurements. We find anisotropic magnetic domain structures with weak c-axis correlation at low fields. The magnetic inhomogeneities are removed above the metamagnetic transition field. These changes of magnetic structure support the phase-separation scenario for the ultra-sharp steps in magnetoresistance.

The thermopower S and electrical resistivity ρ of YbMn₂ Ge₂ were measured under pressures up to 2 GPa in the temperature range from 1.5 to 300 K. The magnetic transition temperature increases with increasing pressure, having maximum at P_c ≈ 1.3 GPa, and decreases with increasing pressure above P > 1.3 GPa. The temperature dependences of S and ρ change their features at critical pressure P_c ≈ 1.3 GPa, where reported that the mean Yb valence value changes from +2.4 to +2.8 at room-temperature. The pressure dependences of S and ρ at 4.2 and 300 K show the abrupt changes at the critical pressure, which indicate the changes of the electronic state near the Fermi level and the magnetic structure of Mn 3d electron.

The magnetic state of the Co 3d-electron subsystem of RCo₃ compounds (R=rare-earth elements) with the rhombohedral PuNi₃-type structure strongly depends on external parameters. In order to clarify the effect of pressure on the magnetic state of the itinerant Co 3d-electrons, we have measured the electrical resistivity and thermopower at temperatures from 2 K to 300 K under hydrostatic pressures up to 2 GPa. Both, ρ and S show anomalies at critical temperature of metamagnetic transition T_m. With increasing pressure T_m, determined from the temperature-dependent resistivity and thermopower, decreases and apparently vanishes at P ≈ 0.7 GPa. The electrical resistivity and thermopower at low temperatures show abrupt changes at P ≈ 0.7 GPa, indicating a pressure-induced phase transition.

¹³C nuclear magnetic resonance (NMR) investigations were performed on the one-dimensional organic conductor (TMTTF)₂SbF₆ to clarify its electronic properties in the proximity of the ground states. An abrupt broadening of ¹³C NMR absorption lines below 8 K (T_N = 8 K), confirmed a long-range antiferromagnetic phase transition. Below T_N, the absorption lines are composed of four distinct broad lines, indicating a commensurate magnetic structure. The amplitude of staggered magnetization, ρ, is comparable to (TMTTF)₂Br, and on the order of 0.1 μ_B according to the splitting of ¹³C NMR lines at 3 K.

We report ultrasonic measurements on the high quality single crystal of the Yb-based heavy fermion compound YbRh₂Zn₂₀ over a temperature range from 200 K to 0.5 K. A shallow, but clear minimum was observed in the temperature dependent elastic constants C₁₁, (C₁₁ – C₁₂)/2 and C₄₄ around 15 K, probably attributed to the ground state and low-lying excited states of Yb³ in the cubic CEF. We discuss the low-temperature elastic properties and possible energy level scheme of localized 4f state of Yb³ ions in YbRh₂Zn₂₀.

CEF ground state developed at the low temperatures and physical parameters relating to a quadrupolar moment in YbRh₂Zn₂₀

We measured the thermal expansion and magnetostriction of a polycrystalline concentrated Kondo compound CePtSi₂, which has an orthorhombic CeNiGe2-type structure and the characteristic Kondo temperature T_k of 3 K. The linear thermal expansion dl/l decreases smoothly with decreasing temperature down to 2.5 K, and shows a rapid increase below 2 K due to antiferromagnetic ordering. The magnetostriction at 0.7 K shows an anomaly below 0.5 T, suggesting the disappearance of antiferromagnetic ordering.

We report a ⁵⁹Co NMR study on a quasi-one-dimensional cobalt oxide Ca₃Co₂O₆ having the in-chain ferromagnetic and out-of-chain antiferromagnetic interactions. We have found an increase of the ⁵⁹Co nuclear transverse relaxation rate 1/T₂ at 6.105 T on cooling temperature below 200 K, much higher than the spin freezing temperature of 25 K at 0 T. The result suggests that the unusual slowing down of spin fluctuations in a wide temperature range, probably arising from the spin frustration on the triangular lattice of the ferromagnetic chains. The NMR signals wiped out below 80 K due to the fast relaxation relive below 17 K, the spin freezing temperature measured at the NMR time window.

Two dimensional anti-ferromagnetic solid ³He adsorbed on graphite, so called 4/7 phase, is a strongly frustrated quantum spin system and the ground state is considered to be a gapless spin liquid. To clarify the magnetic behavior, the magnetization curve has been investigated below 1 mK in high magnetic fields, using NMR over the wide frequency region up to 360 MHz. The magnetization is found to reach full saturation at around 10 T.

In order to investigate a heavy-fermion like behavior on Ce-rich side of amorphous Ce_xMn_100−x alloys system, the ⁵⁵Mn NMR measurements on the powdered sample of amorphous Ce₆₂Y₁₉Mn₁₉, being diluted with non-magnetic Y, have been carried out in the temperature range from 1.8 to 270 K. A broadened resonance spectrum containing five NQR lines is observed in the measuring temperature region. Temperature dependences of Quadrupole coupling constant (e²Q_q) and the line width of ⁵⁵Mn NMR are obtained by means of analysis of each spectrum. Temperature dependence of the line width is expressed by the Curie-Weiss law with T_c = −12.0 K. The 1/(T₁T) is rapidly increased in proportion to inverse square root of temperature with decreasing temperature, but it keeps almost constant below about 6 K, which shows the heavy-fermion state in Ce₆₂Y₁₉Mn₁₉.

Thermoelectric power S(T) of CeRhGe3 and CeIrGe3 were measured from 6 to 280K and under pressure up to 1GPa. For both compounds, S(T) shows a positive maximum of 10–14μV/K at around 60K due to the kondo effect under influence of the crystalline electric field. At low temperatures, S(T) becomes to be negative and shows a clear negative peak at 10.5K for CeRhGe3 and a distinct kink at 8.7K for CeIrGe3. These anomalies are corresponding to the antiferromagnetic transitions. As pressure is applied, the positive maximum and the low temperature anomalies slightly shift to higher temperatures. At the same time intensities of the maximum and the peak are enhanced. It is suggests that both the Kondo effect and the magnetic correlation are increased by pressure.

Using the Kotliar and Ruckenstein slave-boson formalism we consider the finite-U Anderson lattice. We study the appearance of ferromagnetic instabilities and their stabilities. We study both the groundstate and the finite temperature regimes as a function of the Coulomb repulsion, density and f-level location. In the insulating regime there is a ferromagnetic phase that competes with the paramagnetic phase. In the metallic regime there are two ferromagnetic phases as the coupling grows. We calculate the critical temperatures as a function of coupling.

Electronic states of the two-orbital Hubbard model are investigated by means of the composite opeator method. In addition to the transfer within the same kind of orbital, we introduce the off-diagonal transfer t', which provides the mixing of orbitals. In the t' = 0 case, the system shows the orbital selective Mott transition at U = 4. Upon adding t', the band gap goes wider. This increase of the gap originates from the crossover between the Mott-insulator and the band-insulator.

We have succeeded in growing BaAl₄-type CeCu_xAl_4−x (x ∼ 0.7) and ThMn₁₂-type CeCu₄Al₈ by Al self-flux method for the first time, and the magnetic susceptibility χ and electrical resistivity ρ have been measured. These measurements indicate that CeCu_xAl_4−x is a ferromagnet with the Curie temperature of 3.7 K, and another unknown phase transition appears at T₀ = 6.8 K as an abrupt increase in ρ. This latter transition is probably not due to magnetic origin, because χ does not show any anomalies at around T₀. CeCu₄Al₈ orders antiferromagnetically below 4.7 K. We have performed crystalline electric field analysis from the anisotropy in χ, and clarified that Kondo effect plays an important role especially for CeCu₄Al₈.

Raman scattering spectra of single crystalline PrRu₄P₁₂ have been measured in the temperature region between 4.6 K and 300 K. Additional peaks appear below metal-insulator transition temperature. Among these peaks, CEF excitations for two different Pr ion sites can be assigned from the results of magnetic field (H⩽8T) dependence and polarization dependence. From the energy of CEF excitations in Raman spectra, we have re-determined CEF energy levels for Pr1 and Pr2 ions. From the result of CEF level schemes and the different atomic displacement of Ru around Pr1 and Pr2, Pr1 ion is affected by a strong positive point charge potential of Ru, and Pr2 ion feels strong p-f hybridization.

Kagome staircase compound Co₃V₂O₈ (S = 3/2) has a structure very similar to multiferroic compound Ni₃V₂O_g (S = 1), but their magnetic phase diagrams differ noticeably. We present the results of the first NMR study in Co₃V₂O₈ single crystal. From ⁵¹V-NMR spectra, the components of electric field gradient (EFG) tensor and of magnetic shifts tensor, K_i, are obtained. The temperature dependences of NMR shifts ⁵¹K_i for each main crystal axis direction are well described by a spin contributions in the paramagnetic phase. In ferromagnetic phase the zero field ⁵¹V-NMR spectrum is observed in the temperature range of 1.5–6.3 K.

Magnetic twist driven magnetoresistance in Fe/Tb magnetic multilayer is studied. The negative MR ratio is observed to be 24.6 % on [Fe(12 nm)/Tb(15 nm)]₂₅ at 4.2 K, which is much larger than that of Tb monolayer film, 9.9 %. Such large MR has never reported in multilayer systems including rare earth metal. Assuming that the MR of Tb can also be explained by the AMR effect, it means that the MR effects of the Tb layer is enhanced by spin polarization of the Fe layers.

Low-temperature magnetic properties of condensed matter system is investigated microscopically with high-frequency electron spin resonance (ESR). Experiments are carried out conventionally with a transmission method, but its sensitivity is not often sufficient to detect ESR signals of systems with small number of spins such as newly synthesized microcrystals. To attain better sensitivity, we focus on a novel technique of multi-frequency ESR system utilizing a microcantilever. In this method, ESR signal is detected as a torque change associated with the absorption of electromagnetic wave. Its sensitivity is greatly increased due to mechanical resonance when the modulation frequency of electromagnetic wave coincides with the eigenfrequency of the cantilever. In this study, we have succeeded in mechanical detection of ESR signals in the millimeter wave region up to 240 GHz. The achieved signal-to-noise ratio was greater than 10³ for 1-μg sample of Co-Tutton salt, corresponding to the spin sensitivity of 10⁹ spins/G.

Natural mineral Dioptase (Cu₆Si₆O₁₈ ⋅ 6H₂O) is an S = 1/2 antiferromagnet with a unique spin network which consists of helical chains along the c axis connected by inter chain exchange interactions. In order to study the ground state of dioptase, we have performed the magnetization measurement for B//c (easy axis) at 1.5 K and have found the spin-flop transition at 13 T. Moreover, the pressure effect of the magnetic susceptibility is investigated up to 1.78 GPa using the SQUID magnetometer where T_N increased while the broad maximum reflecting the low dimensionality of the system decreased by pressure. The results will be discussed in connection with our previous antiferromagnetic resonance (AFMR) measurement results and the phase diagram obtained by Gros et al.

We theoretically study the properties of the photo-carriers created in a one-dimensional Mott insulator subject to strong AC electric fields. The time-dependent density matrix renormalization group method is used to calculate the nonlinear optical conductivity, for which a metallic, gapless mode is found to emerge. The mode has linear dispersion with a charge velocity that differs from the free electron velocity, which indicates that the photo-doped carriers behave collectively as in the Tomonaga-Luttinger liquid.

We have performed the specific heat and the electrical resistivity measurements under high magnetic fields for Heusler compounds Ru₂CrGe and Ru₂CrSn. We found that the electrical resistivity for Ru₂CrGe shows an upturn in the vicinity of 130 K and changes a metallic to a semiconducting-like behavior with decreasing temperature. By applying magnetic field, a negative magnetoresistance was observed below 200 K. On the other hand, the electrical resistivity for Ru₂CrSn shows a semimetallic-like behavior. The results of the electrical resistivity in Ru₂CrGe suggest that the compound has a gap-like structure in the electronic structure at the Fermi level. Moreover, it was found that another phase transition occurs in Ru₂CrGe and Ru₂CrSn, in addition to the magnetic transition.

We have studied the magnetic properties of Ba-doped layered cobalt oxysulfide (Sr,Ba)₂Cu₂CoO₂S₂ with CoO₂ squre-planes. The antiferromagnetic nature was considered to originate from the CoO₂ layer dominantly. However, the effect of the inter-layer of CoO₂ planes has not been clear. We therefore synthesized polycrystalline (Sr, Ba)₂Cu₂CoO₂S₂, and investigated the effect of inter-layer distance on the magnetic properties in detail. When Sr was substituted by Ba, the difference between zero-field-cooled and field-cooled magnetization was found to decrease clearly, and such difference was ascribed to reflect the increase of the distance between CoO₂ layers owing to the difference of the ionic radii between smaller Sr and lager Ba. There is a possibility that two dimensional magnetic order has strengthened.

⁵¹V NMR measurements have been made on powdered samples to investigate the metal-insulator (MI) transition and the local magnetic properties of the Hollandite vanadate K₂V₈O₁₆ which undergoes the MI transition at T_MI∼170 K. An asymmetric ⁵¹V NMR spectrum in the metallic phase has the T-dependent negative Knight shift K. The two NMR spectra appears around T_MI, showing the coexistence of the metallic and insulating phases in consistent with the two-step first-order transition. The temperature dependence of K and the ⁵¹V nuclear spin-lattice relaxation rate indicates the presence of the ferromagnetic spin fluctuations in the metallic phase. A ⁵¹V NMR spectrum observed below T_MI has the temperature-independent K∼0.35%, showing the presence of the nonmagnetic ground state.

Magnetic properties of Cu₃(OH)₄SO₄, which is a model substance of an S = 1/2 triple chain system, have been investigated by the magnetization and the high field ESR measurements. The magnetization measurements of powder sample have been performed up to 7 T at 2 K. The derivative magnetization curve shows a jump at 1.2 T due to a magnetic phase transition. The high field ESR measurements have been performed using the pulsed magnetic field up to 16 T in the temperature range from 1.8 K to 300 K. The resonance shifts due to low dimensionality are observed below 40 K far from T_N = 5.3 K. The frequency-field diagram at 1.8 K below 5 T shows AFMR modes, which can be described by conventional AFMR theory of two sublattice with an uniaxial anisotropy, and the spin flop transition field is estimated to be 1.2 T. The ESR modes above 5 T deviate from the conventional AFMR modes.

We investigated the substitution effects of Mg²⁺, Ca²⁺, and Al³⁺ for Cr³⁺ on the magnetic, transport and thermal properties of delafossite oxide CuCrO₂, which possesses a quasi-2D Heisenberg triangular antiferromagnetic (AF) lattice. Magnetization and specific heat measurements indicated that AF ordering is promoted by the substitution of nonmagnetic Mg²⁺ for Cr³⁺ (S = 3/2), despite the fact that a spin vacancy is introduced in a 120° spin structure at the Cr sites. These results suggest that the residual magnetic frustration is partially broken by spin fluctuations, which are enhanced through the interaction between the itinerant hole introduced by Mg doping and the localized spin at the Cr site.

We study the phase structure of mixtures of strongly interacting bosonic atoms in a combined potential of a parabolic trap and an optical lattice. We apply a mean-field approximation and a local density approximation to the Bose-Hubbard model for the Bose-Bose mixture, and calculate the superfluid order parameter and the number of particles per site for both species. We find that the mixture is phase-separated due to the strong onsite inter-species repulsion. Considering a zero-hopping limit, we find that the coexisting phase can appear when onsite inter-species repulsion interaction is weak.

Magnetic properties of the S = ½ Heisenberg spin-chain material (6MAP)CuCl₃ have been probed by means of magnetization, specific-heat and electron spin resonance (ESR) measurements. A pronounced low-temperature Curie-like tail in the magnetic susceptibility was found at low temperatures. As recently suggested, such a behavior could originate from staggered magnetization effects, responsible also for the field-induced gap opening in S = ½ chains with broken translational symmetry. We found a clear anomaly in the specific heat having a broad maximum at T_max ∼ 2.3 K, accompanied by a significant ESR line broadening and g-factor shift. The specific-heat anomaly is magnetic-field independent, indicating that its origin is not related to the field-induced transverse staggered magnetization effects.

NdCo₂ undergoes a second order paramagnetic to ferromagnetic transition at T_C ≈ 100 K accompanied with a cubic to tetragonal transition. It transforms from tetragonal to orthorhombic structure along with a spin orientation when the temperature is decreased below T* ≈ 42 K. The heat capacity and resistivity measurements across the magneto structural transition at 42 K in NdCo₂ are presented. Effect of magnetic field on the transition is discussed in light of magneto elastic coupling. As field increases, the peak in heat capacity across the transition at T* shifts to high temperatures with decreased intensity. The peak height vanishes for fields ≥ 3 T. Apart from this, we also show the validity of Kadowaki-Woods plot for the present system.

We present point-contact spectroscopy (PCS) study of CeNi_0.4Cu_0.6 at low temperatures (1.5 – 10 K) and in magnetic fields up to 6 T. We have observed point-contact (PC) spectra with spectroscopic features, which are probably connected with scattering of conduction electrons on clusters and quasiparticles present in this alloy. We obtained reproductible symmetric dependencies in both polarities of applied voltage.

The magnetic susceptibility measurements in the temperature range from 2 K to 300 K have been performed on a novel genuine organic TCNQ salt (N-Me-2,5-di-Me-Pz)(TCNQ)₂. Based on previous study of similiar material spin-ladder like behaviour was expected. However, obtained experimental data are analyzed in terms of the Heisenberg linear chain model. Transition with spin-Peierls features occurs due to the peculiarities of crystalline structure.

RCrSb₃ crystallizes in a quasi two dimensional structure with RSb and CrSb₂ layers stacked along a axis. This system displays rich magnetic structure due to complex magnetic interaction between 3d moments of Cr and 4f moments of rare earth ion. In this work, we present a comparative study on the heat capacity of R(La, Ce, Pr, Nd)CrSb₃ which exemplifies the interplay between two magnetic species (Cr³⁺ and R³⁺ions) leading to a complex magnetic phase diagram of RCrSb₃ system. The nature of ordering of the rare earth ion is quite different in all the compounds. Moreover,the ferromagnetic ordering temperature T_C of Cr spins decreases with the decrease of the size of the rare earth ion.

We report on scanning tunneling microscopy and spectroscopy (STM/S) studies of electron doped La_0.7Ce_0.3MnO₃/LaAlO₃ thin films grown by pulsed laser deposition. Atomic force microscopy of these films reveals an average grain size of ∼100 nm. Spatially resolved STS maps in the metallic state, i.e., well below the metal-insulator transition temperature, show phase separation on a length scale of several nanometers. The conductance maps indicate a correlation between the phase separation and the microstructure of the films. The results demonstrate that the local strain as well as the morphology of thin films have a strong influence on the local conductivities which complicates the search for an intrinsic phase separation in manganite thin films.

We present quantum oscillation measurements of YbRh₂Si₂ at magnetic fields above the Kondo-suppression scale H₀ ≈ 10 T. Comparison with electronic structure calculations is complicated because the "small" Fermi surface, where the Yb 4f-quasi-hole is not contributing to the Fermi volume, and "large" Fermi surface, where the Yb 4f-quasi-hole is contributing to the Fermi volume, are related by a rigid Fermi energy shift. This means that spin-split branches of the large Fermi surface can look like unsplit branches of the small surface, and vice-versa. Thus, although the high-field angle dependence of the experimentally-measured oscillation frequencies most resembles the electronic structure prediction for the small Fermi surface, this may instead be a branch of the spin-split large Fermi surface.

The heat capacity, thermal conductivity and heat release of the glass-like crystalline materials NbTi-D and (ZrO₂)_0.87(CaO)_0.13 were investigated. Both materials show all typical for structural glasses low temperature anomalies. However, the maximum value of the heat release after a rapid cooling is essentially larger the value observed in other glasses. The analysis of the data within the tunnelling model indicates that this giant heat release is caused by tunnelling systems with unexpected high barrier heights.

The equilibrium heat capacity of the 1D system (TMTTF)₂Br with spin density wave and the 2D system 1T-TaS₂ with charge density wave were investigated in the temperature range between 0.07K and 10K and in a magnetic field up to 10T. For both materials an additional contribution to the heat capacity was found below 1K, which is proportional to T^-2. This contribution is caused by low energy excitations with a broad spectrum of their relaxation time. The relaxation time is strongly temperature dependent and follows the Arrhenius law, i.e. the relaxation is a thermal activated process. For both materials the same absolute value of the T^-2 term and the same activation energy E_aa/k_B = 0.5K was found. In difference to (TMTTF)₂Br, where the heat capacity is strongly field dependent, the heat capacity of 1T-TaS₂ is constant up to 5T.