Table of contents

The Strongly Correlated Electron Systems Conference (SCES) 2011, was held from 29 August–3 September 2011, in Cambridge, UK. SCES'2011 was dedicated to 100 years of superconductivity and covered a range of topics in the area of strongly correlated systems. The correlated electronic and magnetic materials featured include f-electron based heavy fermion intermetallics and d-electron based transition metal compounds.

The meeting welcomed to Cambridge 657 participants from 23 countries, who presented 127 talks (including 16 plenary, 57 invited, and 54 contributed) and 736 posters in 40 sessions over five full days of meetings.

This proceedings volume contains papers reporting on the science presented at the meeting. This work deepens our understanding of the rich physical phenomena that arise from correlation effects. Strongly correlated systems are known for their remarkable array of emergent phenomena: the traditional subjects of superconductivity, magnetism and metal-insulator transitions have been joined by non-Fermi liquid phenomena, topologically protected quantum states, atomic and photonic gases, and quantum phase transitions. These are some of the most challenging and interesting phenomena in science.

As well as the science driver, there is underlying interest in energy-dense materials, which make use of 'small' electrons packed to the highest possible density. These are by definition 'strongly correlated'. For example: good photovoltaics must be efficient optical absorbers, which means that photons will generate tightly bound electron-hole pairs (excitons) that must then be ionised at a heterointerface and transported to contacts; efficient solid state refrigeration depends on substantial entropy changes in a unit cell, with large local electrical or magnetic moments; efficient lighting is in a real sense the inverse of photovoltaics; the limit of an efficient battery is a supercapacitor employing mixed valent ions; fuel cells and solar to fuel conversion require us to understand electrochemistry on the scale of a single atom; and we already know that the only prospect for effective high temperature superconductivity involves strongly correlated materials. Even novel IT technologies are now seen to have value not just for novel function but also for efficiency.

While strongly correlated electron systems continue to excite researchers and the public alike due to the fundamental science issues involved, it seems increasingly likely that support for the science will be leveraged by its impact on energy and sustainability.

The conference owes its success to the large number of devoted workers for the cause, which includes the organising and programme committees and a considerable number of workers on the ground who contributed to the smooth running of the meeting.

The conference received major sponsorship from CamCool Research Limited, the International Institute for Complex Adaptive Matter, from the European Science Foundation through the program INTELBIOMAT, and the Cambridge Central Asia Forum.

On behalf of Conference Chairs: P B Littlewood and G G Lonzarich Secretary: S Saxena Treasurer: M Sutherland Local Organising Committee Chair: S E Sebastian Programme Committee Chairs: E Artacho, F M Grosche, Z Hadzibabic

(The PDF file also contains photographs from the conference.)

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We have measured the susceptibility χ and resistivity ρ on single-crystalline Ce_1−xLa_xRu₂Si₂ for 0 ≤ x ≤ 0.8 in directions parallel (//) and perpendicular (⊥) to the c-axis. The χ// for 0.1 ≤ x ≤ 0.7 indicates a Curie-Weiss form at T > 20 K and a typical AF maximum at T_N. A significant crossover between T_N and the magnitude of the Weiss temperature |Θ| occurs at about x = 0.2. The ρ⊥ shows a gap-type upturn at T_N for 0.1 ≤ x ≤ 0.185 with rather a downward bend ρ// at around T_N. The upturn in ρ⊥ at T_N disappears for x = 0.2 with a sharp maximum ρ// at T_N. A sharp peak in ρ⊥ takes place at T_N for 0.3 ≤ x ≤ 0.7 with the sharp maximum ρ// at T_N for 0.4 ≤ x ≤ 0.6. These results can be interpreted as the evolution from the itinerant SDW in the heavy-fermion band for 0.1 ≤ x < 0.2 to the AF ordering with the local moment for x > 0.2 up to x = 0.7.

We have measured the magnetic susceptibility χ, specific heat C_p and resisitivity ρ on sputtered structure-disordered Ce_80−yLa_yRu₂₀ alloys. The χ for y ≤ 70 follows the Curie-Weiss law in the measurement temperature region. The paramagnetic effective magnetic moment p_eff is about 1.8 μ_B/Ce-atom for y = 0 and decreases with increasing the La-concentration (0.6 μ_B/Ce-atom for y = 70). The C_p follows linear relation of temperature above 7 K for all the samples. The specific heat coefficient γ shows about 200 mJ/molK² for y = 0 and decreases rapidly with increasing La-concentration. We found the -log T dependence of the ρ in the La-substituted samples. Magnitude and temperature range of the -log T contribution to the whole resistivity increase with increasing the La concentration. On the other hand, we have observed superconducting behaviors of the resistivity ρ = 0 and a large diamagnetization at low temperature (T < 3 K) for y = 80.

It has recently been found that composite multiplets are formed by the hyperfine coupling between the triplet crystalline-electric-field ground state of the 4f electrons and the ¹⁴¹Pr nuclear spin in Pr ions of the filled skutterudite PrRu₄P₁₂. Entropy analysis of the characteristic Schottky-type peak structure appearing in the low-temperature specific heat demonstrates that it is actually caused by thermal excitations of not only 4f electrons but also ¹⁴¹Pr nuclei. The hyperfine coupling constant for Pr ions in nonmetallic PrRu₄P₁₂ is determined to be A = +0.050(2) K, which is almost the same as in metallic Pr-based compounds. This fact corroborates that the intra-ion hyperfine coupling of Pr ions is independent of the type of material.

We theoretically investigate effects of a vortex lattice structure on antiferromagnetic (AFM) ordering induced inside a d-wave superconducting (SC) phase due to a strong Pauli-paramagnetic pair-breaking (PPB) effect. It is shown that a PPB-induced AFM order is spatially modulated due to the vortex lattice structure and that, in contrast to the familiar competitive nature of AFM and SC orders, the modulated AFM order is not localized in vortex cores but coexistent with the SC order. Discussion on effects of PPB-induced AFM fluctuation on the spin magnetization is also given.

Hall effect measurements were carried out under high pressures up to 4.9 GPa in a single crystal of CeCu₂Si₂. The temperature dependence of the Hall coefficient is interpreted to be composed of two peaks below room temperature. The high-temperature peak shows strong pressure dependence, where the peak shifts from 20 K at ambient pressure to 125 K at 4.9 GPa. This peak is a consequence of the anomalous Hall effect due to skew scattering. The low-temperature peak shifts from 5 K at ambient pressure to 15 K at 4.9 GPa and the magnitude of the peak shows the maximum around 4.1 GPa at which the superconducting transition temperature also reaches the maximum.

We report measurements of the specific heat, electrical resistivity, and magnetic susceptibility for CePt_1−xNi_xSi₂ from which we develop a T − x phase diagram that includes a quantum critical point near x_cr ≈ 0.125 and accompanying non-Fermi-liquid behavior in a "v"-shaped region. This phase diagram is strikingly similar to that of CePtSi₂ under applied pressure P, suggesting that CePt_1−xNi_xSi₂ provides a model system in which a T − P − x phase diagram can be smoothly generated, thereby allowing a systematic study of the influence of disorder on quantum criticality.

Cu-NMR measurements in a random powder of UCu₄Ni reveal two types of spectral lines for each of the two isotopes of naturally abundant Cu in the material. These lines, which we label L₁ and L₂, point to the existence of two inequivalent Cu sites in the sample. We present a study of the NMR line shape in UCu₄Ni at three different frequencies (in the range from 40-70 MHz) and two temperature values (10 K and 150 K), that allow us to assign the lines to particular Cu sites. L₁ is strongly broadened as the frequency decreases, but changes less with increasing temperature. In contrast, the width of L₂ grows in proportion to frequency and decreases noticeably with increasing temperature. This behavior indicates that the crystallographic site corresponding to L₁ is exposed to electric field gradients and has lower point symmetry than the site corresponding to L₂, which displays some anisotropy but no discernible quadrupole effects. By comparison with the Cu-NMR spectra in UCu₄Pd, where only one type of Cu-NMR line has been observed clearly, we can associate L₁ with Cu^(16e) nuclei: Cu nuclei sitting at the 16e site (Wyckoff notation) in the AuBe₅ structure of the parent compound UCu₅. This leaves L₂ as originating from Cu^(4c) nuclei; i.e., those sitting at the 4c site of the same structure. Unlike in UCu₄Pd, the appearance of signal from Cu^(4c) nuclei in the Ni compound is clear evidence of site disorder in UCu₄Ni.

The crystal structure and the physical properties at low temperatures of the novel stannide Yb₃Pd₂Sn₂ have been studied. The compound crystallizes in a new structure type, space group Pbcm, with Yb occupying four different positions in the lattice. Measurements of DC susceptibility and ¹⁷⁰Yb Mössbauer suggest a close to divalent Yb-ions behaviour, ruling out a scenario of heterogeneous mixed valence for the Yb-ions.

We have successfully grown the single crystals of RMg₃ (R = La, Ce and Nd) compounds by Bridgman method, in a sealed molybdenum crucible. These compounds crystallize in the cubic BiF₃-type structure with the space group Fmm. The powder x-ray diffraction pattern clearly revealed that the samples were single phase without any impurity. The crystals were oriented by means of Laue back reflection method for the magnetic and transport measurements. It has been found that CeMg₃ orders antiferromagnetically at T_N = 2.6 K, while NdMg₃ shows two magnetic orderings with T_N1 = 6.8 K and T_N2 = 2.8 K respectively. The magnetic part of the electrical resistivity of CeMg₃ exhibited a double peak structure and the magnetization measurement revealed a reduced magnetic moment of 0.5 μ_B/Ce. The heat capacity measurement of CeMg₃ measured down to 0.5 K exhibited an enlarged Sommerfeld coefficient of 370 mJ/K² mol. From the Schottky heat capacity we have estimated the crystal electric field split energy levels of CeMg₃ and NdMg₃.

We present magnetic phase diagrams to determine Néel critical field on the basis of electrical resistivity, specific heat and magnetic susceptibility measurements at magnetic fields. The magnetic phase diagrams indicate that Néel temperatures were suppressed completely at 35T for [100] and 31T for [001], respectively. These results indicate Quantum Critical Point exists in a range from 30 to 40T. This phase diagrams should be powerful guides for investigating electronic states near QCP.

We present Shubnikov-de Haas measurements on LuRh₂Si₂, the non-magnetic reference compound to the prototypical heavy-fermion system YbRh₂Si₂. We find an extensive set of orbits with clear angular dependences. Surprisingly, the agreement with non-correlated band structure calculations is limited. This may be related to an uncertainty in the calculations arising from a lack of knowledge about the exact Si atom position in the unit cell. The data on LuRh₂Si₂ provide an extensive basis for the interpretation of measurements on YbRh₂Si₂ indicative of discrepancies between the high-field Fermi surface of YbRh₂Si₂ and the "small" Fermi surface configuration.

We have measured ⁶³Cu-NQR spectral lines and nuclear spin-lattice relaxation time T₁ under pressure up to 5.4 GPa. Daphne oil 7474 is used as the pressure-transmitting medium to obtain good hydrostaticity. NQR frequency ⁶³νq above 4 GPa suddenly decreases from the linear pressure dependence in the low pressure range (P ≤ 3.5 GPa). The observed sudden downward deviation of ⁶³νq is associated with an increase of Ce valence. Above 4.5 GPa, the linear pressure dependence of ⁶³νq is observed again, most likely related to the sharp crossover to the high valence state.

An inelastic neutron scattering study of the filled and partially-filled skutterudite compounds RFe₄Sb₁₂ and R_0.5Fe_2.75Ni_1.25Sb₁₂ (where R = Ce and La) was carried out to understand the nature of the spin dynamics. Strong magnetic scattering was observed in Ce_0.5Fe_2.75Ni_1.25Sb₁₂ at ~ 5 meV. The integrated intensity of this peak does not follow the Ce³⁺ form factor, but exhibits a maximum at a momentum transfer (|Q|) of 2 Å⁻¹. We attribute this feature to a Ce³⁺ crystal field excitation in the presence of magnetic exchange interactions. This picture is supported by thermodynamic and magnetic properties. Finally, we confirm the presence of a spin gap in CeFe₄Sb₁₂ suggested by our previous work.

For the first time found that the Curie-Weiss law depends on the crystallographic direction along which the measuring field is directed. The ESR measurements of the EuB_6-xC_x (x < 0.1) single crystal indicated that the sample with x = 0.02 behaves in fields along the [111] -direction as a ferromagnetic, while for field along the [100] - the anti-ferromagnetic behavior.

The magnetic properties of the Zintl compounds EuZn₂As₂ and EuCd₂Sb₂ have been studied. The measurements of resistance, magnetic susceptibility and the ESR (X band) was performed in the range 4 − 300 K. The symmetric Eu²⁺ ESR lines had the g-factors 2.00 (EuZn₂As₂), 2.03 (EuCd₂Sb₂) and a Lorentzian shape above 120 K. The change of shape to Gaussian indicates on the magnetic fluctuations. EuZn₂As₂ and EuCd₂Sb₂ order antiferromagnetically with the T_N 16.5 and 7.4 K, respectively; but with positive Curie-Weiss temperatures. The study revealed metallic conductivity for EuCd₂Sb₂ and semiconductor type for EuZn₂As₂.

Lattice vibrations of LaRu₂Zn₂₀, and LaIr₂Zn₂₀ have been investigated by the local density approximation. Although these compounds have a cage structure in which a La atom is surrounded by Zn, the calculated results show that La vibrations have large dispersion and relatively high frequencies. In both compounds, it is found that the vibrations of Zn at 16c site in the plane perpendicular to the three-fold axis have very low frequencies and can be unstable. This result suggests that the transitions in LaRu₂Zn₂₀ at 150 K and in LaIr₂Zn₂₀ at 200 K are caused by these unstable modes.

We have performed ⁵⁹Co NQR / NMR measurements on the single-crystalline UCoGe, in order to investigate the relationship between ferromagnetism and superconductivity. The measurements of Knight-shift and nuclear spin-lattice relaxation rate provide clear evidence that both static and dynamic susceptibilities are ferromagnetic with Ising anisotropy. In addition, H_c2 also shows extremely large anisotropy which can not be explained by the GL model with anisotropy of effective mass. These data suggest intimate relationship between Ising magnetization and anisotropic superconductivity in UCoGe.

The presence of magnetic clusters has been verified in both antiferromagnetic and ferromagnetic quantum critical systems. We review some of the strongest evidence for strongly doped quantum critical systems (Ce(Ru_0.24Fe_0.76)₂Ge₂) and we discuss the implications for the response of the system when cluster formation is combined with finite size effects. In particular, we discuss the change of universality class that is observed close to the order-disorder transition. We detail the conditions under which clustering effects will play a significant role also in the response of stoichiometric systems and their experimental signature.

We have measured the electrical resistivity of clathrate Ce₃Pd₂₀Ge₆ under high pressure up to 3.86 GPa in order to investigate pressure effects on two successive phase transitions: O⁰₂-type ferroquadrupole order on Ce-8c site at T_Q = 1.2 K and antiferromagnetic order on Ce-4a site at T_N = 0.75 K. With applying pressure, T_Q stays nearly constant at low pressures and gradually decreases above 2 GPa, while T_N first increases and then decreases above 2.5 GPa. From these results, critical pressures, where T_N and T_Q become 0, are roughly estimated to be around 5−6 GPa. In addition, we found that the magnetic resistivity, which is obtained by subtracting the resistivity of La₃Pd₂₀Ge₆ from that of Ce₃Pd₂₀Ge₆, exhibits –InT dependence in two different temperature ranges, suggesting presence of two different Kondo temperatures originated in the two crystallographic Ce-sites.

We have measured the electrical resistivity ρ(T) of heavy fermion antiferromagnet YbNi₃Al₉ under hydrostatic pressure up to 8 GPa. We found that the antiferromagnetic ordering temperature T_N is suppressed with increasing pressure, whereas the localized character of the trivalent Yb state becomes stronger inferred from the reduction of the Kondo scattering. We will discuss the possible origin of the suppression of T_N under pressure.

The metamagnetic behavior, which is typical in heavy fermion compounds, is observed in UCo₂Zn₂₀ and UIr₂Zn₂₀ at H_m = 80 and 20 kOe, respectively. Reflecting the metamagnetic behavior, the magnetoresistance is suppressed in magnetic fields larger than H_m, and the corresponding A-value of a Fermi liquid relation in the electrical resistivity ρ = ρ₀+AT² indicates a peak at around H_m. Pressure suppresses this behavior in UIr₂Zn₂₀.

We investigate the mechanism underlying the suppression of heavy-fermion mass enhancement in the presence of a magnetic field. In the framework of statistically consistent Gutzwiller method (SGA) we study the periodic Anderson model in the strong correlation limit. The finite-U corrections are included systematically allowing to describe the coexistence of Kondo compensation effect and ferromagnetic ordering, as well as weak delocalization of the f-electrons. In particular, we observe that the resulting mass enhancement factor of spin-up electrons and that of spin-down are not equal in ferromagnetic phases and depend strongly on the applied field and the f-level occupancy. We predict that mass enhancement for the spin-up quasiparticles is larger then that of spin-down and both of them decrease in the applied magnetic field. We argue that above features, as well as a nonmonotonic variation of the quasiparticle effective masses observed in our model are in good agreement with earlier experimental measurements for Ce_xLa_1−xB₆.

We performed ultrasound velocity measurements in single-crystalline YNi₂B₂C (T_c = 15.6 K) in the vortex state. The elasticity of the flux line lattice was probed in temperature and magnetic-field dependence of the ultrasound velocities. Magnetic-field dependence of the ultrasound velocities revealed elastic anomalies due to a first-order 45° reorientation transition of the hexagonal flux line lattice, and a second-order hexagonal-to-square lattice transition of the flux line lattice.

We report the electron spin resonance (ESR) measurements in the new phosphides YbRh₆P₄ and CeIr₂P₂. Details of preparation of both materials which proceeds by different techniques are given. Electronic spin-lattice relaxation (SLR) processes with an involvement of the first excited Stark sublevel of the Yb³⁺ ion with an energy Δ ≈ 82.1 K describe well a drastic broadening and vanishing of the ESR signal in YbRh₆P₄ above 10 K. The observed ESR behavior provides evidence for the relatively weak f-p hybridization effects in both phosphides in contrast to silicides YbRh₂Si₂ and YbIr₂Si₂.

Neutron scattering studies reveal that a crystal-field-splitting ground state of Pr³⁺ 4f² electrons in a partially filled Pr_xFe₄Sb₁₂ (x < 1) is a triplet. A magnetically ordered structure below 4 K consists of both magnetic moments at the Pr and Fe sites, in contrast to the x = 1 system without any magnetic ordering due to the 4f-electron singlet ground state.

Inelastic neutron scattering experiments on poly crystalline sample of heavy-fermion compound YbCo₂Zn₂₀ were carried out in order to obtain microscopic insights on the ground state and its magnetic field response. At zero field at 300 mK, inelastic response consists of two features: quasielastic scattering and a sharp peak at 0.6 meV. With increasing temperature, a broad peak comes up around 2.1 meV, whereas quasielastic response gets broader and the peak at 0.6 meV becomes unclear. By applying magnetic field, the quasielastic response exhibits significant broadening above 1T, and the peak at 0.6 meV is obscure under fields. The peaks in inelastic spectra and its temperature variation can be ascribed to the suggested crystal-field model of Γ₆ − Γ₈ − Γ₇ with the overall splitting of less than 3meV. The observed quasielastic response and its rapid broadening with magnetic field indicates that the heavy-electron state arises from the ground state doublets, and are strongly suppressed by external field in YbCo₂Zn₂₀,

SmS undergoes a pressure-induced phase transition from a semiconducting to a mixed-valent state. Although the mixed valence state has been extensively studied, the semiconducting state with a divalent ionic state is not well understood yet. Here we report transport properties of the low-pressure semiconducting phase. Combining the thermoelectric power with the electrical resistivity, we observe that the mobility of electrons excited onto the conduction band is much greater than that of holes left behind in the valence band, the electron effective mass is nearly equal to the hole effective mass, and the relaxation time of the electron is much larger than that of the hole.

The Hall resistivity ρ_H has been measured between 2 K and 300 K for pseudoternary compounds Ce(Ru_1−xFe_x)₂Al₁₀ (x=0.375, 0.625, 1) single crystals up to 7 T. The ρ_H shows linear dependence against magnetic field except low temperatures. We calculated Hall coefficient R_H by the gradient of ρ_H under the field between 5 T and 7 T, where the anomalous Hall effect expected to be small. While R_H for CeRu₂Al₁₀ drastically increases below mysterious antiferromagnetic ordering temperature (T₀ ~ 27 K), R_H decreases by replacing Ru with Fe. The R_H becomes negative at lowest temperatures for x = 0.375, x = 0.625 and x = 1. The reversal of the sign suggests that Ce(Ru_1−xFe_x)₂Al₁₀ is in a multicarrier system.

We measured specific heat of NdRu₂Al₁₀ single crystal under magnetic field along a-axis in order to investigate basic physical properties of this compound. We found three anomalies at T₁ ~ 2.4 K, T₂ ~ 1.2 K and T₃ ~ 0.9 K in the temperature dependence of specific heat. These anomalies are attributed to antiferromagnetic order. In addition, we observed a Schottky-type anomaly, which reflects splitting of crystalline electric field (CEF). The entropy reaches Rln2 just above T₁ and almost reaches Rln10 at room temperature. These results indicate that these three phase transitions occur in a doublet ground state and that all CEF levels of J = 9/2 are below 300K. The results of specific heat are consistent with the CEF level scheme proposed by magnetization measurement.

We report an anomalous energy gap formation in CeM₂Al₁₀ (M = Ru, Os) by using a polarized optical conductivity [σ(ω)] measurement. Near 28 K (= T₀), both compounds undergo an antiferromagnetic transition order, the origin of which is not yet clear. We found that the σ(ω) spectra along the orthorhombic b-axis have characteristic peak structure at the energy gap edge, which implies the existence of the band nesting due to a charge instability. The peak structure does not originate from the conventional Kondo semiconducting property but can be explained by the charge-density wave (CDW) fluctuation based on the mean-field theory. This suggests that the opening of energy gap along the b-axis originates from the CDW or spin-density wave formation.

In order to identify the gap structure of a spin-triplet superconductor UPt₃, we performed field-angle-resolved specific heat (C_ϕ) measurements in a magnetic field rotated around the [0001] axis. Although various theories predict fourfold symmetry of the gap in the superconducting C phase, we observed no angular variation in C_ϕ in a temperature range down to 88 mK (~ 0.17T_c). This absence of the variation in C_ϕ implies that the field-angle-dependent quasiparticle excitations, reflecting the gap anisotropy, are averaged out in the bulk of UPt₃.

We report on transport and magnetic measurements between 1.8 and 400 K on single crystalline YbNi₄P₂, which was recently reported to be a heavy fermion system with a low lying ferromagnetic transition at T_C = 0.17K, based on data from polycrystals. The tetragonal crystal structure of YbNi₄P₂ presents quasi-one-dimensional Yb chains along the c direction. Here we show that at high temperatures, the magnetic anisotropy of YbNi₄P₂ is dominated by the crystal electrical field effect with an Ising-type behaviour, which gets more pronounced towards lower temperatures. The electrical resistivity also reflects the strong anisotropy of the crystal structure and favours transport along c, the direction of the Yb chains.

We report the magnetic properties of a single crystal of a new compound U₂Os₃Al₉ which crystallizes in the well known Y₂Co₃Ga₉ type orthorhombic structure with space group Cmcm. The susceptibility of U₂Os₃Al₉ shows a peak at 7 K typical of antiferromagnetic ordering. The susceptibility in the paramagnetic state is anisotropic, the easy axis of magnetization lying in the ab-plane of the orthorhombic crystal lattice. The magnetization at 2 K, measured up to a maximum field of 160 kOe, shows a metamagnetic transition near 118 kOe when the field is aligned along [010] in addition to a small metamagnetic transition near 25 kOe. The bulk antiferromagnetic ordering of the uranium ions at T_N = 7 K is confirmed by a peak in the heat capacity with ΔC nearly 7 J/U.mol K. An extrapolation of the heat capacity data from the paramagnetic regime to T = 0 gives an enhanced electronic specific heat coefficient of 120 mJ/U.mol K². The electrical resistivity of U₂Os₃Al₉ shows a negative temperature coefficient between 300 and T_N which is a signature of spin fluctuations in a narrow band or a Kondo type of interaction. The data thus suggest the presence of strong electron correlations in this compound.

We have investigated the temperature-pressure phase diagram of the heavy fermion compound CeCoGe_2.2Si_0.8 by DC magnetic susceptibility measurements, χ_DC(T), under high pressure. The Néel temperature of T_N = 4 K in zero pressure is reduced by pressure up to 3 kbar. At higher pressures antiferromagnetic order appears to gradually transform into a spin glass like-state. Magnetic field decreases both T_N and the spin glass freezing temperature T_f. At 3 T and 6.5 kbar a divergence of χ_DC(T) is observed with a power law that is consistent with a disorder-dominated quantum criticality.

The intermetallic compound CeAgSb₂ is an unusual example of a ferromagnetically ordered heavy fermion system. Ferromagnetism sets in below the Curie temperature T_c=9.6 K at ambient pressure. We have investigated the magnetisation of CeAgSb₂ under applied hydrostatic pressure of up to 45 kbar. T_c is suppressed rapidly, and at pressures > 35 kbar it is replaced by an unidentified ordered phase, possibly antiferromagnetism. The ordered magnetic moment in CeAgSb₂ is aligned along the c-axis. We investigate the effect of transverse field tuning on T_c, and show that magnetic order at low temperature is suppressed by in-plane fields exceeding about 3 T.

The compound Ce₄Pt₁₂Sn₂₅ has a rather complex cubic crystal structure in which the Ce atoms are encapsulated in cages of Pt and Sn resulting in a large Ce-Ce distance. Recently, specific heat and magnetic susceptibility measurements revealed a phase transition from a paramagnetic to an antiferromagnetic (AFM) ground state with a low Néel temperature of T_N = 0.19K. Application of a magnetic field was shown to weaken the AFM phase and ultimately suppress it at a critical field of 0.7 T. Here we investigate the observed antiferromagnetic phase transition by electrical resistivity measurements in applied magnetic fields in the vicinity of this field-tuned quantum critical point.

Energy band structures of PuX₂ (X=Ru, Rh, and Ir) are investigated by a relativistic linear augmented-plane-wave method with the exchange-correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Pu 5f and X d electrons.

We study here the influence of a random applied magnetic field on the competition between the Kondo effect, the spin glass phase and a ferromagnetic order in disordered cerium systems such as CeNi_1−xCu_x. The model used here takes an intrasite Kondo coupling and an intersite random coupling; both the intersite random coupling and the random magnetic field are described within the Sherrington-Kirkpatrick model and the one-step replica symmetry breaking procedure is also used here. We present phase diagrams giving Temperature versus the Kondo exchange parameter and the random magnetic field makes decrease particularly the importance of the spin glass and ferromagnetic phases.

We report the results of the electrical resistivity ρ(T) and the ³¹P-NMR measurements on filled skutterudite CeOs₄P₁₂ in order to investigate the magnetic properties at low temperatures from a microscopic point of view. For the polycrystalline sample synthesized under high pressure (HP), ρ(T) increases with decreasing temperature. On the other hand, for the single crystal (SC), ρ(T) reveals a positive temperature dependence between room temperature and 200 K, and increases with decreasing temperature below 200 K. Also, 1/T₁ obeys the activated temperature dependence 1/T₁ ∝ exp(−Δ/k_BT) above 160 K with an energy gap Δ/f_B ~ 500 K and 540 K for the HP and the SC samples, which are slightly larger than that of a previous report.

We have grown single crystals of a low carrier heavy fermion YbPtSb and studied low temperature properties by means of the electrical resistivity (ρ), dc and ac magnetic susceptibility (χ), and specific heat (C). The clear evidence of the antiferromagnetic order at T_N ~ 0.34 K was observed by bulk measurements. At lower temperatures below T_N, extrapolating C/T to T=0 yields γ ~ 9 J/mole K² and ρ(T) shows T² temperature dependence with a large A coefficient, as observed in YbPtBi. We will discuss the Fermi liquid state realized in the low carrier systems.

Here we provide the first clear evidence of Fermi-liquid breakdown in an intermediate valence system. We employ high precision magnetization measurements of the valence fluctuating superconductor β-YbAlB₄ to probe the quantum critical free energy down to temperatures far below the characteristic energy scale of the valence fluctuations. The observed T/B scaling in the magnetization over three decades not only indicates unconventional quantum criticality, but places an upper bound on the critical magnetic field |B_c| < 0.2 mT, a value comparable with the Earth's magnetic field and six orders of magnitude smaller than the valence fluctuation scale. This tiny value of the upper bound on B_c, well inside the superconducting dome, raises the fascinating possibility that valence fluctuating β-YbAlB₄ is intrinsically quantum critical, without tuning the magnetic field, pressure, or composition: the first known example of such a phenomenon in a metal.

We report the first observation of the continuous Fermi surface (FS) variation from La compound (LaRu₂Si₂) with no f electron to Ce compound (CeRu₂Si₂) with itinerant f electron via the de Haas-van Alphen (dHvA) effect. The dHvA frequency smoothly varies with Ce concentration and there is no discontinuous change with Ce concentration. It is found that the effective mass and signal amplitude with Ce concentration depends strongly on the Fermi surface sheet, and the effective mass is enhanced toward x_c (= 0.91) and the signal amplitude reduces around x_c or somewhere between x_c and x ∼ 0.8.

The origin of the spin resonance observed in CeCoIn₅ with Inelastic Neutron Scattering is subject to debate. It has been shown recently that in this heavy electron compound at low temperature an instability to a ground state with coexisting d_x²-y²-wave superconductivity and Spin Density Wave (SDW) order in a magnetic field is a corollary of the consideration of a collective spin excitation mode in a quasi-2D d_x²-y²-wave Pauli-limited superconductor. This provides a natural scenario for the occurence of the puzzling high-field-low-temperature phase highlighted in CeCoIn₅. We present perspectives on this ground state transition and propose directions for future experiment.

In order to study local symmetries at the 4e(Si)- and 4d(Ru)-sites above and below hidden order transition temperature T_O = 17.5 K in URu₂Si₂, we have investigated the nuclear quadrupole interaction by ⁷³Ge-nuclear magnetic resonance (NMR) measurement on a 10% ⁷³Ge-substituted sample URu₂(Si_0.9Ge_0.1)₂, and by ¹⁰¹Ru-nuclear quadrupole resonance (NQR) measurement on a pure single crystal sample. The present ⁷³Ge-NMR measurements do not give any evidence for change in the local symmetry at 4e-site within experimental accuracy. On the other hand, the precise measurement of ¹⁰¹Ru-NQR frequency has detected an anomaly just below T_O as sensitively as the thermal expansion measurement dose. Its temperature dependence shows a linear relation with that of the in-plane lattice parameter, which may be a clue to clarify a modification in microscopic charge distribution at T_O.

Precise electrical resistivity and ac calorimetry measurements under pressures have revealed the (P, T) phase diagram of a mixed-valent compound YbPd. The diagram is rather complicated, reflecting the valence and magnetic instability. In addition to the reported phase transitions at T_N = 1.9 K and T_MH = 0.5 K, the lower temperature transition at T_ML = 0.3 K has been confirmed thermodynamically. T_N decreases monotonically up to 1.7 GPa and disappears discontinuously at P_c ~1.9 GPa, accompanying the drastic enhancement of the effective mass and the residual resistivity, although the ordered phase below T_ML still remains. At P_c, the both of magnetic and valence fluctuations seems to develop.

Ac susceptibility (χ_ac) measurements on Pr(Os_1−xRu_x)₄Sb₁₂ single crystals for x = 0.05 and 0.1 were performed by mutual inductance method. A peak structure caused by anomalously enhanced flux pinning force, which is so-called the peak effect, appears in the H dependence of χ_ac. The peak structure shifts to lower fields as T increases and disappears in T > 1 K in both samples, while in PrOs₄Sb₁₂ it is observable up to near T_c. This fact indicates that the Ru substitution suppresses the peak effect. We demonstrate that the observed T dependent behavior of the peak structure can be explained roughly by the synchronization model although there remains deviation from the model curve, suggesting some modification may be needed in the model to be applied to Pr(Os_1−xRu_x)₄Sb₁₂.

We report the new fabrication method of single crystalline EuO thin film along the (100) plane and its crystal structure and magnetic property. EuO thin film was fabricated on SrO buffer layer created on SrTiO₃ substrate. The obtained EuO thin film was formed a single phase of the rock-salt structure with the lattice constant of 0.516 nm and showed ferromagnetism below the Curie temperature of 71 K. The examined physical properties are consistent with the bulk materials.

⁹Be NMR measurements have been carried out for a single crystal UBe₁₃ with T_c =0.85 K, in order to clarify unusual properties in the normal state. For the applied field parallel to [111] direction, the quadrupole split Be(II) lines gather around the central line. ⁹Be nuclear spin-lattice relaxation rate was measured for Be(II) sites for H||[111] at 0.85, 7, 15 T. 1/T₁ does not depend on applied fields above T^*_NMR ≈ 5 − 9 K and weakly depends on temperature. 1/T₁ at 0.85 and 7 T is proportional to Tⁿ with n = 0.5 − 0.6 down to T = 2 K, suggesting that antiferromagnetic spin fluctuations exist. On the other hand, 1/T₁ is suppressed by applied magnetic field. The present field dependence of 1/T₁ at low temperatures would give important information about the formation of the heavy quasiparticles in UBe₁₃.

We report on the magnetic susceptibility, electrical resistivity and specific heat measurements of the alloys Ce(Os_1−xFe_x)₂Al₁₀ (0 ≤ x ≤ 1) in order to understand the anomalous phase transition in CeOs₂Al₁₀ at T₀ = 28.5 K. With increasing x, the unit cell volume decreases linearly and the c-f hybridization is enhanced as indicated by the systematic change in the magnetic susceptibility. On the other hand, T₀ decreases slowly to 25 K at x = 0.3 then sharply to 20 K at x = 0.5, and eventually disappears at x = 0.6. However, the jump of C_p at T₀, which is proportional to the entropy released by the phase transition, decreases quickly from 0.22 J/Kmol at x = 0 to 0.08 J/Kmol at x = 0.3, and then linearly to 0 at x = 0.6. These behaviours of T₀ resemble the pressure dependence of T₀ for CeOs₂Al₁₀ as well as the x dependence of T₀ in Ce(Ru_1-xFe_x)₂Al₁₀ for 0.25 ≤ x. This result suggests that the critical concentration where T₀ disappears depends strongly on the c-f hybridization strength.

Phonon spectra of LaRu₂Al₁₀ and CeRu₂Al₁₀ have been studied by Raman scattering from 3.4K to 300K. Polarization dependence at room temperature gives the precise mode assignment for the sake of first principles calculation results. The measured Raman spectra are well explained by the space group Cmcm and there is no remarkable difference between LaRu₂Al₁₀ and CeRu₂Al₁₀. Since we observed no anomaly at T₀ = 27K in CeRu₂Al₁₀, the lattice distortion due to the structural transition has not been detected. From the temperature dependence of the vibrational frequency of Ce and La, their vibrational amplitude is not large in CeRu₂Al₁₀ and LaRu₂Al₁₀, that is, close to harmonic case.

We have prepared single crystals of PrRh₂Zn₂₀, which is isostructural and isoelectronic to PrIr₂Zn₂₀ where coexistence of superconductivity and antiferroquadrupolar order has been recently found. The electrical resistivity ρ, magnetic susceptibility χ, isothermal magnetization M, and specific heat C have been measured. Hysteretic behavior appears in ρ(T) for 170 K < T < 470 K, suggesting a structural phase transition. On cooling below 10 K, χ approaches a constant value, indicating a non-magnetic ground state. Anisotropic behavior in M(B) at 1.8 K for B > 2 T and a broad peak in C(T) at around 10 K are reproduced by the two crystal-field levels split by 32 K; a nonmagnetic Γ₃ doublet ground state and a magnetic Γ₄ triplet excited state.

We prepared single crystals of Yb₂Ni₁₂P₇ using a Sn-flux method and measured their resistivity under hydrostatic pressure up to 8 GPa. At ambient pressure, the magnetic part of resistivity ρ_mag(T) shows a peak around T_max ~ 100 K and T² dependence below the characteristic temperature of the Fermi liquid, T_FL = 3 K. When pressure increases up to 2.06 GPa, the value ρ_mag(T) above T_max decreases slightly but ρ_mag below T_max increases slightly. T_FL decreases with increasing pressure, and is expected to become zero at 2.5 GPa. Around this pressure, the coefficient of T² in ρ_mag significantly increases, suggesting the existence of a quantum critical point. Above 2.5 GPa, overall ρ_mag gradually decreases with increasing pressure up to 6 GPa, but it below 15.5 K increases above 6 GPa.

Shubnikov-de Haas oscillations were measured in α-YbAlB₄ at fields up to B = 16 T. Quantum oscillations were used to directly extract the quasiparticle mean free path by fitting to the Dingle decay yielding a mean free path ~ 550 Å. We also describe a novel fitting procedure used to extract a low frequency oscillation from the non-oscillatory background magneto-resistance.

We report the specific heat measurement of single crystalline Ce₃Pd₂₀Si₆ and the resultant magnetic phase diagram for the direction of B // [100]. In zero field, we confirmed two successive phase transitions at T = 0.24 K and 0.4 K. The magnetic phase diagram clearly shows a phase boundary of paramagnetic phase I, quadrupole ordered phase II and II'.

CePt₃B exhibits two magnetic phases, an antiferromagnetic and a weak ferromagnetic one. To determine these magnetic structures of CePt₃B, neutron diffraction and μSR experiments have been carried out. Neutron diffraction experiments provided no evidence of either antiferromagnetic or ferromagnetic order. In contrast, in zero field muon-spin relaxation (μSR) experiments, magnetic transition temperatures T_N = 7.8 K and T_C ~ 6 K are observed from the onset of spontaneous muon precession.

There is evidence that a number of heavy-fermion/mixed-valent materials show strongly renormalized hybridization gaps either at the Fermi-energy or close to the Fermi-energy. In the former case, a heavy-fermion semiconducting state ensues and in the later case, the system remains metallic at low temperatures. Due to the temperature-dependence of the electronic correlations, the magnitudes of the hybridization gaps decrease with increasing temperatures. The existence of a temperature-dependent low-energy electronic energy scale opens up the possibility that the Born-Oppenheimer approximation may fail and that there may be a resonant coupling between the phonons and the electronic excitations. It is argued that such a mechanism may be the cause of the anomalous phonon mode observed in α-uranium at high temperatures.

Zero-field ¹¹⁵In-nuclear magnetic resonance (NMR) has been used to study single crystals of CePt₂In₇, which is a heavy-fermion antiferromagnet with a Néel temperature (T_N) of 5.2 K at ambient pressure. The NMR spectra under zero field near the 3ν_Q line of the orthorhombic In(3) sites are taken at 1.6 K under hydrostatic conditions at ambient pressure and 2.4 GPa. These data reveal the coexistence of commensurate and incommensurate antiferromagnetic (AFM) orders at ambient pressure and that the commensurate ordering is stabilized by increasing pressures. The nuclear spin-lattice relaxation rates (1/T₁) for In(3) sites indicate the localized nature of f electrons far above T_N. The values of 1/T₁ in the paramagnetic state decrease by applying pressure. In contrast, the residual values of 1/T₁ much below T_N increase by pressure.

We report the results of the specific heat measurements for PrTr₂Al₂₀ (Tr = Ti and V), both of which have the cubic nonmagnetic Γ₃ ground state of the crystal electric field scheme. Sharp peak anomalies observed at T = 2.0 K for PrTi₂Al₂₀ and 0.6 K for PrV₂Al₂₀ indicate the second order phase transition due to the multipole degree of freedom. The application of the magnetic field along [111] does not change the anomalies observed in the specific heat of either PrTi₂Al₂₀ or PrV₂Al₂₀, suggesting the quadrupole nature of the phase transition. A large Sommerfeld coefficient γ found above the ordering temperature in PrTi₂Al₂₀ suggests heavy quasi particle formation due to the scattering process of conduction electrons by quadrupole moments.

The components-separated magnetic transition in DyRhIn₅ was investigated by measuring the magnetic susceptibility and elastic constants. The magnetic susceptibility along the [001] direction indicates the antiferromagnetic ordering of c-component of the magnetic moments at T_N = 28.1 K whereas the susceptibility along those of [100] and [110] direction show the paramagnetic behavior. The results of the elastic constant measurements suggest that the degeneracy of quadrupolar degrees of freedom exists in spite of the formation of magnetic order because the elastic softenings are observed below T_N. The quadrupolar effect in DyRhIn₅ is discussed in terms of the symmetry classification.

We have measured the magnetic field dependence of the specific heat in PrPb₃ for H || [001] and [110] up to H = 8T. At H = 0T, a sharp λ-type peak due to antiferro-quadrupolar(AFQ) ordering and a small anomaly appears at T_Q = 0.43K and T^* = 0.25K, respectively. The field dependence of T_Q shows a reentrant behavior and a high-field phase appear above H ~ 6 T in the both field directions. T^* shows a large difference between H || [001] and [110], suggesting that the anomaly is related to the anisotropic feature of the quadrupolar moment in magnetic fields. From these results, we describe the T-H phase diagram PrPb₃ in H || [001] and [110].

Sm-site substitution effect on filled skutterudite compound SmRu₄P₁₂, which shows an intriguing metal-insulator (MI) transition, has been studied by electrical resistivity, magnetic susceptibility and specific heat measurements. Single-phase polycrystalline (Sm_1−xLa_x)Ru₄P₁₂ (x=0.1, 0.2, 0.3) and (Sm_1−xY_x)Ru₄P₁₂ (x=0.1, 0.2, 0.3) have been prepared at high temperature and high pressure. Both La and Y substitutions cause a large suppression of the semiconductor-like behavior at low temperature. The MI transition temperature (T_MI) shifts to lower temperature with increasing La concentration. This change is qualitatively consistent with external pressure experiments. It could be due to a negative chemical pressure effect induced by La substitution. However, T_MI also shifts to lower temperature with increasing Y concentration, which is expected for a positive chemical pressure effect. The results suggest that introducing a disorder into the periodicity of Sm affects sensitively the MI transition in this system.

Low temperature magnetic and thermal (C_m) properties of the ferromagnetic (FM) alloys Ce_2.15(Pd_1−xRh_x)_1.95In_0.9 were investigated in order to explore the possibility for tuning towards a quantum critical point (QCP) by doping Pd with Rh. As expected, the magnetic transition decreases from T = 4.1K at x = 0 with increasing Rh concentration. However, the phase boundary splits into two transitions, the upper being antiferromagnetic (AF) whereas the lower FM. The AF phase boundary extrapolates to T_N= 0 for x_cr ≈ 0.65 whereas the first order FM transition vanishes at x ≈ 0.3. The quantum critical character of the T_N → 0 point is inferred from the divergent T dependence of the tail of C_m/T observed in the x = 0.5 and 0.55 alloys, and the tendency to saturation of the maximum of C_m (T_N)/T currently observed in exemplary Ce compounds when T_N → 0. Beyond the critical concentration the unit cell volume deviates from the Vegard's law in coincidence with a strong increase of the Kondo temperature.

Thermal variations of magnetization under various magnetic fields and magnetization processes at various temperatures have been studied on a HoRh₂Si₂ single crystal compound. Three anomalies at T_N1=29.1 K, T_t=27.3 K and T_N2=12.0 K exist at low field in the thermal variation. The thermal dependence of M/B and the magnetization change drastically with magnetic field. Step-like metamagnetic processes appear along the all directions below T_N2; two-step ones appear along the [001] and [100] directions, and a one-step one appears along the [110] direction. For T_N2<T<T_t, a two-step metamagnetic one persists along the [001] direction whereas the processes along directions in the basal plane become a paramagnetic like one. The B₀₀₁-T magnetic phase diagram is constructed, in which there are at least five ordered phases. Two field-induced magnetic phases appear for low and high temperatures.

The form factor of elastic neutron scattering is calculated for the quadrupole order phase of CeB₆ in magnetic fields. It is shown that the dependence of the form factor on the direction of neutron momentum transfer is very small for the Bragg reflections, whereas the scattering due to field-induced octupoles gives rise to a significant anisotropy of the form factor for the super-lattice reflections. These results are discussed in quantitative comparison with recent exprimental results. The similarity and dissimilarity with the results for phase IV in Ce_1−xLa_xB₆ is also discussed.

We have studied anomalous upper critical field B_c2(T) of a single crystal of UBe₁₃ by DC magnetization measurements down to 0.14 K and up to 8 T. We have observed a broad anomaly and a peak effect below B_c2 in magnetization curves. Although a possibility of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been proposed as an origin of the unusual upper critical field by previous works, our results have shown that thermal equilibrium magnetization curves reveal no step-like behavior, which could be observed at 1st-order phase transition from a superconducting mixed state to the FFLO state. We have also investigated anisotropy of B_c2 for B || <001> and B || <110>, and found that B_c2 for B || <001< is larger than B_c2 for B || <110> below ~ 0.5 K. These results might indicate that the anomalous upward curvature of B_c2 in UBe₁₃ is related to anisotropy of B_c2 rather than the FFLO state.

DC Magnetization of the heavy-fermion compound YbCo₂Zn₂₀ was examined at very-low temperatures down to 0.07 K in the field up to 14.5 T. In addition to the nearly-isotropic metamagnetic behavior at 0.6 T, a new metamagnetic behavior is observed in the field variation of magnetization M(H) at 6 T only for H || [111]. For the same field direction, a clear kink appears in the temperature dependence of the magnetization M(T) for fields above 6 T. These results strongly suggest the existence of a new ordered phase induced by magnetic fields. The H - T phase diagram as well as the anisotropy in M(H) above ~ 2 T are best explained by a level crossing of the low-lying crystalline-electric-field states by magnetic fields.

We present the Ce 3dx-ray photoemission (XPS) spectra for CeM₂Al₁₀ (M=Ru, Os, Fe) from which we determined the on-site hybridization between the f and conduction electron states, Δ_cf, and the 4f-level occupancy, n_f. Those parameters have been obtained using the Gunnarsson-Schönhammer approach. We found Δ_cf stronger for the Kondo insulator CeFe₂Al₁₀ than for the remaining compounds with Ru and Os. We discuss the type of behaviour of CeM₂Al₁₀ on the base of the earlier theoretical phase diagram obtained within the Anderson-lattice model.

We report low temperature magnetization measurements on several samples of CeCoSi₃ synthesized by induction melting and the Czochralski method. We aim to investigate whether the noncentrosymmetric material CeCoSi₃ is a superconductor as there are conflicting reports in the literature. Induction melted samples were generally found to display superconducting signals with up to a 12% superconducting volume fraction. However most samples grown by the Czochralski method produced no superconducting signal although the boules were generally inhomogeneous. Superconducting impurity phases could not be identified from powder x-ray diffraction studies. Further work is necessary to determine whether CeCoSi₃ may be classified as another noncentrosymmetric superconductor in the same series as CeRhSi₃, CeIrSi₃ and CeCoGe₃ and the first at ambient pressure.

We carried out ultrasonic measurements on NdRu₂Al₁₀ single crystals in order to clarify what kind of phase transition occurs in this compound. The elastic moduli were measured as a function of temperature from 0.5 K to 150 K using the phase comparison-type pulse echo method. As the temperature is decreased from 150 K, longitudinal elastic modulus C₁₁ and transverse elastic modulus C₄₄ show a monotonic enhancement. With further decreasing temperature, both elastic moduli show abrupt hardening at 2.4 K and then show two-step softening at 1.3 K and 1.0 K. We found obvious hysteresis around both transitions at 1.3 K and 1.0 K in the temperature sweep, whereas no hysteresis was observed around the transition at 2.4 K within our experimental accuracy. The abrupt hardening at 2.4 K is very similar to that found in CeRu₂Al₁₀ and CeOs₂Al₁₀, suggesting that the transition at 2.4 K originates from the same mechanism of the transitions in CeRu₂Al₁₀ and CeOs₂Al₁₀. This is the first report that NdRu₂Al₁₀ undergoes the successive phase transitions.

The magnetic susceptibility of single crystalline samples of CeT₂Al₁₀ (T=Ru, Os) showing unusual phase transitions at T₀ = 27 K and 29 K, respectively, has been measured up to 700 K along the three principal axes of the orthorhombic structure. The crystal field analysis of the data led to the crystal field scheme with excited levels at 326 K and 530 K for T = Ru and 433 K and 704 K for T = Os. For the ground state, the butterfly-like wave function elongates along the b-axis in the distorted hexagon composed of Al(3) and Al(4) atoms in the b-c plane. This charge distribution allows the 4f electron state to hybridise with the 3p states of the Al(3) and Al(4) atoms. We propose that this hybridisation plays an important role in both the formation of the hybridisation gap and the enhancement of the antiferromagnetic ordering temperature.

We report the magnetic susceptibility and the low-temperature specific heat of Ce₆Ni₆P₁₇ with geometric frustrations and La₆Ni₆P₁₇ as a nonmagnetic counterpart. The magnetic susceptibility of Ce₆Ni₆P₁₇ decreases monotonically with decreasing temperature and the specific heat shows a broad peak around 1.4K. The evaluated magnetic entropy is about a half of Rln2 at 5.0 K. This result suggests that the frustrations persist down to very low temperatures and the residual entropy originate from frustration is present.

We studied the magnetic field versus temperature phase diagram of the heavy-fermion compound YbCo₂Zn₂₀ with the cubic CeCr₂Al₂₀-type structure by measuring the specific heat, ac-susceptibility, and electrical resistivity under magnetic fields. In a low magnetic field region, a metamagnetic behavior was observed at H_m = 6 kOe for three principal field directions below T_χmax = 0.32 K, at which the temperature dependence of ac-susceptibility shows a local broad maximum. Two crossover lines were found below and above H_m. At high magnetic fields, the heavy-fermion state was rapidly suppressed with increasing magnetic field, and the field-induced ordered phase appeared in a very limited field direction around H || ⟨111⟩ above the transition field H_Q= 60 kOe in the temperature range below T_Q = 0.6 K.

Energy band structures of PuX₃ (X=Rh, Pd, and Pt) are investigated by a relativistic linear augmented-plane-wave method with the exchange-correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Pu 5f and X d electrons.

Inelastic X-ray scattering experiments were carried out in the series of filled skutterudites ROs₄Sb₁₂ (R: light rare-earth). Phonon energy of the rare-earth guest modes in the longitudinal (1 0 0) direction decreases with increase of atomic number of rare-earth elements from La to Sm inserted into a Sb cage. This fact demonstrates that the guest mode energy is affected by the ionic radius of the rare-earth atom. In addition, comparison between inelastic X-ray scattering and X-ray diffraction experiments reveals the correlation of the guest mode energy in the longitudinal (1 0 0) direction and the Einstein temperature obtained by the temperature dependence of the atomic displacement factor.

We report the pressure effect on the ferromagnetism of type-I clathrate Eu₈Ga₁₆Ge₃₀ with T_C = 36 K by means of the resistivity ρ and Hall coefficient R_H measurements. With increasing pressure up to 11.4 GPa, T_C increases up to 45 K. The concomitant decrease of ρ is attributed to the increase of the carrier concentration because the normal part of R_H decreases with the ratio of −3.5%/GPa. At ambient pressure, ρ(T) has broad peak at T^*=23 K. Upon pressurizing, the peak is weakened but remain visible at T^* = 27 K even at P = 11.4 GPa. Our findings suggest that the anomaly at T^* is not so sensitive to the cage size up to 11 GPa, where the cage volume is compressed to 87% of the value at ambient pressure.

To investigate the role of multipoles in the phase transitions of NdPd₃S₄, measurements of specific heat, magnetization, and electrical resistivity were performed for single-crystalline samples. We also carried out powder neutron diffraction experiments to identify a magnetically ordered structure. At zero magnetic field, an antiferromagnetic (AFM) transition occurs at T_N = 1.8 K. When a magnetic field is applied, the AFM ordered phase, characterized by a propagation vector k = (1, 0, 0), switches into field-induced phases. We propose that the field-induced phases are associated with quadrupolar ordering.

The intermetallic compound CeRu₄Sn₆ has tentatively been classified as anisotropic Kondo insulator. Here we present electrical resistivity measurements on CeRu₄Sn₆ single crystals, both along the main directions a and c of the tetragonal crystal structure, and v along the diagonal c' of the a-a plane. This direction was selected because c' = differs from c by only 0.2 %, suggesting that the coumpound might alternatively be regarded as quasi-cubic. Amazingly, strong anisotropy is observed not only between a and c but also between c and c'. We analyse the temperature dependent resistivities both with a simple semiconductor model and within a Kondo insulator picture.

We have measured the specific heat of heavy fermion YbCo₂Zn₂₀ down to 0.1 K under magnetic field up to 3 T. At zero-field, the electronic contribution to the specific heat C_e/T shows logarithmic increase with decreasing temperature and tends to saturate at lower temperature, reflecting the Fermi-liquid state. With increasing magnetic field, C_e/T exhibits a power-law divergence at H_m ~ 0.6 T, which corresponds to the critical field of the metamagnetic crossover. The field dependence of C_e/T shows the two peak structure and the local minimum point of this structure coincides with H_m as observed in prototypical metamagnetic compound CeRu₂Si₂. We will compare the characteristic energy scale relevant to the metamagnetic behavior of YbCo₂Zn₂₀ and CeRu₂Si₂.

Ultrasonic investigations of the elastic constants of non-5f compound ThRu₂Si₂ were performed in order to highlight possible quadrupolar response in the elastic constants of heavy fermion compound URu₂Si₂, by using the ThRu₂Si₂ for backgrounds to subtract the phonon contributions in the elastic constants of URu₂Si₂. We confirmed that the all-elastic constants of ThRu₂Si₂ show no elastic anomaly, i.e., increasing monotonically and saturating toward low temperatures, which can be understood regular phonon contributions for the solid states. The background-subtracted (C₁₁-C₁₂)/2 of URu₂Si₂ keeps decreasing with decreasing temperatures from 300 K, while other share modes C₄₄ and C₆₆ show little change. These contrasts suggest that the Γ₃-type quadrupole moment O₂² will be still dominant and active in URu₂Si₂ at low temperatures.

We carried out an ¹²¹Sb nuclear magnetic resonance (NMR) measurement on CeOs₄Sb₁₂ to investigate its electric state. The precise temperature (T) dependence of the Knight shift was obtained by using an aligned powdered sample and NQR parameters. The T dependence of the Knight shift below 300 K can not be explained by the c-f hybridization gap model with 2D = 3000 K and 2Δ = 320 K obtained by the 1/T₁. This result indicates that there is q-dependence in the gap. It was revealed that a ferromagnetic correlation develops with decreasing temperature below 50 K from an increase of the Knight shift. Considering this result and previous 1/T₁ measurements, there are both ferromagnetic and antiferromagnetic fluctuations in CeOs₄Sb₁₂.

SrHo₂O₄ is a geometrically frustrated compound because the Ho³⁺ ions are arranged in a network of triangles and hexagons and coupled with antiferromagnetic exchange interactions. Powder neutron diffraction measurements show that long-range magnetic order is established at T_N = 0.68 K. The low temperature magnetic structure is made up of two distinct components: a k = 0 long-range structure as well as a shorter-range low-dimensional structure with scattering intensity around the (0 0 ½) position. The diffuse component of the magnetic scattering from SrHo₂O₄ remains very intense at temperatures well above T_N.

High field magnetization measurement were performed up to 45 T using a long pulse magnet on a GdPd₂Si₂ single crystal compound having the tetragonal ThCr₂Si₂-type crystal structure and an antiferromagnetic ordering of T_N=16.7 K. The magnetization processes are almost typical antiferromagnetic ones with a weak magnetic anisotropy though small anomalies exist in the processes. The magnetization saturates 7 μ_B/f.u. above B_s=32 T. The B₀₀₁-T and B₁₀₀-T magnetic phase diagrams were constructed. There are three phases, the phase I, II and III, under non-magnetic field in the ordered state for both diagrams. The field-induced phase, phase IV, appears below the paramagnetic phase by the [100] magnetic field, B₁₀₀, which is accompany with a re-entrant behaviour. The B₁₀₀ makes the phase II be stable and the phase I and III be instable. These strange behaviours indicate the direction of the magnetic moment changes in each phase.

We report on magneto-optical studies of an epitaxial thin film of GdMnO₃ grown on SrTiO₃. Significant differences were found between the magneto-optical and absorption spectra, which highlight the importance of magneto-optics in determining the magnetic character of the optical transitions. Two main features were identified in the magnetic circular dichroism (MCD) spectra: the charge transfer transition between Mn d states at ~2eV and the band edge transition from the oxygen p band to the d states at ~3eV. The strength of the MCD at ~2eV correlates well with the Mn spin ordering, indicating the applicability of magneto-optics for understanding the magnetic properties.

The spin singlet and spin triplet superconducting states can coexist in noncentrosymmetric crystal structures like Li₂T₃B (T: Pd, Pt), while it is prohibited in ordinary crystal structures. In this study, we focused our research on non-magnetic impurity effect in Li₂T₃B(T: Pd, Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We investigated the effect of non-magnetic impurity on s-wave dominant parity mixing superconductor Li₂Pd₃B by substituting B with Al. In this substitution process, we controlled the quality of the samples and showed their quality deteriorations through the measurement of residual resistivity at T_c. Furthermore, we checked the relations of residual resistivity and T_c as well as its superconducting phase diagram. Our results showed that s-wave dominant parity mixing superconductor Li₂Pd₃B exhibits the small T_c suppression attributed by the non-magnetic impurities and defects, while the H_c2 value clearly increased about 1.5 times in the poor quality samples. Such behaviours are similarly observed in ordinary s-wave superconductors.

We investigate the zero-temperature phase diagrams of the bilayer square-lattice Kugel-Khomskii (d⁹) model involving entangled and singlet phases using mean-field cluster approach. This diagram includes interlayer singlet phase observed in copper fluoride K₃Cu₂F₇ and exotic entangled spin-orbital phases. For a monolayer case, realized in K₂CuF₄, we perform similar calculations at finite temperature and show that the alternating-orbital ferromagnet decays first to an entangled uniform ferromagnet and then to a paramagnet.

We present magnetoresistance studies on a series of homogenously disordered 3-dimensional NbN thin films with disorder ranging from the moderately clean limit (k_Fl~10.12) to the extremely dirty limit where the superconducting ground state is completely destroyed (k_Fl~0.42). We find that for samples with k_Fl >1, the magnetoresistance is positive up to 12T and as disorder increases it decays more slowly with temperature. On the other hand, for samples with k_Fl<1, we observe a peak in the magnetoresistance which vanishes at a temperature close to the pseudogap temperature of disordered superconducting samples. These observations are consistent with the idea of the disorder-driven non-superconducting state being comprised of pre-formed Cooper pairs.

The magnetic properties of one-band metals are discussed within the single site approximation, which is a picture to take into account electron correlations beyond the standard mean-field approach. The metal is described by a Coulomb correlated band. The Roth band shift arising from a two pole approximation is included. Here we present results for the infinite Coulomb correlation limit.

The electronic and magnetic properties of Mn:Ge(111) interfaces have been investigated by photoelectron spectroscopy and SQUID magnetometry. An ordered, metallic and ferromagnetic, Mn:Ge(111) interface and a disordered, semiconducting and paramagnetic, Mn_xGe_1-x surface alloy have been considered. An analysis of the Mn 2p X-ray photoemission core line shows that the former interface can be described by a single-configuration Mn 3d⁶ initial state, while the latter presents satellite features typical of Mn-based diluted magnetic semiconductors, characterized by relevant ligand-to-metal charge transfer effects.

The magnetic Bragg reflections of charge-stripe ordered La_1.725Sr_0.275NiO₄ where investigated by neutron diffraction to determine the intensity ratio of first to third order harmonics. The third harmonic Bragg reflections were not observed at temperatures across that magnetically ordered phase, creating an upper bound for their intensity of 0.16% of the first harmonic intensity.

We report here the magnetic susceptibility and electrical resistivity measurements of W_5-xTa_xSiB₂. W_5−xTa_xSiB₂ crystallizes in a tetragonal (the Mo₅SiB₂ (T₂ phase) type) structure. We evaluated the effect of the partial substitution of Ta in W₅SiB₂. The superconductivity was observed below x = 2 in W_5−xTa_xSiB₂. As substituted Ta for W, the superconducting transition temperature (T_c) clearly enhanced from 5.8 K to 6.5 K. The largest volume fraction exhibits at x = 0.2 sample. Thus, this work shows, for the first time, the superconductivity in W_5−xTa_xSiB₂. Moreover, we determined that the dominant carrier sign of W₅SiB₂ are the hole from the thermoelectric power measurement and discussed about the enhancement of T_c of W_5−xTa_xSiB₂ system.

We study the microwave properties of SrRuO₃ and CaRuO₃ thin films in the frequency range 45 MHz to 40 GHz, and at temperatures between 80 K and 300 K, using a broadband Corbino technique. The films are grown on SrTiO₃ and MgO substrates, and we discuss how dielectric substrate resonances complicate the study of the film properties, in particular in the case of SrTiO₃ with its large dielectric constant. For SrRuO₃ on MgO substrate we find signatures of the ferromagnetic transition throughout our spectral range.

Structural and magnetic properties in spinel type Fe_1−xZn_xV₂O₄ have been investigated for x ≤ 0.4. Successive structural transitions of cubic-to-tetragonal (c < a), tetragonal-to-orthorhombic and orthorhombic-to-tetragonal (c > a) are observed for x ≤ 0.2. On the other hand, the only cubic-to-tetragonal (c > a) structural transition is observed for 0.3 ≤ x ≤ 0.4. The result of the magnetic measurements indicates that the ferrimagnetic transition is observed for x ≤ 0.4 and the transition temperature is close to the tetragonal-to-orthorhombic transition temperature for x ≤ 0.2, and the cubic-to-tetragonal transition one for 0.3 ≤ x ≤ 0.4. These results are discussed in terms of Jahn-Teller distortions of FeO₄ tetrahedron and VO₆ octahedron, and spin-orbital interaction at Fe²⁺ ion.

We studied effects of Fe-site substitutions on superconductivity in the iron silicide Lu₂Fe₃Si₅ by investigating electric and magnetic properties of the mixed crystal Lu₂(Fe_1−xM_x)₃Si₅ (M = Mn, Co and Ru). The Mn and Co substitutions induce Curie tails in temperature dependence of magnetic susceptibilities indicating the enhancement of the localized character of 3d electrons. The Ru substitution reveals the disorder-sensitive superconductivity, suggesting the sign-reversed superconducting gap function in Lu₂Fe₃Si₅.

In the present study, we propose a new simple technique based on Faraday low of induction for the ac magnetic susceptibility measurement under high pressure. Although the technique needs a simple additional change from the conventional method, it performs certain separation a real (χ') from imaginary (χ") variable of magnetic susceptibility and free from the change of the ac field amplitude. A typically result obtained from this technique and the shielding effect of ac magnetic field by a pressure cell are shown.

Spinel Compound CuV₂S₄ exhibits incommensurate charge-density wave (CDW) transition at T_t ~ 90 K accompanied by a structural phase transition. We have performed synchrotron powder diffraction experiments in order to determine the crystal structure for low temperature phase. The CDW phase has an orthorhombic symmetry with the superspace group Imm2(0β0). It was found that the crystal structure is characterized by the significant change of bond lengths between vanadium atoms along a-axis. We suggested that this change is related to the orbital ordering of t_2g orbitals, which is the origin of the formation of the CDW.

We have investigated magnetic properties of the A-site ordered perovskite manganite LaBaMn₂O₆ by muon spin relaxation. In the ferromagnetic metal (FM) phase below T_C ~ 330 K, no muon spin oscillation is observed in the time spectra, where the muon spin relaxation significantly deviates from a single exponential form. These results may suggest a large spatial distribution of spin dynamics, i.e. the inhomogeneous magnetic nature in the FM phase of LaBaMn₂O₆, which has been also observed in the FM phase of A-site disordered perovskite manganites. It is, therefore, suggested that there exists the spatial distribution of spin dynamics in the FM phase of the perovskite manganites irrespective of the order/disorder at the A-site of perovskite structure.

We found a phase transition in the normal state of superconducting carbide, Mo₃Al₂C by nuclear magnetic resonance (NMR) measurement. At ~130 K the sharp central NMR line broadens and has additional broad components at the foot. In addition, the Knight shift and the spin-lattice relaxation rate exhibit a significant decrease, suggesting the reduction in the conduction electron density. From these results, Mo₃Al₂C undergoes the phase transition which can be understood as the charge-density wave ordering. The decrease in 1/T₁ confirms the superconductivity appears in the partially quenched Fermi surface.

We studied magnetic properties of Ge-based spinel oxides Ge(Fe_1−xCo_x)₂O₄ to verify the frustration effect. We discovered that the spin-glass-like behavior in Ge(Fe_1−xCo_x)₂O₄ exists in 0 ≤ x ≤ 0.9, and coexists with the antiferromagnetic order in 0.1 ≤ x ≤ 0.9. Taking into account the coexistence of spin molecular excitations (frustration effect) and antiferromagnetic order in GeCo₂O₄ (x = 1), the spin-glass-like behavior coexisting with the antiferromagnetic order in Ge(Fe_1−xCo_x)₂O₄ might be attributed to the freezing of the spin molecules by the substitution of Fe for Co.

Ferroelectricity was studied in magnetic fields up to 14 T on the XY-like perfect-triangular-lattice antiferromagnet RbFe(MoO₄)₂, which exhibits a magnetically induced ferroelectric polarization along the c axis in the low-field 120°structure phases P1 and P2. We have experimentally confirmed that the high-field P4/P5 phases of RbFe(MoO₄)₂ for B || a are nonferroelectric. The results favor the scenario that the ferroelectricity in the low-field phases of this system is driven by the in-plane spin chirality.

A long-time variation of magnetic structure has been observed in a geometrically frustrated magnet Ca₃Co₂O₆. In order to clarify that the long-time variation in this material does not originate in randomness due to impurities, magnetization measurements have been made on single crystals of diluted compounds Ca₃(Co-M)₂O₆ (M=Ga, Mg and Al). The longtime variations of the magnetization were detected in these compounds and characteristics of the time variation behavior were not much modified by the introduction of disorder produced by the substitution of non-magnetic atoms. These results show that the long-time variation of the magnetic structure in Ca₃Co₂O₆ is not caused by randomness or imperfections.

The spinel CuV₂S₄ exhibits two-step anomalies at about 92 K (T_t1) and 56 K (T_t2), reflecting the CDW state. In order to investigate the CDW state on V rich side in Cu(Ir_1−xV_x)₂S₄ microscopically, NMR measurements of ⁵¹V for Cu(Ir_1−xV_x)₂S₄ (x = 1.00, 0.98 and 0.95) have been carried out between 4.2 and 300 K. The line width of ⁵¹V for x = 1.00 is very narrow above T_t1 and become wider with decreasing x. For x = 1.00 and 0.98, the line width is suddenly broadened at T_t1 due to an electrical quadruple interaction between ⁵¹V nucleus and CDW charge modulation, but for x = 0.95, the line width is continuously increased around T_t1. The Knight shifts of ⁵¹V for x = 1.00, 0.98 and 0.95 show almost the same negative shift due to the influence of d-electrons above T_t1. At T_t1, the values of the Knight shift for x = 1.00 and 0.98 suddenly change to positive values, but for x = 0.95 continuously shifts to a positive one. The 1/T₁T's of ⁵¹V for x = 1.00, 0.98 and 0.95 obey the Korringa law above T_t1. Below T_t1, the 1/T₁T values of ⁵¹V for x = 1.00, 0.98 and 0.95 take constant values, which shows the Korringa law resulting from Pauli paramagnetic state.

To clarify a crucial role of a spin-orbit coupling in the emergence of novel spin-orbital states in 5d-electron compounds such as Sr₂IrO₄, we investigate ground state properties of a t_2g-orbital Hubbard model on a square lattice by Lanczos diagonalization. In the absence of the spin-orbit coupling, the ground state is spin singlet. When the spin-orbit coupling is strong enough, the ground state turns into a weak ferromagnetic state. The weak ferromagnetic state is a singlet state in terms of an effective total angular momentum. Regarding the orbital state, we find the so-called complex orbital state, in which real xy, yz, and zx orbital states are mixed with complex coefficients.

The electrical resistivity ρ(T) of dilute Pd-Ni alloys has been measured at high pressure up to 3 GPa. It is found that the ρ(T) of the ferromagnetic Pd-Ni alloy shows an anomalous temperature dependence near the critical pressure P_c, where the ferromagnetism disappears. The results are analysed in the framework of quantum critical behaviour induced by pressure. The effect of magnetic field on the ρ(T) is also examined. Different behaviour of magnetoresistance against pressure was found depending on the alloy concentration and discussed in connection with an instability of ferromagnetism.

State of the art density functional theory (DFT) calculations within the generalized gradient approximation (GGA) are reported for the (metastable) cubic MnGe with chiral B20 structure under applied pressure. It is found that the ambient pressure metallic (nominally) ferromagnetic high spin state (2 μ_B/ f.u.) in MnGe undergoes a transition into a low spin state (1 μ_B/ f.u.). From the calculations, the pressure at this first-order transition is about 18 GPa. The low moment state is halfmetallic, i.e., a small band-gap opens in the density of states for the majority spin electrons at the Fermi edge. Calculations for decreasing lattice parameter show that a fully metallic ferromagnetic low spin state should be recovered, before at still higher pressures the magnetic moment collapses and a non-spin polarized state arises. For the zero, low and high spin states, the equation of state has been determined. As the non-centrosymmetric crystal structure enables chiral Dzyaloshinskii-Moriya couplings, the magnetic order in MnGe is long-period modulated. The theoretical results suggest that half-metallic transport properties could be observed on the background of chiral complex spin-structures in MnGe under presssure or in strained thin films.

We have investigated the spin dynamics of the paramagnetic region of -FeGe by means of Electron Spin Resonance (ESR) spectroscopy. The high-quality single crystal showed a strong paramagnetic resonance line which is well-described by a Lorentzian shape above 287 K. The close relation of the linewidth to the magnetic susceptibility indicates a resonance of a strongly coupled 3d / conduction electron system.

The magnetic and structural properties of non-stoichiometric La_0.5Ba_0.5CoO_3-δ cobaltites have been studied by neutron and synchrotron powder diffraction. The oxygen content decrease leads to transformation of the magnetic structure from ferromagnetic to G-type antiferromagnetic through the two-phase state. Both coexisting phases have a cubic crystal structure with different volume of the unit cell.

Possible orbital orders below the metal-insulator transition temperature in quasi-one-dimensional BaVS₃ have been investigated in relation to the monoclinic lattice distortion in the insulating phase using a one-dimensional two-band Hubbard model. Orbital occupations in the ground state and optical conductivity have been calculated by means of exact diagonalization of finite size clusters. Depending on the size of crystal field splitting of the t_2g levels, two different orbital orders in the t_2g orbitals with the periodicity of four V ion sites along the c-axis are found to be stabilized with the lattice distortion. The on-site exchange interaction plays important role in the formation of these orbital orders.

One of the interesting issues on the spin-triplet superconductor Sr₂RuO₄ (T_c = 1.5 K) is the origin of the T_c enhancement up to about 3 K in the eutectic system Sr₂RuO₄-Ru. We have recently revealed a striking fact that T_c of pure Sr₂RuO₄ is also enhanced up to 3.2 K by uniaxial pressure along the c axis (P_||c). When P_||c is applied to Sr₂RuO₄-Ru, there is a crossover at P^*_||c ~ 0.4 GPa from behavior attributable to interfacial superconductivity (SC) to behavior similar to that in pure Sr₂RuO₄ under P_||c. We focus on the field-temperature phase diagrams of Sr₂RuO₄-Ru above and below P^*_||c. We revealed that the H-T curves of Sr₂RuO₄-Ru both above and below P^*_||c are concave-up. This fact suggests that the P_||c-originated SC above P^*_||c as well as the interfacial SC below P^*_||c are granular-like. We also found that the H-T curves of these SCs do not scale with T_c's, which is possibly related to a difference in the spatial distribution of superconducting regions.

We make the electronic structure calculations of transition-metal oxides with a hollandite-type crystal structure A₂M₈O₁₆ using the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction is taken into account (GGA+U). We first examine the electronic structure of the 3d series (M=Ti, V, Cr, Mn) as well as the 4d series (M=Mo, Ru, Rh) to discuss their generic features. We then discuss in particular the origins of the metal-insulator transition observed in K₂Cr₈O₁₆ and quasi-one-dimensional electron conduction observed in K₂Ru₈O₁₆. We also consider the observed metal-insulator transition in K₂V₈O₁₆ where the effect of electron correlations may play an essential role.

Electrical resistivity ρ and thermopower S of the Laves phase compound of NdCo₂ has been investigated at temperatures from 2 K to 300 K. ρ has been measured under pressures up to 8 GPa and S has been measured under pressures up to 3 GPa. The magnetic transition temperature T_C obtained by ρ measurement decreases with increasing pressure. The temperature T_mim where the thermopower S takes minimum at high temperature region increases linearly with increasing pressure. The high-temperature minimum of S is associated with a sharp peak in density of states related mainly to the Co 3d-electron density. Since the width of an itinerant electronic band depends on the extent of the corresponding overlapping of 3d orbitals, the pressure variation of T_min can be attributed to the broadening of the peak width of 3d electron density of states.

We have investigated the effect of hydrostatic pressure on 3-K phase superconductivity in Sr₂RuO₄-Ru eutectic crystals by means of AC magnetic susceptibility measurements. We have found that the application of hydrostatic pressure suppresses the superconducting transition temperature T_c of the 3-K phase with a pressure coefficient of dT_c/dP ≈ −0.2 K/GPa, similar to the case of the 1.5-K phase. We have also observed that the effect of hydrostatic pressure on the 3-K phase seems to be elastic whilst that of uniaxial pressure is plastic.

The 5d transition metal oxide Cd₂Os₂O₇ having a pyrochlore lattice of Os atoms exhibits a metal-insulator transition with a magnetic ordering at 227 K, but the magnetic structure in the ordered state and the origin of the transition have not yet been specified. The commensurate magnetic reflection with the propagation vector k = (0, 0, 0) was first observed in the resonant X-ray scattering using a high-quality single crystal. We also clarified the only one kind of magnetic moment via the synchrotron radiation-based Mössbauer effect. We propose that a strong spin-orbit interaction is notably apparent in the spin arrangement and the insulating mechanism in this compound.

We performed inelastic neutron scattering experiments on a three-dimensional magnetic system MnP, which is a ferromagnetic inter-metallic compound below T_C = 291 K and transitions into a screw state at T^* = 47 K. Using newly developed high resolution chopper spectrometer (HRC) and triple axis spectrometer LTAS (for E = 0–2 meV), 6G (for E = 2–8 meV) TAS1 (for E = 10-35 meV), we succeeded in observing the excitations from low energy to Brillouin zone center. We assumed isotropic 6 exchange parameters, and calculated dispersion relation using Heisenberg model for 2-sub lattices. We confirm that nearest neighbor (n.n) exchange parameters J_m are J > 0 (ferromagnetic interaction), the next nearest neighbor (n.n.n) exchange parameters are J < 0 (anti-ferromagnetic interaction), We conclude that the competition between n.n and n.n.n would be the cause of helimagnetic structure in MnP.

Low-temperature specific heat of CaRu_1−xMn_xO₃ was measured to clarify the role of d electrons in ferromagnetic and antiferromagnetic orders observed above x = 0.2. Specific heat divided by temperature C_p/T is found to roughly follow a T² function, and relatively large magnitudes of electronic specific heat coefficient γ were obtained in wide x range. In particular, γ is unchanged from the value at x = 0 (84 mJ/K² mol) in the paramagnetic state for x ≤ 0.1, but linearly reduced with increasing x above x = 0.2. These features of γ strongly suggest that itinerant d electrons are tightly coupled with the evolution of magnetic orders in small and intermediate Mn concentrations.

We have investigated the electronic structures of the parent compound of an iron chalcogenide superconductor TlFe₂Se₂ by bulk-sensitive photoemission spectroscopy (PES). Valence-band PES demonstrates the energy gap opening at the Fermi level due to the shift of Fe 3d state to the higher binding energy side as compared to the calculated density of states, being consistent with the insulating behaviour in transport measurements.

The low temperature electrical and thermal transport properties of the itinerant ferromagnet ZrZn₂ were investigated in order to explore the nature of the Fermi-liquid breakdown in this material. We have implemented electrical and thermal conductivity measurements down to temperatures of 100 mK and in high magnetic field. In zero field and above 2 K the electrical and effective thermal resistivities take a T^5/3 and T-linear form, respectively. These are the signatures of the marginal Fermi-liquid, predicted to occur close to a ferromagnetic quantum critical point by spin fluctuation theory. In contrast, we find that below 2 K and in external magnetic field the electrical resistivity assumes a quadratic temperature dependence, consistent with a return to conventional Fermi-liquid behaviour.

Transmission electron microscopy offers the possibility of imaging flux vortices in superconductors quantitatively and at video rate. Here we use the technique to image flux vortices in MgB₂ and investigate their magnetic structure.

We present a structural investigation of the organic charge-transfer salt superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br at a temperature of 90 K using high energy (90 keV) powder x-ray diffraction. With this approach we access the scattering profile over a wide range in Q-space (from ~ 0.3 to 19 Å⁻¹). Overall, the observed scattering intensity is understood as a superposition of Bragg diffraction from a crystalline phase superimposed onto a large quasi-amorphous signal, indicative of short range order effects in our powder material. We discuss implications of these findings regarding the structural properties of this material.

We calculate the electronic structure and exchange interactions in a copper(II)phthalocyanine (Cu(II)Pc) crystal as a one-dimensional molecular chain using hybrid exchange density functional theory (DFT). In addition, the intermolecular exchange interactions are also calculated in a molecular dimer using Green's function perturbation theory (GFPT) to illustrate the underlying physics. We find that the exchange interactions depend strongly on the stacking angle, but weakly on the sliding angle (defined in the text). The hybrid DFT calculations also provide an insight into the electronic structure of the Cu(II)Pc molecular chain and demonstrate that on-site electron correlations have a significant effect on the nature of the ground state, the band gap and magnetic excitations. The exchange interactions predicted by our DFT calculations and GFPT calculations agree qualitatively with the recent experimental results on newly found η-Cu(II)Pc and the previous results for the α- and β-phases. This work provides a reliable theoretical basis for the further application of Cu(II)Pc to molecular spintronics and organic-based quantum information processing.

The fullerides A_nM_mC₆₀ (A=K, Rb, Cs; M = Be, Mg; B, Al, Ga. In; n = 1, 2; m = 1, 2) have been synthesized by exchange reactions of alkali metal fullerides with metal chlorides (groups 2 and 13), by method using liquid alloys of metals with mercury (amalgams), and by thermal decomposition of aluminum hydride. The samples are studied by X-ray diffraction, nuclear magnetic resonance, electron paramagnetic resonance, Raman scattering, and differential scanning calorimetry. It was found that the fulleride K₂GaC₆₀ is superconductor with transition temperature T_c = 22 K that exceeds T_c for K₃C₆₀ (19 K). Fullerides with In and B are not supercomductors. The fullerides with composition Cs_nHg_xC₆₀ (n = 2; 3) are not superconductors and crystallize in orthorhombic lattices.

We present a calorimetric study on single crystals of Ca(Fe_1−xCo_x)₂As₂ (x = 0, 0.032, 0.051, 0.056, 0.063, and 0.146). The combined first order spin-density wave/structural transition occurs in the parent CaFe₂As₂ compound at 168 K and gradually shifts to lower temperature for low doping levels (x = 0.032 and x = 0.051). It is completely suppressed upon higher doping x ≥ 0.056. Simultaneously, superconductivity appears at lower temperature with a transition temperature around T_c ~ 14.1 K for Ca(Fe_0.937Co_0.063)₂As₂. The phase diagram of Ca(Fe_0.937Co_0.063)₂As₂ has been derived and the upper critical field is found to be μ₀H^(c)₂ = 11.5 T and μ₀H^(ab)_c2 = 19.4 T for the c and ab directions, respectively.

We study the two-dimensional p-d model by means of a four-pole approximation within the Composite Operator Method framework. Many results of numerical simulations have been correctly reproduced by considering as basic composite field a four-component spinor field, including the p field, the two Hubbard operators for the d field, and a composite operator describing the p field dressed by the spin excitations of the d field. In this manuscript, we propose an alternative choice for the latter field, which takes into account charge and pair fluctuations too. The results of this study show that near half-filling one can safely neglect the contribution of charge and pair fluctuations and safely use the previously proposed field.

We have investigated the isovalently substituted system BaFe₂(As_1−xP_x)₂ by high-resolution thermal expansion using a home-built capacitive dilatometer. Accurate measurements succeeded despite the very small size of the available single crystals (~ 500 × 500 × 100μm³). Information on the uniaxial pressure derivatives of the transition temperatures is obtained using thermodynamic relations. In-plane and out-of-plane pressure derivatives have opposite sign, which demonstrates the sensitivity of the compound to uniaxial pressure. The structural and the superconducting transition always respond oppositely to uniaxial pressure, which signals their coupling and competition.

Superconductivity (SC) and long-range ferromagnetism (FM) are mutually antagonistic, thus in general SC does not coexist with FM. In this mini-review, however, we will show that such a coexistence can inhabit several iron pnictide systems, including EuFe₂(As_1−xP_x)₂, Eu(Fe_1−xCo_x)₂As₂, Eu(Fe_1−xRu_x)₂As₂, CeFe(As_1−xP_x)O_0.95F_0.05 and Sr₂VFeAsO₃. We will briefly discuss the possible reasons and consequences of the coexistence of 3d-electron SC and 4f-electron FM.

We report on systematic magnetization experiments in an Y_1-xCa_xBa₂Cu₃O_7−δ (x = 0.25 at%) single crystal. The magnetization experiments were made using a superconducting quantum interference device magnetometer (SQUID). Magnetic moments were measured as functions of the temperature according to the zero-field cooling (ZFC), field-cooled cooling (FCC), and field-cooled warming (FCW) prescriptions. The time-dependence of the FC magnetization at fixed magnetic fields was studied. Magnetic fields up to 50 kOe were applied and a paramagnetic response related to the superconducting state was observed when strong enough fields were applied parallel to the c axis. The magnitude of the high field paramagnetic moment (HFPME) increases when the field is augmented. The effect shows strong and anomalous time dependence, such that the paramagnetic moment increases as a function of the time. An YBa₂Cu₃O_7−δ single crystal exhibiting the same effect was used for comparison. We discuss our results in terms of the flux compression scenario into the sample modulated by Ca concentration.

We report the orbital character of the Fermi surfaces for the Fe-based superconductor LiFeAs using the polarization-dependent three-dimensional angle-resolved photoemission spectroscopy and the angular-dependent dipole selection rule analysis. The evaluated orbital character is well reproduced by the band calculation. The results demonstrate that the polarization-dependent ARPES combined with the selection rule analysis is useful to identify the orbital character of the Fe-based unconventional superconductivity.

We report the electrical resistivity measurements of (Fe₂As₂)(Ca₆(Al, Ti)₄O_y) polycrystalline sample under hydrostatic pressure using a cubic-anvil apparatus. The temperature dependence of the normal state resistivity exhibits metallic behavior. The ρ(T) curve is convex upward at low pressure, and it approaches T-linear dependence by applying high pressure. The superconductivity is suppressed by application of pressure. The onset temperature of superconductivity, which is 38 K at ambient pressure, decreases to 28 K at 8 GPa.

³¹P-NMR measurements were performed in the iron pnictides BaFe₂(As_1−xP_x)₂ for 0.07 ≤ x ≤ 0.64 in order to investigate evolution of electronic state with isovalent substitution. In general, the spin part of the Knight shift K_spin and (T₁T)^−1/2 are proportional to the quasiparticle density of states at the Fermi level N(E_F) at high temperature where the Korringa relation holds. We found that K_spin and (T₁T)^−1/2 are almost unaffected by the P-substitution, indicating that N(E_F) is nearly constant. These results are quantitatively consistent with our band calculation. We claim that the slight changes of N(E_F) by P-substitution in BaFe₂(As_1−xP_x)₂ is not enough to explain the Tc variation ranging from 30 K to 0 K, and thus suggest that antiferromagnetic correlations related to the quantum critical character are essential for the high T_c superconductivity.

We have discussed the origin of broad high-frequency spectra in Angle-resoluved photoemission spectroscopy(ARPES) in high-Tc cuprates by using quantized massive gauge fields. It is suggested that the emitted massive gauge fields, which are mediating Cooper pairing in high-Tc cuprates, decay into other excitations such as spin, charge, and phonon waves. The evolution mechanism of the Fermi arc with increasing hole-doping has been discussed, taking into account the restoration of spontaneous symmetry breaking.

We report the crystal structure parameters of clathrate-type copper-oxide Cu₆O₈YCl, which belongs to the Cu₆O₈MX (M = cation, X = anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu₆O₈YCl was identified to be a cubic structure with the space group Fmm. The Cl atom is located at the 4a site, which is the center position of the Cu₆O₈ cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (U_iso) at room temperature, which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu₆O₈ cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds.

⁷⁵As nuclear spin-lattice relaxation rate 1/T₁ was measured in Zn-free and Zn-substituted La(Fe_1−xZn_x)AsO_0.85 (x = 0, and 0.05), T_c of which is 25 K and 0 K, respectively. Although the temperature dependence of 1/T₁T above T_c is the same between two samples, 1/T₁T of x = 0 shows T^3.0±0.1 dependence (1/T₁ ∝ T⁴) in the superconducting (SC) state, but 1/T₁T of non-SC x = 0.05 sample continuously decreases below 50 K. The normalized 1/T₁T [(1/T₁T)_x=0/(1/T₁T)_x=0.05], which is related to the SC gap structure, is proportional to T^2.4±0.1 below T_c. These results indicate that the temperature dependence of 1/T₁T in the SC state can be affected by the temperature dependence of the normal-state 1/T₁T.

Variation of superconducting transition temperature T_c for the pseudoternary Sr(T_1−xT'_x)₂Ge₂ layer systems (T, T' = Co, Ni, Rh, Pd; 0 ≤ x ≤ 1) are reported. For the system Sr(Pd_1−xRh_x)₂Ge₂, T_c decreases from 3.12 K for x = 0, to 2.92 K for x = 0.1, to 2.50 K for x = 0.2, and extrapolated to 0 K for x = 0.65. No T_c is expected for SrRh₂Ge₂. For the system Sr(Pd_1−xCo_x)₂Ge₂, T_c decreases sharply to 2.80 K for x = 0.01, 2.58 K for x = 0.03, and extrapolates to 0 K for x = 0.15. No T_c is also expected for SrCo₂Ge₂. In these systems, stronger T_c suppression for Co(3d⁷) and Rh(4d⁷) were due to lower density of states in d_xz,yz bands with one less conduction electron per transition metal. Weaker T_c suppression for isoelectronic Ni is due to shift from Pd-4d⁸ to Ni-3d⁸ band. T_c decreases monotonically from 3.12 K for SrPd₂Ge₂ to 0.92 K for SrNi₂Ge₂. The lower T_c of the present electron-overdoped (nd⁷ or nd⁸) compounds are due to dispersive 3D-like nd_xz,yz conduction bands with weaker electron correlation, as compared to the less-electron-doped (3d^6.1) 22 K superconductor BaFe_1.8Co_0.2As₂ or the hole-doped (3d^5.9) 38 K superconductor Ba_0.6K_0.4Fe₂As₂ with additional less dispersive 2D-like 3d_xy band contribution and stronger electron correlation.

We have performed electrical resistivity measurements of EuFe₂As₂ under hydrostatic pressures (P) up to 3.2 GPa and magnetic fields up to 15 T. In zero field, the superconducting (SC) ground state with a sharp transition to zero resistivity, indicative of bulk superconductivity, emerges at T_c ~ 30 K in a pressure range from 2.5 GPa to ~ 3.0 GPa. Under applied field of 15 T, the SC transition is suppressed to lower temperatures with increasing pressure, suggesting that the application of pressure reduces the upper critical field B_c2 for the P-induced superconductivity. Meanwhile, the antiferromagnetic (AF) order of the Eu²⁺ moments survives up to 3.2 GPa, the highest pressure in the experiments, with no significant change in the AF ordering temperature and the magnetic field effect.

We have investigated the upper critical field B_c2 of the stoichiometric Fe-based superconductor LiFeAs, via the magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B_c2 at 0.3K is obtained to be 26.4 T and 15.5 T for the applied field B_a || ab and B_a || c, respectively, indicating nearly-isotropic superconductivity. The detailed analyses show that, while B_c2 for B_a || c is limited by the orbital effect, the spin-paramagnetic effect is evident in the temperature dependence of B_c2 for B_a || ab.

Transmission electron microscopy was used to investigate the tetragonal-to-orthorhombic phase transitions in SrFe₂As₂ and Ba(Fe_0.985Co_0.015)₂As₂ which are members of the recently discovered iron-based superconductors. We use a method which measures the spatially-resolved order parameter and show that whereas the transition in SrFe₂As₂ is a first-order martensitic phase change, Ba(Fe_0.985Co_0.015)₂As₂ shows a continuous change of the order parameter as the phase transition takes place. This would normally indicate a second order phase transition but the situation seems to be more complicated as, during the transition, different regions of the sample had different values of the order parameter: something which should only happen in a first order change.

We perform self-consistent calculations in the framework of the Bogoliubov-deGennes theory with a model Hubbard Hamiltonian with strong local correlations. The calculations are performed in a charge density disordered system to study the competition of charge inhomogeneity, spin disorder and superconductivity These calculations are appropriate to describe the cuprate superconductors' phase diagram with the competition of different phases. We find that, in the presence of charge disorder the spin density wave (SDW) order occurs at higher temperatures above the opening of the superconducting gap. This is in opposition to the homogeneous systems where the SDW phase appears in the low doping compounds in the superconducting phase. These finding provides an explanation to the non-Fermi liquid behavior of the cuprates normal phase.

The effect of oxygen isotope substitution on T_c has been analyzed for heavily underdoped and overdoped La_2-xSr_xCu_1-yZn_yO₄ compounds with different Zn contents in the CuO₂ plane. The effect of Zn on the isotope exponent, α, was more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with in-plane disorder can be described quite well within a model based solely on impurity induced Cooper pair-breaking in the case of the underdoped compounds. This model, on the other hand, fails to describe the behavior of α(y) for the overdoped samples, even though Zn still breaks pairs quite effectively at this hole (Sr) content. We discuss the implications of these finding in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as number of charge carriers in the CuO₂ plane, p (≡ x), is varied.

Unoccupied electronic structure of iron-based superconductors NdFeAsO_0.7 (T_c=51 K), BaFe₂As₂ and Ba(Fe_0.89Co_0.11)₂As₂ (T_c=23 K) has been investigated by means of inverse-photoemission spectroscopy (IPES). The unoccupied Fe 3d states are observed around 1 eV above the Fermi level for all compounds. The Fe 3p-3d resonant IPES suggests that the unoccupied Fe 3d states of Ba(Fe_1−xCo_x)₂As₂ have more localized character compared with those of NdFeAsO_0.7. The unoccupied Nd 4f states of NdFeAsO_0.7 are located around 5 and 7 eV, and the Ba 5d and Ba 4f states of Ba(Fe_1−xCo_x)₂As₂ are located around 5 and 12 eV, respectively.

Current-voltage characteristics and dynamic conductance of the superconductor -constriction - superconductor junctions in GdO(F)FeAs polycrystalline samples with critical temperatures T^local_C = 46 ÷ 53 K were investigated. Two superconducting gaps, the large Δ_L = 10.5±2 meV, and the small one Δ_s = 2.3 ± 0.4 meV were clearly observed at T = 4.2 K. The 2Δ_L/k_BT^local_C = 5.5 ± 1 ratio gives support to the strong coupling mechanism which is responsible for the high T_c value. Temperature dependence of the large gap Δ_L(T) indicates the presence of intrinsic proximity effect (in κ-space) between two superconducting condensates.

We calculate the magnetic excitation spectrum of the J₁ − J₂ model for iron pnictides within the framework of the nonlinear sigma model. We find, in addition to the usual acoustic spin-wave branch, a second, optical spin-wave branch, not captured by previous theoretical studies. We also find that the spin-wave velocity has a planar directional anisotropy, resulting from the collinear/striped structure of the antiferromagnetic ground state. Finally, we include single ion anisotropy, which fully gaps all branches, and we discuss the relevance of the modified, multi-branched, fully gapped character of the spectrum to the understanding of neutron scattering results for SrFe₂As₂

We have studied experimentally by electrical magnetoconductivity measurements the influence of the site disorder, introduced by chemical doping, on the superconducting paring transition of the YBa₂Cu₃O_7-δ superconductor. The measurements were performed in YBa₂Cu₃O_7-δ and YBa_1.9Sr_0.1Cu₃O_7-δ single crystals with low magnetic fields applied either parallel or perpendicular to the superconducting Cu-O atomic planes. The in-plane electrical conductivity results show the systematic occurrence of tree-dimensional Gaussian (3D), tree-dimensional XY (3DXY) and beyond tree-dimensional XY fluctuation regimes, as the temperature is decreasing toward to the T_c of our samples. We propose that the observed beyond tree-dimensional XY fluctuation regime is precursory to a weak first-order superconducting transition driven by antiferromagnetic excitations related to the pseudo gap phenomenon. The experimental results shows that chemical introduced disorder, by Sr doping, does not hardly affect the stability of the beyond tree-dimensional XY fluctuation regimes in the YBa₂Cu₃O_7-δ superconductor when H ≤ 600Oe are applied.