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We have measured the correlated electron pair emission from a Cu(001) surface by both direct and core-resonant channels upon excitation with linearly polarized photons of energy far above the 3p threshold. As expected for a single-step process mediated by electron correlation in the initial and final states, the two electrons emitted by the direct channel continuously share the sum of the energy available to them. The core-resonant channel is often considered in terms of successive and independent steps of photoexcitation and Auger decay. However, electron pairs emitted by the core-resonant channel also share their energy continuously to jointly conserve the energy of the complete process. By detecting the electron pairs in parallel over a wide range of energy, evidence of the core-resonant double photoemission proceeding by a coherent single-step process is most strikingly manifested by a continuum of correlated electron pairs with a sum energy characteristic of the process but for which the individual electrons have arbitrary energies and cannot meaningfully be distinguished as a photoelectron or Auger electron.

We consider a weakly interacting finite wire with short and long range interactions. The long range interactions enhance the 4k_F scattering and renormalize the wire to a strongly interacting limit. For large screening lengths, the renormalized charge stiffness Luttinger parameter K_eff decreases to , giving rise to a Wigner crystal at T = 0 with an anomalous conductance at finite temperatures. For short screening lengths, the renormalized Luttinger parameter K_eff is restricted to . As a result, at temperatures larger than the magnetic exchange energy we find an interacting metal which, for , is equivalent to the Hubbard model, with the anomalous conductance .

In this work Raman excitation profiles of metallic carbon nanotubes have been calculated and thoroughly analyzed. Suppression and vanishing of the high-energy resonance is completely confirmed by our calculations. The presented results clearly show that the suppression, and finally the absence, of the resonance is caused by electron–phonon interaction and interference effects. Electron–phonon coupling for low-energy resonance is significantly larger than for high-energy resonance. Furthermore, the transition energies of those two transitions are close enough to make interference effects important. The type of interference is determined by the sign of the electron–phonon interaction matrix elements. Constructive interference makes the low-energy resonance more intensive and destructive interference destroys the high-energy resonance for most of the metallic tubes.

NeHe₂ was compressed to about 90 GPa using a diamond anvil cell technique. The crystal structure was confirmed to be stable with hexagonal symmetry in the investigated pressure range and its p–V equation of state was determined by angular dispersive x-ray diffraction with synchrotron radiation. With the help of ab initio calculations, the compressibility and inter-atomic distances of NeHe₂ were compared with those of a helium and neon mixture of the same composition. This study shows that the bulk modulus of NeHe₂ is between those of neon and helium and that linear compressibilities of the inter-atomic distances are different from those of the elementary solids. This material can be a pressure-transmitting medium, providing both a large sample space and good quasi-hydrostatic conditions.

The pressure-induced crystal structure of lead sulfide (PbS) above 2.2 GPa has been studied with single-crystal x-ray diffraction in a diamond anvil cell at room temperature. It has been found to be twinned and of the TlI type (Cmcm, Z = 4), in which the Pb atoms are surrounded by seven S atoms in a capped trigonal prism coordination. The twin laws in relation to the parent B1 (NaCl) type structure (, Z = 4) at atmospheric pressure have been discussed.

The absorption of fast quasi-transverse modes during anharmonic scattering processes in cubic crystals with positive (Ge, Si, diamond and InSb) or negative (KCl and CaF₂) anisotropies of the second-order elastic moduli is studied. Mechanisms underlying the relaxation of the fast quasi-transverse mode by two fast (the FFF mechanism) or two slow (the FSS) modes are discussed in the long-wavelength approximation. Angular dependences of the ultrasound absorption for the FFF, FSS and Landau–Rumer relaxation mechanisms are analyzed in terms of the anisotropic continuum model. The full absorption of the fast quasi-transverse mode is determined. The problem of the scattering of collinear and noncollinear phonons in cubic crystals and their role in the ultrasound absorption of the fast quasi-transverse modes is considered. It is shown that the FFF and FSS relaxation mechanisms are due to the cubic anisotropy of the crystals, leading to the interaction between noncollinear phonons. In crystals with a considerable anisotropy of the elastic energy (InSb and KCl), the total contribution of the FFF and FSS relaxation mechanisms to the full absorption is one to two orders of magnitude larger than the contribution from the Landau–Rumer mechanism, depending on the direction. Much of the dominance of the former relaxation mechanisms over the Landau–Rumer mechanism is explained by second-order elastic moduli. The role of the Landau–Rumer mechanism in ultrasound absorption may be considerable in cubic crystals with a smaller anisotropy of the elastic energy. It is demonstrated that when anharmonic scattering processes play the dominant role, the inclusion of one of the relaxation mechanisms (the Landau–Rumer mechanism or the FFF or FSS mechanisms of relaxation) is insufficient for the quantitative description of the anisotropy of the full absorption of the fast quasi-transverse modes in cubic crystals.

We use electronic structure calculations based upon density functional theory to search for ideal plasmonic materials among the alkali–noble intermetallics. Importantly, we use density functional perturbation theory to calculate the electron–phonon interaction and from there use a first order solution to the Boltzmann equation to estimate the phenomenological damping frequency in the Drude dielectric function. We discuss the necessary electronic features of a plasmonic material and investigate the optical properties of the alkali–noble intermetallics in terms of some generic plasmonic system quality factors. We conclude that at low negative permittivities, KAu, with a damping frequency of 0.0224 eV and a high optical gap to bare plasma frequency ratio, outperforms gold and to some extent silver as a plasmonic material. Unfortunately, a low plasma frequency (1.54 eV) reduces its utility in modern plasmonics applications. We also discuss, briefly, the effect of local fields on the optical properties of these materials.

CeOs₄Sb₁₂ and CeFe₄P₁₂ are classified as Kondo semiconductors, which show coupled changes in electrical transport, thermodynamic and magnetic properties with a low-temperature semiconductor-like electrical resistivity. We have carried out core level and valence band photoemission spectroscopy on single crystal CeOs₄Sb₁₂ and CeFe₄P₁₂ to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (∼10–300 K). The Ce 3d core level spectra show the presence of f⁰, f¹ and f² final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of n_f = 0.97 and 0.86 per Ce atom, suggestive of a low- and high-T_K (= single ion Kondo temperature) for CeOs₄Sb₁₂ and CeFe₄P₁₂, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy ∼ 50 meV and ∼ 110 meV in the valence band density of states (DOS) of CeOs₄Sb₁₂ and CeFe₄P₁₂, respectively. The temperature dependence of the DOS at the Fermi level follows the change in effective magnetic moment estimated from magnetic susceptibility for both materials, confirming the Kondo nature of the pseudogap in CeOs₄Sb₁₂ and CeFe₄P₁₂. A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps Δ_C for Ce-based Kondo semiconductors and provides a direct relation with T_K given by Δ_C ∼ 2k_BT_K. In conjunction with the known behaviour of the spin gaps Δ_S ∼ k_BT_K, the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors.

Effects of compression on the structural and electronic properties of liquid lithium are investigated with first-principles molecular dynamics calculations. Within a large pressure range up to 60 GPa, along isotherms from 600 to 1000 K, several structural transformations were found. The liquid structures at high pressure are found to be not sensitive to the temperature within this range. It is shown that the radial distribution functions broadly resemble the corresponding solid phases, particularly at low pressures. The evolution of the electronic structure under pressure also shows a remarkable similarity to the underlying solid. However, detailed analyses of the temporal liquid inherent structures show that the instantaneous short-range order may differ significantly from the underlying known solid phase.

We implement a bottom-up multiscale approach for the modeling of defect localization in C_6n²H_6n islands, i.e. graphene quantum dots with a hexagonal symmetry, by means of density functional and semiempirical approaches. Using the ab initio calculations as a reference, we recognize the theoretical framework under which semiempirical methods adequately describe the electronic structure of the studied systems and thereon proceed to the calculation of quantum transport within the nonequilibrium Green function formalism. The computational data reveal an impurity-like behavior of vacancies in these clusters and evidence the role of parameterization even within the same semiempirical context. In terms of conduction, failure to capture the proper chemical aspects in the presence of generic local alterations of the ideal atomic structure results in an improper description of the transport features. As an example, we show wavefunction localization phenomena induced by the presence of vacancies and discuss the importance of their modeling for the conduction characteristics of the studied structures.

We studied the electronic structure of the band-filling CaVO₃ and LaVO₃ compounds. The experimental techniques were photoemission (PES) and x-ray absorption (XAS) spectroscopy. The experimental results were analyzed using an extended cluster model. The ground states of CaVO₃ and LaVO₃ are highly covalent and contain a considerable 3d^{n + 1}L contribution. The CaVO₃ compound is in the charge transfer regime (Δ < U), whereas the LaVO₃ material is in the intermediate regime (Δ ∼ U). The spectral weight distributions reveal that CaVO₃ is a coherent metal and that LaVO₃ is a p–d insulator. The photoemission of CaVO₃ shows the coherent peak (3d¹C) and the incoherent feature (3d¹L). The spectrum of insulating LaVO₃ presents only the incoherent structure (3d²L), whereas the coherent peak is replaced by the Mott–Hubbard screening (3d²D). This transfer of spectral weight is responsible for the opening of the experimental bandgap. The incoherent feature contains a considerable O 2p character and cannot be attributed to the lower Hubbard band. Further, the relative V 3d–O 2p cross section helps to explain the photon energy dependence of the PES spectra. The addition spectra of both CaVO₃ and LaVO₃ are dominated by the 3d^{n + 1} final state configuration. The distribution of spectral weight is mainly dictated by intra-atomic exchange and crystal field splittings. The coherent contribution is less important than in photoemission, and is greatly diminished in the O 1s x-ray absorption spectra.

We study electronic phase transitions in the half-filled ionic Hubbard model with an on-site Coulomb repulsion U and an ionic energy Δ by using the coherent potential approximation. For a fixed and finite Δ two transitions from the band insulator via a metallic state to a Mott insulator are found with increasing U. The values of the critical correlation-driven metal–insulator transitions U_c1(Δ) and U_c2(Δ) are estimated. Our results are in reasonable agreement with the ones obtained by single-site dynamical mean-field theory and determinant quantum Monte Carlo simulation.

We present the results of an experimental study of the nucleation of superconductivity at the surface of a single-crystal YB₆ in a tilted dc magnetic field. The developed experimental approach allowed us to measure H_c3 at each side of the sample as a function of the angle between the dc magnetic field and the surface. Experiment showed that the ratio H_c3/H_c2≈1.28 when the dc field became perpendicular to the surface while the expected value of this ratio is 1. This sharp distinction with theory cannot be ascribed to the surface roughness.

The Si-B3 spectrum, observed in neutron-irradiated p-type silicon after annealing at T_an≈400 °C, has previously been extensively studied using electron spin resonance (ESR). It has been assigned to a silicon tetra-interstitial (I₄), based on the symmetry of the defect and resolved hyperfine (hf) structure doublets. However, additional ESR measurements carried out here at three frequency bands show that one of these doublets would not originate from the hf interaction since the doublet spacing is found to be dependent on the applied microwave frequency f. This casts doubt on the previous assignment of the Si-B3 spectrum to I₄ based on ESR data obtained at one observational f. Despite profound investigation, the origin of the f dependence of the satellite doublet could not be traced, disabling any progress on the Si-B3 defect modeling. The observation (re)emphasizes the necessity of the multi-frequency approach in coming to a correct interpretation of ESR parameters and correlated defect modeling.

In this paper the complex dielectric permittivity of gallium doped Cd_0.99Mn_0.01Te mixed crystals is studied at different temperatures. We observe a two-power-law relaxation pattern with m and n, the low- and high-frequency power-law exponents respectively, satisfying the relation m < 1 − n. To interpret the empirical result we propose a correlated-cluster relaxation mechanism. This approach allows us to find origins of both power-law exponents, m and n.

Using a nonequilibrium Green function approach, we systematically investigate the current induced spin polarization (CISP) in a two-dimensional heavy-hole system with cubic Rashba spin–orbit coupling, driven by in-plane electric and magnetic fields. We find that when a magnetic field is applied along the direction of electric field, the longitudinal conductivity drops monotonously with an increase of magnetic-field strength or of hole density. The spin polarization along the electric-field direction is just the Pauli paramagnetism and it quadratically increases with an increase of hole density. The nonvanishing out-of-plane component of spin polarization emerges for both short-range and long-range disorders, and it changes sign with the variation of magnetic field, especially for long-range hole-impurity scattering. In the magnetic-field dependences of this out-of-plane CISP and of the in-plane CISP perpendicular to the electric field, there are singular magnetic fields, below or above which the effects of magnetic field are completely different.

Strontium stannate is under study as an ultra-stable dielectric material for microelectronic applications at low temperatures. It is known to have a remarkably temperature-independent dielectric constant from 27 K to room temperature. However, we report anomalies in the Raman spectra, dielectric response, and differential thermal analysis of strontium stannate compatible with a structural phase transition at 160 K. Further anomalies are seen in calorimetric and Raman data (but not dielectric data) that suggest another phase transition at 270 K. A preliminary x-ray powder diffraction study confirms a small change in the pseudo-cubic lattice constant a(T) at the lower transition.

The anomalous birefringence and circular differential reflection of NH₄H₂PO₄ (point group ), cut on the optic axis, have been found to cause an additional signal in measurements of the optical rotation employing polarized light technology, with the sample between crossed and slightly modulated linear polarizers (tilting high accuracy universal polarimetry). The azimuthal rotation of the linearly polarized light, up to 100 times larger than expected, is described in terms of a circularly polarized light mode along the optic axis of varying amplitude. Experimental evidence leading to our conclusion is given and a qualitative model for the effect is presented.

We have investigated the magnetoelastic effects in CoF₂ associated with the antiferromagnetic phase transition temperature T_N≈39 K by means of neutron powder diffraction. The temperature variation of the lattice parameters and the unit cell volume has been determined accurately with small temperature steps. From the temperature variation of the lattice parameter c we extracted the lattice strain Δc associated with the antiferromagnetic phase transition. Rietveld refinement of the crystal and magnetic structure from the diffraction data at 2.2 K gave a magnetic moment of 2.57 ± 0.02 μ_B per Co ion. We determined the temperature variation of the intensity of the 100 magnetic Bragg reflection, which is proportional to the square of the order parameter of the phase transition. We established that the lattice strain Δc couples linearly with the square of the order parameter of the antiferromagnetic phase transition in CoF₂.

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni₂Mn_1.48Sb_0.52 have confirmed that it is ferromagnetic below 350 K and undergoes a structural phase transition at T_M≈310 K. The high temperature phase has the cubic L2₁ structure with a = 5.958 Å, with the excess manganese atoms occupying the 4(b) Sb sites. In the cubic phase above ≈310 K the manganese moments are ferromagnetically aligned. The magnetic moment at the 4(a) site is 1.57(12)  μ_B and it is almost zero (0.15(9) μ_B) at the 4(b) site. The low temperature orthorhombic phase which is only fully established below 50 K has the space group Pmma with a cell related to the cubic one by a Bain transformation a_orth = (a_cub + b_cub)/2; b_orth = c_cub and c_orth = (a_cub − b_cub). The change in cell volume is ≈2.5%. The spontaneous magnetization of samples cooled in fields less than 0.5 T decreases at temperatures below T_M and at 2 K the magnetic moment per formula unit in fields up to 5.5 T is 2.01(5)  μ_B. Neutron diffraction patterns obtained below ≈132 K gave evidence for a weak incommensurate magnetic modulation with propagation vector (2/3, 1/3, 0).

The optical and magneto-optical properties of ferromagnetic La_{1 − x}Ba_xMnO₃ single crystals with x = 0.15, 0.20 and 0.25 are studied. The components of the permittivity tensor are obtained by spectral ellipsometry techniques and transverse Kerr effect measurements. The Kerr effect spectra depend substantially on the Ba content. The plasma frequency is estimated. In the paramagnetic semiconductor state, the small polarons contribute to conductivity in La_0.85Ba_0.15MnO₃, in La_0.75Ba_0.25MnO₃ no evidence for polarons is found even in the semiconductor state. For La_0.85Ba_0.15MnO₃ (T_C = 214 K), the metallic phase is estimated to occupy less than 1% of the total volume at T = 190 K. It is shown that in La_0.75Ba_0.25MnO₃, the energy gap vanishes and the metal–semiconductor transition occurs somewhat below T_C rather than at T = T_C.

Nanocrystallites of nominal composition (La_0.5Sr_0.5)TiO₃ 12–15 nm in diameter exhibit a diamagnetic susceptibility that is greater by a factor of three than that of bulk ceramic material, due to a much-reduced Pauli paramagnetic contribution associated with oxidation (cation deficiency) of the material. Doping the nanocrystallites with 1.5 or 2.0% of every transition metal from V–Ni adds a Curie term to the susceptibility. Exchange coupling between these paramagnetic ions is very weak. In the cases of Fe, Co and Ni there is an additional hysteretic ferromagnetic magnetization, with a moment equivalent to a few tenths of a Bohr magneton per dopant atom, which is attributed to a secondary ferromagnetic impurity phase. Mössbauer analysis of samples prepared with ⁵⁷Fe reveals the presence of some metallic iron. Metallic nickel is detected by x-ray diffraction, but no direct evidence of metallic cobalt was found in the ferromagnetic Co-doped material. The possibility of high temperature defect-related ferromagnetism in a metallic oxide is discussed.

We present low field thermoremanent magnetization (TRM) measurements in granular CrO₂ and composites of ferromagnetic (FM) CrO₂ and antiferromagnetic (AFM) Cr₂O₃. TRM in these samples is seen to display two distinct timescales. A quasi-static part of remanence, appearing only in the low field regime, exhibits a peculiar field dependence. TRM is seen to first rise and then fall with increasing cooling fields, eventually vanishing above a critical field. Similar features in TRM have previously been observed in some antiferromagnets that exhibit the phenomenon of piezomagnetism. Scaling analysis of the TRM data suggest that presumably piezomoments generated in the AFM component drive the FM magnetization dynamics in these granular systems in the low field regime.

The 0 K pressure-induced magnetic phase transformations of face-centered cubic (FCC) and hexagonal close packed (HCP) Co have been examined using first-principles calculations. Issues of fitting an equation of state to the first-principles energy versus volume data points containing a magnetic transformation and comparing to experimental phase equilibria are discussed. It is found that a fitting scheme employing only data where the magnetic moment decreases linearly with volume offers a physically meaningful behavior for the equation of state at metastable volumes. From this fitting, the ferromagnetic to nonmagnetic transformations with increasing pressure at 0 K are at 77 GPa and 123 GPa for FCC and HCP, respectively, and are first order and second order, respectively, on the basis of an unambiguous measure proposed in the paper. In addition to the HCP/FCC structure transformation at 99 GPa, another transformation at negative pressures is predicted, at − 31 GPa. These results are shown to be consistent with the extrapolations of the experimental pressure–temperature phase diagram to 0 K.