On the low-temperature thermal properties and low-energy Fermi excitations in CeNiSn

The thermal expansion of a single crystal of the intermetallic compound CeNiSn has been measured at low temperatures 0.3 K<T<12 K and in a magnetic field up to 8 T. A large anisotropy of the linear expansion is observed which is strongly influenced by the magnetic field. These data are interpreted within the theory explaining the origin of the quasigap in the heavy fermion spectrum of CeNiSn by the interplay between the heavy fermions and low-energy excitations in non-cubic Kondo lattices.     

A planar model for interacting fermions

One-particle Green function in the paramagnetic phase of a model of interacting fermions is obtained in the planar approximation. The model is zero-dimensional, in that thermal fluctuations are the only source of kinetic energy.     

Calculated electronic structure of the sandwichd 1 metals LaI2 and CeI2: Application of new LMTO techniques

The electronic structures of the cubic layeredd ¹ metals LaI₂ and CeI₂ were calculated using local density-functional theory and the linear muffin-tin orbital method. Special care was taken in the sphere packing used for the atomic spheres approximation. the band structure and the bonding were analysed in terms of projections of the bands onto orthogonal orbitals. The conduction-band structure could be calculated with a down-folded two-orbital basis which then served for the construction of an analytical 2×2 orthogonal, two-center tight-binding Hamiltonian. The conduction band has almost pure Ln-Ln 5d e _g character. Thex ²–y ²contribution dominates and is two-dimensional and short ranged. Strong hybridization with the 3z ²–1 orbital occurs near the saddle point, which is thereby lowered in energy and bifurcated due to thek _z -dispersion provided by the 3z ²–1 orbital. This strengthens the metal-metal bonds and prevents the nesting instability of the Fermi surface of the half filledx ²–y ²band. Within the limited accuracy of the LDA, the band structure of CeI₂ was found to be identical to that of LaI₂. The conduction-band 4f hybridizationV _{d f} ² (0) was analysed and found to be several times smaller than in fcc -Ce, in qualitative agreement with recent photoemission results [1]. Of importance for this reduction seems to be that the conduction band is formed by essentially only one orbital, ,that the number of Ce nearest-neighbors is small, and that the Ce–Ce distance is relatively large.     

The three-particle ladder approximation as a new approach towards a general theory of electron-correlation effects inCVV Auger-electron and appearance-potential spectroscopy

Due to their sensitivity to electron-correlation effects,CVV Auger-electron (AES) and appearance-potential spectroscopy (APS) can provide useful information on the electronic structure of solids. Correlations among the valence-band electrons (VV correlations) as well as correlations between the valence-band and the core electrons (CV correlations) are responsible for a variety of effects. StrongVV correlations are well known to give rise to sharp satellites in the spectra, which are related to localized two-hole (electron) final states. On the other hand, the screening of the core-hole potential in the initial state for AES, the sudden response of the valence-band electrons after the destruction of the core hole, and, for APS, the scattering of the valence-band electrons at the core hole are all consequences ofCV correlations. Up to now, however, little is known about the combined influence of both types of correlations on the spectra. We present a new theoretical approach that refers to the general case of a model system with arbitrary band-filling and arbitrary strengths ofVV as well asCV correlations. Remaining restrictions and simplifications concerning the degeneracy of the valence band, the transition matrix elements, etc. can be improved systematically. Of course, this generality can only be achieved at the expense of inevitable approximations in the theoretical formulation. The AES and APS intensities are given by properly defined three-particle Green functions, which are determined by use of a diagrammatic vertex-correction method that is based on the three-particle ladder approximation, which is the main idea of our approach. It is a direct generalization of the two-particle ladder approximation, which in the past has been applied for the calculation of two-particle Green functions that are related to the AES and APS intensities, ifCV correlations can be neglected.     

Variational pair theory of the attractive and extended Hubbard model ind-dimensions

We present a variational approach for treating the Hubbard Hamiltonian in one, two and three dimensions. It is based on 2M-fermion wavefunctions which are allowed to form correlated spin-singlet pairs. Expressions for the ground state energy and correlation functions are derived in terms of general pair coefficient functions. The presented approach offers a convenient starting point for improved variational treatments that allow to include different specific types of pair correlations. We present first applications to the attractive and to the extended Hubbard model using a very simple ansatz for the pair coefficient functions. The ground state energy, chemical potential, order parameter, momentum distribution as well as spin-spin and density-density correlation functions follow from a system of coupled nonlinear equations that has to be solved selfconsistently. All quantities are given for arbitrary band-filling in one, two and three dimensions. Our results are compared with those of other approximations and for the one-dimensional case with the exact results of Krivnov and Ovchinnikov.     

Strong correlation, electron-phonon interaction and critical fluctuations: isotope effect, pseudogap formation, and phase diagram of the cuprates

Within the Hubbard-Holstein model with long-range Coulomb forces, we revisit the charge-ordering (CO) scenario for the superconducting cuprates and account for the presence or the absence of a formed stripe phase in different classes of cuprates. We also evaluate the mean-field and the fluctuation-corrected critical lines for CO and we relate them with the various pseudogap crossover lines occurring in the cuprates and we discuss a mechanism for their peculiar isotopic dependence. Considering the dynamical nature of the CO transition, we explain the spread of T∗ and of its isotopic shift, obtained with experimental probes with different characteristic time scales.     

Electronic structure and magnetic properties of Y4Co3

The electronic structure of Y₄Co₃ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that Y₄Co₃ is very close to ferromagnetic instability. The Fermi surfaces are composed mainly of 3d electrons of Co and 4d electrons of Y.     

The tight-binding approach to the dielectric response in the multiband systems

Starting from the random phase approximation for the weakly coupled multiband tightly-bounded electron systems, we calculate the dielectric matrix in terms of intraband and interband transitions. The advantages of this representation with respect to the usual planewave decomposition are pointed out. The analysis becomes particularly transparent in the long wavelength limit, after performing the multipole expansion of bare Coulomb matrix elements. For illustration, the collective modes and the macroscopic dielectric function for a general cubic lattice are derived. It is shown that the dielectric instability in conducting narrow band systems proceeds by a common softening of one transverse and one longitudinal mode. Furthermore, the self-polarization corrections which appear in the macroscopic dielectric function for finite band systems, are identified as a combined effect of intra-atomic exchange interactions between electrons sitting in different orbitals and a finite inter-atomic tunneling.     

Single-particle self-energy and optical conductivity of the simplified Hubbard model

It is shown that the single-particle self-energy of the one and two-dimensional simplified Hubbard model exhibits different behavior characterized by Fermi-liquid, non-Fermi-liquid quasiparticle, or non-quasiparticle excitations, as a function of the strength of the on-site Coulomb repulsionU, temperature, and electron filling. For half-filled lattices, results for the optical conductivity indicate that the d.c. conductivity is zero for all temperatures andU>0.     

Quantum self-consistent approach to the charge gap of the quasi-one-dimensional organic conductors

We apply an extended quantum self-consistent method, in which quantum fluctuations are taken into account, to the bosonization Hamiltonian to investigate analytically the charge gap in the quasi-one-dimensional (1D) organic conductors at quarter-filling described by the 1D Hubbard model with dimerization and alternate potential on site. It is shown that either dimerization or alternate potential gives rise to the enhancement of the charge gap. Our results are compared with those of the numerical and the other analytical theories. Our results are also consistent with the experimental data of the actual organic materials when the effect of nearest-neighbor Coulomb interaction is taken into account.     

Electronic structure of nearly ferromagnetic compound HfZn2

The electronic structure of HfZn₂ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that HfZn₂ shows ferromagnetic instability. Large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

Effective masses of electrons, heavy holes and positrons in quasi-binary (GaSb)1−x(InAs)x crystals

The present investigation is focused on the effective masses of electrons, heavy holes and positrons in quasi-binary (GaSb)_1−x(InAs)_x crystals. To the best of our knowledge, this is the first time such quantities have been obtained for such a quasi-binary crystals. Our computations are based on the pseudopotential scheme within the virtual crystal approximation in which the effects of composition disorder are involved, while the positron wave function is evaluated under the point core approximation for the ionic potential. Comparisons are made with the measured values, which are only available for binary parent compounds and showed roughly good agreement. The calculated quantities in the quasi-binary crystals of interest are found to be generally different from those of the conventional quaternary alloys GaInAsSb which may provide more diverse opportunities to describe most carrier transport properties.     

Electronic structure and magnetism of FeGe2

The electronic band structure of FeGe₂ has been calculated using the self-consistent full potential non-orthogonal local orbital minimum basis scheme based on the density functional theory. In the band structure of FeSn₂, Fe 3d and Sn 5p states play important roles near the Fermi level. Our calculations show that large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

First-principles studies on the electronic structure of ScPd3

The electronic structure and magnetic properties of ScPd₃ have been studied based on the density functional theory within the local-density approximation. In the band structure of ScPd₃, Sc 3d and Pd 4d states play dominant roles near the Fermi level. The fixed spin moment calculation indicates that ScPd₃ has a stable paramagnetic (non-magnetic) state.     

Strong vertex corrections from weak disorder in 2D d-wave superconductors

We show that weak static random potentials have pronounced effects on the quasiparticle states of a 2Dd-wave superconductor close to a node. We prove that the vertex correction coming from the simplest crossed diagram is important even for a nonmagnetic potential. The leading frequency and momentum dependent logarithmic singularities in the self-energy are calculated exactly to second order in perturbation theory. The self-energy corrections lead to a modified low energy density of states which depends strongly on the type of random potential and which can be measured in experiments. There is an exceptional case for a potential with extremely local scatterers and opposite nodes separated by (, ) where an exact cancelation takes place eliminating the leading frequency dependent singularity in the simplest crossed diagram. A comparison of the perturbative results with a self-consistent CPA (coherent potential approximation) for the nonmagnetic disorder reveals qualitative differences in the self-energy at the smallest energies which are due to the neglectance of vertex corrections in CPA.     

An exact renormalization-group approach for local phonon properties of single-atom and double-atom generalized Fibonacci systems

We present an exact renormalization-group approach to study the local phonon properties of the double-atom generalized Fibonacci systems in which the masses and spring constants are all associated with the aperiodic sequences constructed by the inflation rule: {A, B}{A ⁿ B^m, A}.n(m+1) basic transformations and 2n(n+m–1)+1 basic decimations are introduced. By applying the combinations of derived transformations and decimations, we can determine exactly the renormalized local environment, up to infinite order, of any given site in the double-atom systems. Both the single-atom and double-atom phonon models are employed, and the local density of states in several sites of some generalized Fibonacci systems are numerically calculated.     

Dynamic spin and charge susceptibilities in thet−J model

Based on the spin-rotation-invariant formulation of the Kotliar Ruckenstein slave-boson representation, the paramagnetic spin and charge susceptibilities in thet-J model are calculated. Analyzing the static spin susceptibility, the instability of the paraphase towards incommensurate magnetic order is in agreement, with the saddle-point phase diagram recently obtained by some of the authors. The spin dynamics at arbitrary frequencies, wave vectors and band fillings is calculated, where the Fermi-surface and correlation effects are studied. The magnetic instability region is investigated with respect to the formation of a collective spin-fluctuation mode. Near the transition point, a kinetic gap and a sharp peak in the spectral weight ((1,0) paramagnon) are obtained.     

Exchange-correlation and layer-thickness effects in quasi-two dimensional electron liquids

We demonstrate a replacement of the non-uniform sub-band density of quasi-2D electron layers by an effective uniform-slab density. Exchange, correlation and Fermi-liquid properties are determined via a mapping of the electron liquid to a classical fluid, using the hyper-netted-chain equation inclusive of bridge corrections, (i.e. the CHNC), as a function of the density, spin-polarization, layer width and the temperature. Our parameters-free theory is in good accord with quantum simulations, with effective-mass and spin-susceptibility results for 2D layers found in GaAs/AlGaAs structures.     

A new approach to electron-electron interactions in strongly correlated systems at arbitrary spin-polarization and temperature

The uniform electron fluid is the reference model for density functional calculations. Even for this system, many-body perturbation theory, and related methods become questionable when the density parameter r_s exceeds unity. Hence, quantum Monte Carlo (QMC) simulation has been almost the only applicable method. We review a new approach, which uses a mapping of the quantum fluid to a classical Coulomb fluid, based on density-functional concepts. It is applicable at finite temperatures and arbitrary spin polarizations as well, and correctly recovers even the logarithmic terms in the exchange and correlations energies close to T=0. We show by detailed comparison with available QMC data that the method yields accurate pair-distribution functions, spin-dependent energies, static local-field factors, Landau parameter-based quantities like m∗ and g∗, for strongly coupled electron fluids.