Central-cell And Screening Effects On The Binding Energies Of Neutral Chalcogen Impurities In Silicon

The binding energies of D0 states in S-, Se-, and Te-doped silicon crystals are calculated within a variational scheme in the effective-mass approximation and with a Chandrasekhar-type variational function for the two-electron envelopes. Central cells are modeled with a constant core potential within the impurity sphere. Screening effects for the potential and the electron-electron interaction are taken into account by means of a position-dependent dielectric function. Results are compared with recent theoretical and experimental work. It is found that central-cell effects and position-dependent screening are essential to account for the experimental data. © 1986 The American Physical Society.33128765876

New Approach To The Theory Of Intermediate Valence. I. General Formulation

We present a new approach to the theory of intermediate valence, applicable to a specific kind of such solids. It consist of: (i) diagonalization of all intra-atomic terms in the Hamiltonian, including hybridization; (ii) elimination by means of a projection of all states beyond a low-energy manifold; (iii) conversion of the remaining states into equivalent fermion states; (iv) expression of the intra- and interatomic states in terms of the new fermion operators; and (v) treatment of this new Hamiltonian in a mean-field approximation. This approach, although valid in restricted cases, avoids all the problems of the alternative treatments. © 1979 The American Physical Society.20114554455

Falicov-Kimball model and the problem of electronic ferroelectricity

The density matrix renormalization group method is used to examine possibilities of electronic ferroelectricity in the spinless Falicov-Kimball model. The model is studied for a wide range of parameters including weak and strong interactions as well as the symmetric and unsymmetric case. In all examined cases the $$-expectation value vanishes for vanishing hybridization $V$, indicating that the spinless Falicov-Kimball model does not allow for a ferroelectric ground state with a spontaneous polarization.Comment: 9 pages, 4 figures, LaTe

Response Of Hybridized (mixed Valence) Systems To External Probes

The problem of the response of a hybridized system to an external probe is discussed from a completely quantum-mechanical point of view. An exactly soluble model, which describes qualitatively the mixed-valence states of rare-earth compound solids, is presented. We show that it is possible to obtain regimes of "single response"-e.g., isomer shifts-as well as regimes of "two responses"-e.g., photoelectron emission spectra. The calculation does not involve real time fluctuations. We point out that it would be worthwhile to investigate experimentally intermediate regimes where the characteristic energy of the probe is comparable to the energy separation of the hybridized states. © The American Physical Society.1393948395

Short-range-order Effects On The Esr Spectra Of A Spin Cluster

Using the Kikuchi counting for the Ising problem, we present a theory which takes into account short-range-order effects in the formation of spin clusters. The Weiss molecular-field treatment is thus improved, since partial spin alignment is allowed in the paramagnetic phase. The above approach is used to study the influence of short-range order on the electron-spin-resonance (ESR) spectra of a spin system. Two main effects are then predicted: (i) One is related to a shift of resonances towards higher frequencies when the temperature is lowered. This effect is induced by effective local fields generated by short-range order. (ii) The other deals with multiple splitting of lines as a manifestation of transitions between states corresponding to different cluster configurations. These results are in good qualitative agreement with experiments. © 1981 The American Physical Society.2394391439

Spin Waves In Dilute Ferromagnets: Cluster-bethe-lattice Approach

A calculation of the local density of spin-wave states in a dilute ferromagnet is presented. It is based on a Green's-function formulation solved within the cluster-Bethe-lattice approximation. The method is compared with an exact solution for the one-dimensional dilute ferromagnet. We also study clusters of nine atoms in the bcc structure, and clusters of 7, 19, and 27 atoms in the simple-cubic structure. Our study shows: (i) a stability condition for ferromagnetism in agreement with percolation theory; (ii) a local structure dependence of the spin-wave structure; (iii) the existence of localized spin-wave states due to both isolated magnetic clusters and nonpropagating magnon modes. © 1976 The American Physical Society.1431314132

Charge-density-wave order parameter of the Falicov-Kimball model in infinite dimensions

In the large-U limit, the Falicov-Kimball model maps onto an effective Ising model, with an order parameter described by a BCS-like mean-field theory in infinite dimensions. In the small-U limit, van Dongen and Vollhardt showed that the order parameter assumes a strange non-BCS-like shape with a sharp reduction near T approx T_c/2. Here we numerically investigate the crossover between these two regimes and qualitatively determine the order parameter for a variety of different values of U. We find the overall behavior of the order parameter as a function of temperature to be quite anomalous.Comment: (5 pages, 3 figures, typeset with ReVTeX4

Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension

The ground state energy and energy gap to the first excited state are calculated for the attractive Hubbard model in one dimension using both the Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are provided as a function of coupling strength and electron density. While the ground state energies are always in very good agreement, the BCS energy gap is sometimes incorrect by an order of magnitude, particularly at half-filling. Finite size effects are also briefly discussed for cases where an exact solution in the thermodynamic limit is not possible. In general, the BCS result for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure

Electronic dynamic Hubbard model: exact diagonalization study

A model to describe electronic correlations in energy bands is considered. The model is a generalization of the conventional Hubbard model that allows for the fact that the wavefunction for two electrons occupying the same Wannier orbital is different from the product of single electron wavefunctions. We diagonalize the Hamiltonian exactly on a four-site cluster and study its properties as function of band filling. The quasiparticle weight is found to decrease and the quasiparticle effective mass to increase as the electronic band filling increases, and spectral weight in one- and two-particle spectral functions is transfered from low to high frequencies as the band filling increases. Quasiparticles at the Fermi energy are found to be more 'dressed' when the Fermi level is in the upper half of the band (hole carriers) than when it is in the lower half of the band (electron carriers). The effective interaction between carriers is found to be strongly dependent on band filling becoming less repulsive as the band filling increases, and attractive near the top of the band in certain parameter ranges. The effective interaction is most attractive when the single hole carriers are most heavily dressed, and in the parameter regime where the effective interaction is attractive, hole carriers are found to 'undress', hence become more like electrons, when they pair. It is proposed that these are generic properties of electronic energy bands in solids that reflect a fundamental electron-hole asymmetry of condensed matter. The relation of these results to the understanding of superconductivity in solids is discussed.Comment: Small changes following referee's comment

Spectral functions of the Falicov-Kimball model with electronic ferroelectricity

We calculate the angular resolved photoemission spectrum of the Falicov-Kimball model with electronic ferroelectricity where $d$- and $f$-electrons have different hoppings. In mix-valence regimes, the presence of strong scattering processes between $d$-$f$ excitons and a hole, created by emission of an electron, leads to the formation of pseudospin polarons and novel electronic structures with bandwidth scaling with that of $d$-$f$ excitons. Especially, in the two-dimensional case, we find that flat regions exist near the bottom of the quasiparticle band in a wide range of the $d$- and $f$-level energy difference.Comment: 5 pages, 5 figure