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Journal of Physics Condensed Matter

Electron correlation on metallic surfaces

H Hasegawa

Published Jan 27, 1992• 5 citations
Publisher

Abstract

Electron correlation on metallic surfaces is studied for the first time with the use of the slave-boson functional integral method. The ground-state properties of the semi-infinite simple-cubic model in the non-magnetic state are investigated. The double occupancy, delta n , and the band narrowing factor, q n , on layer n are calculated as a function of the electron interaction. It is shown that delta 1 and q 1 on the surface show a peculiar behaviour depending on the ratio U 1 /U b where U 1 (U b ) denotes the interaction on the surface (in the bulk).

DOI
10.1088/0953-8984/4/4/015

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Electron correlation on metallic surfaces is studied for the first time with the use of the slave-boson functional integral method. The ground-state properties of the semi-infinite simple-cubic model in the non-magnetic state are investigated. The double occupancy, delta n , and the band narrowing factor, q n , on layer n are calculated as a function of the electron interaction. It is shown that delta 1 and q 1 on the surface show a peculiar behaviour depending on the ratio U 1 /U b where U 1 (U b ) denotes the interaction on the surface (in the bulk).
The effect of electron correlations in the infinite-dimension (d=∞) Hubbard model at finite temperatures has been investigated with the use of a theory which was recently proposed by the present author generalizing the slave-boson functional-integral method. Sublattice magnetizations, double occupancies and band-narrowing factors are computed as functions of the interaction, U, and the temperature, T. Calculated results are exact in the limits of U→0 and U→∞, and in the limits of T=0 and T→∞ they are expected to be very accurate owing to the nature of our mean-field-type theory for infinite-dimension systems. Similarities and differences between the calculated results for d=3 and d=∞ are also discussed.
The dynamical mean-field theory (DMFT) is employed to study the correlation-driven metal-insulator transition in the semi-infinite Hubbard model at half-filling and zero temperature. We consider the low-index surfaces of the three-dimensional simple-cubic lattice, and systematically vary the model parameters at the very surface, the intralayer and interlayer surface hopping, and the surface Coulomb interaction. Within the DMFT the self-energy functional is assumed to be local. Therewith, the problem is self-consistently mapped onto a set of coupled effective impurity models corresponding to the inequivalent layers parallel to the surface. Assuming that the influence of the high-energy Hubbard bands on the low-energy quasiparticle resonance can be neglected at the critical point, a simplified ``linearized DMFT'' becomes possible. The linearized theory, however, is formally equivalent to the Weiss molecular-field theory for the semi-infinite Ising model. This implies that qualitatively the rich phenomenology of the Landau description of second-order phase transitions at surfaces has a direct analog for the surface Mott transition. Motivated by this formal analogy, we work out the predictions of the linearized DMFT in detail. It is found that under certain circumstances the surface of a Mott insulator can be metallic, while a Mott-insulating surface of a normal metal is not possible. We derive the corresponding phase diagrams, the (mean-field) critical exponents and the critical profiles of the quasiparticle weight. The results are confirmed by a fully numerical evaluation of the DMFT equations using the exact-diagonalization (ED) method. By means of the ED approach, we especially investigate the noncritical parts of the phase diagrams and discuss the U and layer dependence of the quasi-particle weight. For strong modifications of the surface model parameters, the surface low-energy electronic structure dynamically decouples from the bulk.
The spectral weight functions and the optical conductivity of a highly correlated electronic system described by a simple Hubbard model are studied. The calculations are carried out for a small system, a cubic cluster of eight sites, by means of exact diagonalization using symmetrized basis functions. The spectral weight functions of a single hole (or electron) placed in an otherwise half-filled band are obtained. The density of states at different interaction strengths, ranging from weak to strong couplings, is studied. We observe the spreading of the spectral weight associated with some single-particle eigenstates over a large range of energies, and the appearance of satellite structure at higher excitation energies. The nature of band narrowing is also discussed. The frequency dependent (optical) conductivity is studied for the cases of a half-filled band, and for one hole and two hole dopings away from half filling. The interaction strength is varied between the weak- and strong-coupling limits. The formation of a Hubbard gap is observed. Results are related to aspects of the metal-insulator transition in bulk systems.
Slave boson mean-field studies of the ground state of the Hubbard model with correlated hopping were performed. The approach qualitatively recovers the exact results for the case when the hopping integral ttt equals the correlated hopping integral X.X.X. The phase diagram for the strongly correlated state with only singly occupied sites, the weakly correlated state, where single and double occupation is allowed, and for the superconducting state, was determined for any value of XXX and any electron concentration n.n.n. At the half-filled band (n=1)(n=1)(n=1) a direct transition from the superconductor to the Mott insulator was found. In the region of strong correlations the superconducting solution is stable for nnn close to 1, in contrast to the case of weak correlations, in which superconductivity occurs at $n\ensuremath{\approx}0$ and $n\ensuremath{\approx}2.$ We have also found that strong correlations change the characteristics of the superconducting phase, e.g., the gap in the excitation spectrum exhibits a nonexponential dependence on the on-site Coulomb integral UUU close to the phase-transition point.
The variational scheme of Gutzwiller has been reexamined so as to apply it to the case of degenerate narrow bands. The ground-state wave function for electrons is investigated for an arbitrary density of electrons and an arbitrary strength of interaction. The typical features of a metal, particularly the Fermi surface, are preserved in this approach as opposed to some other methods. The Coulomb repulsion among electrons scatters the electrons from below to above the Fermi surface of uncorrelated bands. Under the assumption that only the intra-atomic Coulomb interaction is important, the probability of electrons being scattered is larger for the paramagnetic state than for the ferromagnetic state. Therefore the ferromagnetic ground state is favored if the density of states is large at the band edges. The degenerate-band-model Hamiltonian is spin dependent, hence the results obtained from it do not have to obey the theorem of Lieb and Mattis.