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arXiv:cond-mat/0509764

Fractionally Charged Polarons and Phase Separation in an Extended Falicov-Kimball Model

P. M. R. Brydon, Jun Zhu, A. R. Bishop

Published Sep 29, 2005• 0 citations
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Abstract

We present the first numerical study of the Falicov-Kimball model extended by an on-site hybridization away from the particle-hole symmetry point. Stable polaronic distortions of the charge-density wave (CDW) phase are observed when doping with a single hole. For moderate hole-doping, we find phase separation between a hole-rich homogeneous state and the usual CDW state. Excitonic order is enhanced around the hole-rich region, locally manifesting the global competition between this and the CDW phase. Associated with the hole-doping is a fractionalization of the electron density between the itinerant and localized electrons. The calculated local density of states at the polaron centre reveals this fractionalization to be the result of charge fluctuations induced by the hybridization. We propose a scanning tunneling microscope experiment to test our results.

DOI
10.48550/arxiv.cond-mat/0509764

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We examine the one-dimensional spinless Falicov-Kimball model extended by a hybridization potential between the localized and itinerant electron states. Below half-filling we find a crossover from a mixed-valence metal to an integer-valence phase-separated state with increasing on-site Coulomb repulsion. This crossover regime is characterized by local competition between the strong- and weak-coupling behaviour, manifested by the formation of an electronic polaron liquid. A phase diagram is presented and discussed in detail.
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The spinless Falicov-Kimball model is solved exactly in the limit of infinite dimensions on both the hypercubic and Bethe lattices. The competition between segregation, which is present for large U,U,U, and charge-density-wave order, which is prevalent at moderate U,U,U, is examined in detail. We find a rich phase diagram that displays both of these phases. The model also shows nonanalytic behavior in the charge-density-wave transition temperature when UUU is large enough to generate a correlation-induced gap in the single-particle density of states.
The ground-state density of states of the half-filled Falicov-Kimball model contains a charge-density-wave gap. At finite temperature, this gap is not immediately closed, but is rather filled in by subgap states. For a specific combination of parameters, this leads to a stable phase where the system is in an ordered charge-density-wave phase, but there is high density of states at the Fermi level. We show that this property can be, in finite dimensions, traced to a crossing of sharp states resulting from the single particle excitations of the localized subsystem. The analysis of the inverse participation ratio points to a strong localization in the discussed regime. However, the pronounced subgap density of states can still lead to a notable increase of charge transport through a finite size system. We show this by focusing on the transmission in heterostructures where a Falicov-Kimball system is sandwiched between two metallic leads.
We use a novel approach to analyze the one-dimensional spinless Falicov-Kimball model. We derive a simple effective model for the occupation of the localized orbitals which clearly reveals the origin of the known ordering. Our study is extended to a quantum model with hybridization between the localized and itinerant states: We find a crossover between the well-known weak- and strong-coupling behaviors. The existence of electronic polarons at intermediate coupling is confirmed. A phase diagram is presented and discussed in detail.