The Exotic Barium Bismuthates

TL;DR

This paper reviews the unique properties of lead- and potassium-doped barium bismuthates, focusing on their superconductivity and charge-density-wave behavior, and explores theoretical models suggesting exotic physics and charge transport mechanisms.

Contribution

It introduces a comprehensive theoretical analysis of the negative-U extended-Hubbard model for barium bismuthates, highlighting potential for exotic physics and novel charge transport phenomena.

Findings

Charge $ ext{±}2e$ Cooperon bound states dominate transport properties.

Electronic mechanisms imply unique phases with distinct optical and transport gaps.

Proposes experimental tests for the negative-U charge transport mechanism.

Abstract

We review the remarkable properties, including superconductivity, charge-density-wave ordering, and metal-insulator transitions, of lead- and potassium-doped barium bismuthate. We discuss some of the early theoretical studies of these systems. Our recent theoretical work, on the negative-$U\/$, extended-Hubbard model for these systems, is also described. Both the large- and intermediate-$U\/$ regimes of this model are examined, using mean-field and random-phase approximations, particularly with a view to fitting various experimental properties of these bismuthates. On the basis of our studies, we point out possibilities for exotic physics in these systems. We also emphasize the different consequences of electronic and phonon-mediated mechanisms for the negative $U.\/$ We show that, for an electronic mechanism, the \secin \,\,phases of these bismuthates must be unique, with their transport properties {\it dominated by charge $\pm 2e$ Cooperon bound states}. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport and comment on the effects of disorder.