7 - Polarons and bipolarons in WO_3−x and YBa₂Cu₃O₇

Summary

Abstract

The physical properties of polarons and bipolarons in WO_{3− x} are reviewed and compared with characteristics of carriers in YBa₂Cu₃O₇ and several other high-temperature superconductors, namely (Ca_1−x Y_x)Sr₂(Tl_0.5Pb_0.5)Cu₂O₇, Bi₂Sr₂(Ca_0.9Y_0.1)Cu₂O_8+δ and La₂CuO_4+δ. The fingerprint for (bi)polarons is optical excitations in the spectral near-infrared region. The absorption cross section is drastically reduced in the superconducting phase. The temperature evolution is analysed quantitatively in terms of Bose–Einstein condensation of bipolarons.

Introduction

The physics of polarons and bipolarons has recently been reconsidered because it is believed that condensation of bipolarons is closely related to, or may even be the origin of, superconductivity in oxide materials [1–12]. The justification for such belief is based on several experimental observations such as the absence of the Korringa law in the nuclear spin relaxation rate [3], the heat capacity anomaly [4, 5] and the softening of phonons above the (pseudo-) gap in the superconducting phase [7, 8, 13–15]. Few experimental results point directly to the existence of polarons and/or bipolarons in these materials, however. Probably the most direct indication for the existence of such particles stems from observation of their internal excitations in the infrared and visible spectral range. Such excitations were firmly established in WO_3−x (in its ɛ-phase) and related transition metal oxides [16–30]. Similar excitations were recently observed in YBa₂Cu₃O₇ and other high-temperature superconductors [31–37]. Although their original discovery by Dewing and Salje [32] was contested on experimental grounds, it is now confirmed that the apparently contradictory result that such excitations were not seen in reflection spectra of YBa₂Cu₃O₇ crystals lies in the insensitivity of early reflection measurements and statistical errors introduced by subsequent Kramers–Kronig analysis [38, 39].