Abstract

If lattice polarons exist in high-temperature superconducting oxides then there must be evidence of local lattice distortion associated with polarons. While the distortions are dynamic and subtle, making direct observation difficult, there are numerous indications that some anomalous local deviations from the crystallographic lattice structure exist in superconducting oxides. Based largely upon the results of pulsed neutron scattering measurements, we present an argument in favor of the presence of local lattice distortions consistent with lattice polarons. A few implications of the observation in relation to other physical properties are discussed.

Introduction

Even though polarons have been known for a long time, direct experimental observation of lattice distortions associated with them is surprisingly scarce, largely because the density of polarons is usually low and consequently the lattice distortion is small on average, making observation very difficult. While some observations of lattice distortion associated with polarons have been made for low-dimensional organic conductors in which the periodic lattice distortion (Peierls distortion) can be regarded as an array of localized polarons [1], there are very few such reports for oxides [2]. Moreover, most known cases of polarons are heavy, small polarons, while in high-temperature superconducting (HTSC) oxides the presence of mobile large polarons is suspected. For those reasons, local lattice distortion has been observed so far mostly by nontraditional methods of structural study, while the crystallographic community has largely been skeptical. In this paper we discuss why observation is difficult, whether there is sufficient experimental evidence to support the presence of polarons in high-temperature superconducting oxides or not, and the implications of these observations.