Low-frequency kinetics of high-T c metal oxides

Abstract

A microscopic explanation of the low-frequency kinetic properties of metal oxides is proposed. It is based on a strong electron-phonon interaction, which forms a charged Bose liquid of small bipolarons. The large value, the nonKorringa temperature dependence above To, and the absence below Tc of the coherent peak of the nuclear spin relaxation, as well as an unexpected "'coherent peak"of the low frequency dynamic conductivity and the linear T-dependence of the resistivity are explained.The low-frequency kinetics of high-To metal oxides have revealed several puzzling phenomena (see, for example, Refs. 1 and 2 and papers cited there). A number of related phenomenological models, based on the Fermi-liquid description, have appeared (for a review see Ref.2). It has been demonstrated 2 that the cuprate data are inconsistent with the canonical Fermi-liquid behavior, but if the Fermi liquid is characterized by low-energy scales arising from soft-spin fluctuations, then the canonical behavior is not expected at the relatively "high" temperature of the normal state. However, as argued by Millis et aL 1 some neutron-scattering experiments are inconsistent with the phenomenological analysis, showing no magnetic scattering in the low-frequency range (<25meV) in YBa2Cu307 (Tc=90K) 3 or a practically temperature-independent antiferromagnetic correlation length for T~<300 K in Laa_xSrxCuOn_a.A recent neutron-scattering study of YBa2Cu306+x has revealed a quasi-gap above Tc and a temperature-independent antiferromagnetic correlation length, which led Rossat-Mignod et aL 4 to the conclusion, that "the anomalous behaviour of 1/T1 is not due to a variation with temperature of the AF-correlation length but to the growing of some superconducting correlations or pairing above To."

Nernst Effect in Poor Conductors and in the Cuprate Superconductors

“…In two dimensions, eÿph / T ÿ1 [27], so that aT in Eq. (11) is enhanced by the drag effect as aT / T ÿ6 .…”

Bose–Einstein Condensation in the Pseudogap Phase of Cuprate Superconductors

“…When strong EPI binds holes into intersite oxygen bipolarons [19], the chemical potential remains pinned inside the charge transfer gap, as clearly observed in the tunnelling experiments by Bozovic [20]. The bipolaron binding energy and the singlet-triplet bipolaron exchange energy are thought to be the origin of normal state charge and spin pseudogaps, respectively, as has been predicted by us [21] and later supported experimentally [22]. In overdoped samples carriers screen part of EPI with low frequency phonons.…”

Strong-coupling theory of high-temperature superconductivity

“…Indeed it is a common feature of the normal state of many cuprates. The bipolaron model has described the temperature dependence of 1/T 1 [23]. The conventional contact hyperfine coupling of nuclear spin on a site i with electron spins is given in the site representation by…”

“…There are also thermally excited single polarons and triplets in the model, Fig.12, which are responsible for the crossover regime at T * and normal state charge and spin (pseudo)gaps in cuprates. These pseudogaps were predicted as half of the binding energy ∆ and the singlet-triplet exchange energy J of preformed bipolarons, respectively [23]. T * is a temperature, where the polaron density compares with the bipolaron one.…”