Abstract

Recent experiments on the near-infrared absorption, thermal conductivity and the critical field Hc2 in several high-Tc oxides are interpreted as a manifestation of the Bose–Einstein condensation of small bipolarons.

Basic Model for High-TcOxides

To describe low-energy spin and charge excitations of metal oxides and doped fullerenes with bipolarons, Alexandrov and Mott [1, 2, 3, 4] have suggested that bipolarons are intersite in two possible spin states (S = 0 or 1), and a proportion of bipolarons are in Anderson localised states.

Our assumption is that all electrons are bound in small singlet or triplet bipolarons and they are responsible for the spin excitations. Hole pairs, which appear with doping, are responsible for the low-energy charge excitations of the CuO2 plane. Above Tc, a material such as YBCO contains a non-degenerate gas of these hole bipolarons in a singlet or in a triplet state, with a slightly lower mass due to the lower binding energy.

The low-energy band-structure includes two bosonic bands (singlets and triplets), separated by the singlet–triplet exchange energy J, estimated to be of the order of a few hundred meV. The half-bandwidth w is of the same order. The bipolaron binding energy is assumed to be large (Δ >> T), and therefore single polarons are irrelevant in the temperature region under consideration.

We argue that many features of spin and charge excitations in metal oxides can be described within our simple model.