Abstract

In small-polaron models the hopping amplitude for a carrier from a site to a neighboring site is reduced due to ‘dressing’ by a background degree of freedom. Electron–hole symmetry is broken if this reduction is different for a carrier in a singly occupied site and one in a doubly occupied site. Assuming that the reduction is smaller in the latter case, the implication is that a gradual ‘undressing’ of the carriers takes place as the system is doped and the carrier concentration increases. A similar ‘undressing’ will occur at fixed (low) carrier concentration as the temperature is lowered, if the carriers pair below a critical temperature and as a result the ‘local’ carrier concentration increases (and the system becomes a superconductor). In both cases the ‘undressing’ can be seen in a transfer of spectral weight in the frequency-dependent conductivity from high frequencies (corresponding to non-diagonal transitions) to low frequencies (corresponding to diagonal transitions), as the carrier concentration increases or the temperature is lowered repectively. This experimental signature of electron–hole asymmetric polaronic superconductors as well as several others have been seen in high-temperature superconducting oxides. Other experimental signatures predicted by electron–hole-asymmetric polaron models remain to be tested.

The physics of high-Tc oxides

From the beginning of the high-Tc era there have been indications that small polarons may play an important role in the physics of these materials [1–10]. Among the workers that have not completely abandoned the Fermi liquid framework for this problem most would agree that the physics of the normal state may be described in terms of heavily dressed quasi-particles [11].