Semiconducting conduction in underdoped YBaCuO systems - the problem of the 'pseudo-gap'

The behaviour of the resistivity of underdoped Y1Ba2Cu3Ox samples (x=6.4-6.7) at temperatures above Tc, i.e., in the case when the samples are in a normal, non-superconducting state, is analysed. It is shown that the model of negative-U centres describes the transition correctly at T ∼ T* from the state with metallic conduction to that with semiconducting conduction, which occurs as temperature is lowered. In this model, the state with the so-called pseudo-gap is treated as a state in which the Fermi level, pinned half-way between the levels associated with negative-U centres, lies at a distance on the order of T* above the valence band top