Mechanism of carrier generation and the origin of the pseudogap and 60 K phases in YBCO

The mechanism of hole carrier generation is considered in the framework of a model assuming the formation of negative U centers (NUCs) in HTSC materials under doping. The calculated dependences of carrier concentration on the doping level and temperature are in quantitative agreement with experiment. An explanation is proposed for the pseudogap and 60 K phases in YBa₂Cu₃O_6+δ. It is assumed that a pseudogap is of superconducting origin and arises at temperature T* > T _c∞ > T _c in small nonpercolating clusters as a result of strong fluctuations in the occupancy of NUCs (T _c∞ and T _c are the superconducting transition temperatures of an infinitely large and finite NUC clusters, respectively). The T*(δ) and T _c(δ) dependences calculated for YBa₂Cu₃O_6+δ correlate with experimental dependences. In accordance with the model, the region between T*(δ) and T _c(δ) is the range of fluctuations in which finite nonpercolation clusters fluctuate between the superconducting and normal states due to NUC occupancy fluctuations.