The common origin of the pseudogap- and 60 K-phases in YBCO

Here we assume that the pseudogap observed in various experiments is nothing but the superconducting (SC) gap, formed at a temperature T > Tc as a result of large fluctuations of the negative-U center (NUC) occupation due to electron transitions between oxygen sub-band and NUCs. The mechanism leading to suppression of the gap in our case is filling of NUCs with electrons. Fluctuation-induced variations of NUC occupation will amplify (reduce) the pairing interaction and can result in fluctuation-induced turning on (off) superconductivity at T* > T > Tc∞ (Tc < T < Tc∞). Here, Tc∞ is equilibrium value of Tc for infinite cluster of NUCs. With doping decreasing, the average size of NUC clusters decreases and relative fluctuations in NUC occupation in these clusters increase (i.e., T* increases and Tc decreases). In the frameworks of this model, we have obtained the dependences Tc(δ) and T*(δ) for YBa2Cu3O6+δ as two solutions of one quadratic equation. It is found that both calculated dependences are in a good agreement with experimental results