Spatial inhomogeneity and pseudogap in HTSC Original Research Article

It is shown that spatial inhomogeneity, pseudogap and 60 K-phase in underdoped YBa2Cu3O6+δ apparently have the same nature specified by the existence of finite clusters uniting of various numbers of negative-U centers (NUCs). These NUCs are formed in HTSC under doping on some pairs of Cu ions in CuO2 plane at definite arrangement of the nearest localized doped charges. Electron fluctuations between NUCs and oxygen band in clusters uniting small number of NUCs result in fluctuating transitions of such clusters between superconducting and normal states owing to fluctuations of NUC occupation.