Evolution of the pseudogap and excess conductivity of YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals in the course of long-term aging

Abstract

The temperature dependences of both fluctuation conductivity (FLC) $\sigma^\prime(T)$ and pseudogap (PG) $\Delta^*(T)$ derived from measurements of resistivity $\rho(T)$ of an optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ single crystal subjected to long-term storage have been studied. The as-grown sample S1 exhibits characteristics typical of optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals containing twins and twin boundaries. Analysis of both FLC and PG showed an unexpected improvement in all characteristics of the sample after 6 years of storage (sample S2), indicating that the effect of twin boundaries is somehow limited. After 17 years of storage, all characteristics of the sample changed dramatically, which indicates a strong influence of internal defects formed during the aging process. For the first time, the temperature dependences of both FLC and PG were obtained after 17 years of storage.