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

The temperature dependences of both fluctuation conductivity (FLC) ${\sigma }^{\mathrm{\prime }}(T)$ and pseudogap (PG) ${\mathrm{\Delta }}^{\ast }(T)$ 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.