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) ${\Delta }^{\ast }(T)$ 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.

10 pages, 6 figures

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