FLUCTUATION CONDUCTIVITY IN YBa₂Cu₃O_7-δ SINGLE CRYSTALS UNDER ELECTRON IRRADIATION

The phenomenon of superconductivity in high-temperature superconductors (HTSC's or cuprates) can be understood by studying HTSC's properties in the normal state, which are well known to be very peculiar. The most intriguing property is a pseudogap (PG) observed mostly in oxygen deficient cuprates below any representative temperature $T^* \gg T_c$ [1]. At present, it is believed that the proper understanding of PG physics has to offer the possibility to decipher the basic pairing mechanism in the HTSC's which still remains uncertain.

The effect of 2.5 MeV electron irradiation on the temperature dependences of the resistivity $\rho(T)$ of an optimally doped YBa₂Cu₃O_7-δ single crystal has been studied. Here we show that with an increase in dose $\phi$, the value of $\rho(300K)$ increases linearly, while T_c decreases linearly. The value of $\rho(100K)$ increases nonlinearly, demonstrating a feature for $\phi = 4.3 \cdot 10^{18}$ e/cm², which is also observed on the number of other dose dependent parameters. Regardless of the irradiation dose, in the temperature range from T_c up to T₀₁, the excess conductivity $\sigma'(T)$ obeys the classical fluctuation theories of Aslamazov - Larkin (3D-AL) and Maki-Thompson (2D-MT), demonstrating 3D-2D crossover with increasing temperature. The crossover temperature T₀ makes it possible to determine the coherence length along the c-axis, $\xi_c(0)$, which increases by $\sim 2.6$ times under irradiation. Furthermore, the range of superconducting fluctuations above T_c also noticeably increases. At $\phi = 0$, a "classical" pseudogap dependence $\Delta^*(T)$ is observed with a maximum at T_pair $\sim 120$ K and a clear minimum at T = T₀₁ [2]. It was determined for the first time that at $\phi = 4.3 \cdot 10^{18}$ e/cm² the shape of $\Delta^*(T)$ changes strongly and becomes the same as in optimally doped YBa₂Cu₃O_7-δ single crystals with defects, which is accompanied by a sharp decrease in T_pair and the pseudogap opening temperature T*, while at T_c($\phi$) there are no singularities. With an increase in the irradiation dose up to $\phi = 8.8 \cdot 10^{18}$ e/cm², the shape of $\Delta^*(T)$ is restored and becomes the same as in well-structured YBa₂Cu₃O_7-δ films. Moreover, in this case, T_pair and T* increase noticeably.