Influence of strong electron irradiation on fluctuation conductivity and pseudogap in YBa$_2$Cu$_3$O$_7$--$\delta$ single crystals

Abstract

The effect of high-energy electron irradiation on the temperature dependences of the resistivity $\rho(T)$, fluctuation conductivity (FLC), and pseudogap (PG) $\Delta^{*}(T)$ of YBa$_2$Cu$_3$O$_7$--$\delta$ (YBCO) single crystals without twins was studied. Irradiation causes a linear increase in $\rho(T)$ and a decrease in the superconducting transition temperature $T_c$ with dose $\phi$. For small $\phi$, the reduction of $T_c$ follows the Abrikosov--Gorkov (AG) pair-breaking theory, while for large $\phi$ it is described by the Emery--Kivelson (EK) model, where quantum phase fluctuations dominate. At $\phi_3 = 2.5 \times 10^{19}$ e/cm$^2$, which corresponds to the AG--EK crossover, the spacing between CuO$_2$ planes $d_{01}$, the coherence length $\xi_c(0)$, and the fluctuation region $T_{\mathrm{fl}}$ increase sharply, and the two-dimensional Maki--Thompson (2D--MT) contribution is replaced by the Aslamazov--Larkin (2D--AL) term. Surprisingly, no signatures of the crossover appear in $\rho(\phi)$ or $T_c(\phi)$. At the same $\phi_3$, a sharp rise in the pseudogap opening temperature $T^{*}$ and in $\Delta^{*}$ indicates a possible reduction of the density of states. With further increase in $\phi$, both PG parameters and their energy scale decrease markedly, and $\Delta^{*}(T)$ acquires an unusual form. However, at $\phi_5 = 5.6 \times 10^{19}$ e/cm$^2$, the temperature dependences of FLC and PG again show behavior typical of well-structured YBCO, regardless of defect density.