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

The effect of high-energy electron irradiation on the temperature dependences of the resistivity $\rho (T)$, fluctuation conductivity (FLC), and pseudogap (PG) ${\mathrm{\Delta }}^{\ast }(T)$ 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 $T_{\mathrm{f}\mathrm{l}}$ 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 $T^{\ast }$ and in ${\mathrm{\Delta }}^{\ast }$ indicates a possible reduction of the density of states. With further increase in $\phi$, both PG parameters and their energy scale decrease markedly, and ${\mathrm{\Delta }}^{\ast }(T)$ 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.