Suppression of superconductivity in YNi$_2$B$_2$C at the atomic disordering

The behavior of the electrical resistivity $\rho(T)$, the superconducting transition temperature $T_c$, and the upper critical field $H_{c2}(T)$ of a polycrystalline sample of YNi$_2$B$_2$C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval $T_{\text{ann}}=(100$–$1000)\,^\circ$C has been studied. It has been found that the irradiation of YNi$_2$B$_2$C with a fluence of $10^{19}\,$cm$^{-2}$ leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial $\rho(T)$, $T_c$, and $H_{c2}(T)$ values are almost completely recovered upon annealing at up to $T_{\text{ann}}=1000\,^\circ$C. The quadratic dependence $\rho(T)=\rho_0 + a_2T^2$ is observed for the sample in the superconducting state ($T_c=(5.5$–$14.5)$ K). The coefficient $a_2$ (proportional to the square of the electron mass $m^*$) hardly changes. The form of the dependence of $T_c$ on $\rho_0$ can be interpreted as the suppression of the two superconducting gaps, $\Delta_1$ and $\Delta_2$ $(\Delta_1\sim2\Delta_2)$. The degradation rate of $\Delta_1$ is about three times higher than that of $\Delta_2$. The dependences $dH_{c2}/dT$ on $\rho_0$ and $T_c$ may be described by the relations for a superconductor in the intermediate limit (the coherence length $\xi_0$ is on the order of the electron mean free path $l_{tr}$) under the assumption of a nearly constant electron density of states on the Fermi level $N(E_{\text{F}}$. The observed behavior of $T_c$ obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.