Effect of $3d$ Impurities on the Superconducting Transition Temperature of the Layered Compound Nb${\mathrm{Se}}_2$

The transition temperature of Nb${\mathrm{Se}}_2$ as a function of the concentration of such paramagnetic impurities as Fe, Ni, and Co shows a very strong anomaly close to the critical concentration which destroys superconductivity. The strength of the effect in Nb${\mathrm{Se}}_2$ is believed to be caused both by the weak depairing of the impurities and by their nonstatistical distribution resulting from the layered structure.