On the influence of pressure and a paramagnetic impurity on the superconducting properties of metals

A study is made of a superconductor in which the topology of the Fermi surface changes under the influence of pressure and a paramagnetic impurity; equations are obtained for the ordering parameter $\Delta$ and the critical temperature $T_c$. It is found that $T_c$ increases nonlinearly with the parameter $\varepsilon'_F=\varepsilon_F-\varepsilon_k$ at different impurity concentrations for the case when a new part of the Fermi surface appears and that it decreases nonlinearly for the case when an open Fermi surface becomes closed. An increase in the impurity concentration smoothes the nonlinearity to some extent. The impurity contribution to $T_c$ associated with a change in the topology of the Fermi surface is important. A study is also made of the change of the critical impurity concentration with increasing $\varepsilon'_F$.