Manifestations of 2 1/2-order transitions and Kohn singularities in the pressure dependence of the superconducting transition temperature

A theory is suggested for the pressure behavior of Tc in superconductors which accounts for changes due to pressure not only in the energy spectrum of conduction electrons but also in the phonon spectrum and in the electron-phonon interaction function. It is shown that in a superconductor subjected to pressure ∂Tc/∂P may have two types of singularities: one type due to the 2 1/2-order transition and another due to Kohn singularity. The dependence of ∂Tc/∂P on the impurity concentration is a reliable indicator which can be used to distinguish between these two types of singularities experimentally.

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