Unusual electron–phonon superconductivity in nickel borocarbides?

Abstract

We propose a picture of incomplete superconducting condensation, which is based on the BCS pairing theory, to account for the seemingly contradictory observation of unconventional superconducting state properties including specific heat, magnetic penetration depth, spin–lattice relaxation rate, thermal conductivity, and microwave conductivity in nickel borocarbide superconductors RNi 2B 2C (R=Y, Lu), in which strong electron–phonon interaction plays an important role. We show that the power-law temperature T dependence of these properties at low T can arise in s-wave superconductors when a small fraction of charge carriers remain unpaired at T=0.