Anomaly in the heat capacity of Kondo superconductors

Using numerical renormalization group we study thermodynamic properties of a magnetic impurity described by the Anderson impurity model in a superconducting host material described by the BCS Hamiltonian. When the Kondo temperature in the normal state, T_K, is comparable to the critical temperature of the superconducting transition, T_c, the magnetic doublet state may become degenerate with the Kondo singlet state, leading to a ln3 peak in the temperature dependence of the impurity contribution to the entropy. This entropy increase translates into an anomalous feature in the heat capacity which might have already been experimentally observed.