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 $\text{ln}3$ 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.

Figure 1

(Color online) Thermodynamic properties of an Anderson impurity in a BCS superconductor, assuming a fixed value of $\Delta$. The system is particle-hole symmetric: $ϵ=-U/2$.

Figure 2

(Color online) Closeup of the low-temperature region in the transition regime.

Figure 3

(Color online) Thermodynamic properties of an Anderson impurity in a BCS superconductor, taking into account the temperature dependence of the gap.