Variational cluster approach to s -wave pairing in heavy-fermion superconductors

We study s-wave Cooper pairing in heavy-fermion systems. We analyze the periodic Anderson model by means of the variational cluster approach (VCA) focusing on the interorbital Cooper pairing between a conduction electron (c electron) and an f electron, called the "c-f pairing." It is shown that the s-wave superconductivity appears coexisting with long-range antiferromagnetic order when electrons or holes are doped into the system at half filling. The antiferromagnetic order vanishes when the doping concentration exceeds a certain critical value, leading to a pure s-wave superconducting state. Moreover, the comparative study with different reference systems used in the VCA shows that the interorbital c-f pairing is essential for the appearance of the s-wave superconductivity.