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 bymeans 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.