Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors

Antiferromagnetic superconductors in a magnetic field are studied. We examine a mechanism which significantly reduces the Pauli paramagnetic pair-breaking effect. The mechanism is realized even in the presence of the orbital pair-breaking effect. We illustrate it using a three-dimensional model with an intercalated magnetic subsystem. The upper critical field is calculated for various parameters. It is shown that the upper critical field can reach several times the pure Pauli paramagnetic limit. The possible relevance to the large upper critical field observed in the heavy fermion antiferromagnetic superconductor CePt_3Si discovered recently is briefly discussed. We try to understand the large upper critical field in the compound CePt_3Si and field-induced superconductivity in the compound CePb_3 within a unified framework.