Superconducting Properties of CePt3Si and CeIrSi3 without Inversion Symmetry in the Crystal Structure

The upper critical field Hc(2) in superconductivity was investigated for CePt3Si and CeIrSi3 without inversion symmetry in the tetragonal structure. A significant anisotropy of Hc(2) between H vertical bar vertical bar [0 0 1] and H vertical bar vertical bar [1 0 0] was not observed in CePt3Si even at 0.6 GPa as well as ambient pressure, where the Neel temperature with T-N = 2.3 K at ambient pressure becomes zero at 0.6 GPa. The similar result was observed at 1.95 GPa in CeIrSi3, where antiferromagnetism and superconductivity coexist, while a large anisotropy of Hc(2) in CeIrSi3 was observed with increasing pressure: Hc(2)(0) = 300-400 kOe for H vertical bar vertical bar [0 0 1] and Hc(2)(0) = 95 kOe for H vertical bar vertical bar [1 1 0] at 2.65 GPa, which is higher than the critical pressure P-c = 2.25 GPa. A large value of Hc(2) for H vertical bar vertical bar [0 0 1] in CeIrSi3 might become an experimental evidence of the spin-triplet superconductivity in the non-centrosymmetric structure. It is also remarkable that CeIrSi3 is a strong coupling superconductor and the electronic specific heat coefficient is approximately unchanged as a function of pressure even at P-c, which is highly different from the divergent mass enhancement at Pc in a pressure-induced heavy fermion superconductor CeRhIn5.