Field-induced antiferromagnetic state in non-centrosymmetric superconductor CeIrSi₃

We have carried out the electrical resistivity and ac specific heat measurements under pressure P and magnetic field H in a pressure-induced superconductor CeIrSi3 with the noncentrosymmetric tetragonal crystal structure. Superconductivity is observed in a wide pressure region from 1.9 GPa to about 3.5 GPa. The antiferromagnetic state disappears at P_c = 2.25 GPa under H = 0, but in magnetic field we have found an antiferromagnetic state. The corresponding electrical resistivity in constant magnetic field decreases rather steeply below the Néel temperature Tn under pressures up to about 2.4 GPa, while it shows an upturn anomaly at T_N under pressures larger than 2.4 GPa. The ac specific heat also indicates a clear jump at T _N in magnetic field. When the antiferromagnetic phase exists in magnetic field, the superconducting transition becomes broad, possessing a transition width ΔT_sc = 0.2-0.5 K. On the other hand, it becomes extremely sharp, with ΔT_sc = 0.03 K even at 17 T under P = 2.55 GPa, for example, when the antiferromagnetic phase disappears. The upper critical field Hc2 for H ∥ [001] is also extremely enhanced around 2.5-2.7 GPa, and the H_c2(0) value reaches about 45 T at P ^*_c ≳ 2.63 GPa, even though the superconducting transition temperature T_sc = 1.6 K is very small. The electronic instability is realized in magnetic field at P_c^*. The present characteristic heavy fermion superconductivity in the noncentrosymmetric structure is compared with the similar pressure-induced superconductivity of CeRhIn5.