Field-Induced Antiferromagnetic State in Non-centrosymmetric Superconductor CeIrSi3

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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_{\text{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 T_{\text{N}} under pressures up to about 2.4 GPa, while it shows an upturn anomaly at T_{\text{N}} under pressures larger than 2.4 GPa. The ac specific heat also indicates a clear jump at T_{\text{N}} in magnetic field. When the antiferromagnetic phase exists in magnetic field, the superconducting transition becomes broad, possessing a transition width \Delta T_{\text{sc}}=0.2{\mbox{--}}0.5 K. On the other hand, it becomes extremely sharp, with \Delta T_{\text{sc}}=0.03 K even at 17 T under P=2.55 GPa, for example, when the antiferromagnetic phase disappears. The upper critical field H_{\text{c2}} for H\parallel [001] is also extremely enhanced around 2.5--2.7 GPa, and the H_{\text{c2}}(0) value reaches about 45 T at P_{\text{c}}^{\ast}\simeq 2.63 GPa, even though the superconducting transition temperature T_{\text{sc}}=1.6 K is very small. The electronic instability is realized in magnetic field at P_{\text{c}}^{\ast}. The present characteristic heavy fermion superconductivity in the noncentrosymmetric structure is compared with the similar pressure-induced superconductivity of CeRhIn5.
2011-09-15

Field-Induced Antiferromagnetic State in Non-centrosymmetric Superconductor CeIrSi3

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