Strong-Coupling Superconductivity of CeIrSi${}_3$ with the Non-centrosymmetric Crystal Structure

We studied the pressure-induced superconductor CeIrSi${}_3$ with the non-centrosymmetric tetragonal structure under high pressure. The electrical resistivity and ac heat capacity were measured in the same run for the same sample. The critical pressure was determined to be $P_{\mathrm{c}}$ = 2.25 GPa, where the antiferromagnetic state disappears. The heat capacity $C_{\mathrm{a}\mathrm{c}}$ shows both antiferromagnetic and superconducting transitions at pressures close to $P_{\mathrm{c}}$. On the other hand, the superconducting region is extended to high pressures of up to about 3.5 GPa, with the maximum transition temperature $T_{\mathrm{s}\mathrm{c}}$ = 1.6 K around $2.5-2.7$ GPa. At 2.58 GPa, a large heat capacity anomaly was observed at $T_{\mathrm{s}\mathrm{c}}$ = 1.59 K. The jump of the heat capacity in the form of $\mathrm{\Delta }{C}_{\mathrm{a}\mathrm{c}}\mathrm{/}{C}_{\mathrm{a}\mathrm{c}}(T_{\mathrm{s}\mathrm{c}})$ is 5.7 $\pm$ 0.1. This is the largest observed value among previously reported superconductors, indicating the strong-coupling superconductivity. The electronic specific heat coefficient at $T_{\mathrm{s}\mathrm{c}}$ is, however, approximately unchanged as a function of pressure, even at $P_{\mathrm{c}}$.