Localized-to-itinerant transition preceding antiferromagnetic quantum critical point and gapless superconductivity in CeRh 0.5 Ir 0.5 In 5

fundamental problem posed from the study of correlated electron compounds, of which heavy-fermion systems are prototypes, is the need to understand the physics of states near a quantum critical point (QCP). At a QCP, magnetic order is suppressed continuously to zero temperature and unconventional superconductivity often appears. Here, we report pressure ( P )-dependent 115 In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh 0.5 Ir 0.5 In 5 . These experiments reveal an antiferromagnetic (AF) QCP at P_c^AF=1.2 GPa where a dome of superconductivity reaches a maximum transition temperature T c . Preceding P_c^AF , however, the NQR frequency ν Q undergoes an abrupt increase at P_c^* = 0.8 GPa in the zero-temperature limit, indicating a change from localized to itinerant character of cerium’s f -electron and associated small-to-large change in the Fermi surface. At P_c^AF where T c is optimized, there is an unusually large fraction of gapless excitations well below T c that implicates spin-singlet, odd-frequency pairing symmetry.