Localized-to-itinerant transition preceding antiferromagnetic quantum critical point and gapless superconductivity in CeRh0.5Ir0.5In5

A 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. A quantum critical point describes a phase transition at zero temperature when an order is suppressed, for instance by application of pressure. Here, the authors investigate the pressure dependence of a heavy fermion antiferromagnet using nuclear quadrupole resonance and reveal two quantum critical points (QCP), among which the first one marks a Fermi surface change and triggers unusual superconducting state.