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 115In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh0.5Ir0.5In5. These experiments reveal an antiferromagnetic (AF) QCP at $${P}_{{\rm{c}}}^{{\rm{AF}}}=1.2$$  GPa where a dome of superconductivity reaches a maximum transition temperature Tc. Preceding $${P}_{{\rm{c}}}^{{\rm{AF}}}$$ , however, the NQR frequency νQ undergoes an abrupt increase at $${P}_{{\rm{c}}}^{{\rm{* }}}$$ = 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 $${P}_{{\rm{c}}}^{{\rm{AF}}}$$ where Tc is optimized, there is an unusually large fraction of gapless excitations well below Tc 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.