Strong Coupling between Antiferromagnetic and Superconducting Order Parameters in CeRhIn$_5$ Studied by In-NQR Spectroscopy

We report on a novel pressure ($P$)-induced evolution of magnetism and superconductivity (SC) in a helical magnet CeRhIn$_5$ with an incommensurate wave vector $Q_i=({1/2},{1/2},0.297)$ through the $^{115}$In nuclear quadrupole resonance (NQR) measurements under $P$. Systematic measurements of the $^{115}$In-NQR spectrum reveal that the commensurate antiferromagnetism (AFM) with $Q_c=({1/2},{1/2},{1/2})$ is realized above $P_m \sim$ 1.7 GPa. An important finding is that the size of SC gap and $T_c$ increase as the magnitude of the AFM moment decreases in the $P$ region, where SC uniformly coexists with the commensurate AFM. This result provides evidence of strong coupling between the commensurate AFM order parameter (OP) and SC OP.