Gapless Magnetic and Quasiparticle Excitations due to the Coexistence of Antiferromagnetism and Superconductivity in CeRhIn${}_5$ : A study of ${}^{115}$In-NQR under Pressure

Abstract

We report systematic measurements of ac-susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ($T_1$) on the pressure ($P$)- induced heavy-fermion (HF) superconductor CeRhIn${}_5$. The temperature ($T$) dependence of $1/T_1$ at $P$ = 1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at $T_N=2.8$ K and $T_c^{MF}$ = 0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below $T_c^{onset}=2$ K, but $T_c^{MF}=0.9$ K, followed by a $T_{1}T$ = const law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below $T_c^{onset}=2$ K.

4pages,5figures,revised version