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

TL;DR

This study reveals gapless magnetic and quasiparticle excitations in CeRhIn$_5$ due to the microscopic coexistence of antiferromagnetism and unconventional superconductivity under pressure, challenging traditional gap-opening expectations.

Contribution

It provides experimental evidence of gapless excitations in a heavy-fermion superconductor coexisting with antiferromagnetism, highlighting the need for theoretical models of this coexistence.

Findings

Microscopic coexistence of AFM and SC in CeRhIn$_5$

Absence of gap opening below $T_c^{onset}$

Unconventional gapless excitations observed

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 $T^{MF}_c$ = 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 $T_1T$ = 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.