Gapless magnetic and quasiparticle excitations due to the coexistence of antiferromagnetism and superconductivity in [formula presented]: A study of [formula presented] NQR under pressure

We report systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ([Formula presented]) on the pressure ([Formula presented])-induced heavy-fermion superconductor [Formula presented]. The temperature ([Formula presented]) dependence of [Formula presented] at [Formula presented] has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at [Formula presented] and [Formula presented]. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below [Formula presented], but [Formula presented], followed by a [Formula presented] 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 the low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below [Formula presented].