Evolution of an Unconventional Superconducting State inside the Antiferromagnetic Phase of CeNiGe$_3$ under Pressure: a $^{73}$Ge-Nuclear-Quadrupole-Resonance Study

We report a $^{73}$Ge nuclear-quadrupole-resonance (NQR) study on novel evolution of unconventional superconductivity in antiferromagnetic (AFM) CeNiGe$_3$. The measurements of the $^{73}$Ge-NQR spectrum and the nuclear spin-lattice relaxation rate ($1/T_1$) have revealed that the unconventional superconductivity evolves inside a commensurate AFM phase around the pressure ($P$) where Néel temperature $T_{\rm N}$ exhibits its maximum at 8.5 K. The superconducting transition temperature $T_{\rm SC}$ has been found to be enhanced with increasing $T_{\rm N}$, before reaching the quantum critical point at which the AFM order collapses. Above $T_{\rm SC}$, the AFM structure transits from an incommensurate spin-density-wave order to a commensurate AFM order at $T\sim 2$ K, accompanied by a longitudinal spin-density fluctuation. With regard to heavy-fermion compounds, these novel phenomena have hitherto never been reported in the $P$-$T$ phase diagram.