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 ($1\mathrm{/}{T}_1$) have revealed that the unconventional superconductivity evolves inside a commensurate AFM phase around the pressure ($P$) where N'{e}el temperature $T_{\mathrm{N}}$ exhibits its maximum at 8.5 K. The superconducting transition temperature $T_{\mathrm{S}\mathrm{C}}$ has been found to be enhanced with increasing $T_{\mathrm{N}}$, before reaching the quantum critical point at which the AFM order collapses. Above $T_{\mathrm{S}\mathrm{C}}$, 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.