Anisotropic Superconducting Gap in Transuranium Superconductor PuRhGa₅: Ga NQR Study on a Single Crystal

69,71Ga NMR/NQR studies have been performed on a single crystal of the transuranium superconductor PuRhGa₅ with Tc ~ 9 K. We have observed a ⁶⁹Ga NQR line at ~29.15 MHz, and assigned it to the $4i$ Ga site using the NMR results. The ⁶⁹Ga NQR spin-lattice relaxation rate 1/T₁ shows no coherence peak just below Tc, but obeys a T³ behavior below Tc. This result strongly suggests that PuRhGa₅ is an unconventional superconductor having an anisotropic superconducting gap.