We report the results of ${}^{87}\mathrm{Rb}$ NMR measurements on ${\mathrm{RbOs}}_2{\mathrm{O}}_6$, a member of the family of the superconducting pyrochlore-type oxides with a critical temperature $T_c=6.4\mathrm{K}$. In the normal state, the nuclear spin-lattice relaxation time $T_1$ obeys the Korringa-type relation $T_{1}T=\mathrm{const}$ and the Knight shift is independent of temperature, indicating the absence of strong magnetic correlations. In the superconducting state, $T_1^{-1}\left(T\right)$ exhibits a tiny coherence enhancement just below $T_c$, and decreases exponentially with further decreasing temperatures. The value of the corresponding energy gap is close to that predicted by the conventional weak-coupling BCS theory. Our results indicate that ${\mathrm{RbOs}}_2{\mathrm{O}}_6$ is a conventional $s$-wave-type superconductor.

• Received 7 September 2004

DOI:https://doi.org/10.1103/PhysRevB.71.024524