Evidence for s-wave superconductivity in the β-pyrochlore oxide RbOs₂O₆

We report the results of ⁸⁷Rb NMR measurements on RbOs₂O₆, a member of the family of the superconducting pyrochlore-type oxides with a critical temperature T_c=6.4 K. In the normal state, the nuclear spin-lattice relaxation time T₁ obeys the Korringa-type relation T₁T=const and the Knight shift is independent of temperature, indicating the absence of strong magnetic correlations. In the superconducting state, T₁⁻¹(T) 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 RbOs₂O₆ is a conventional s-wave-type superconductor.