HEBEL-SLICHTER PEAK AND SUPERCONDUCTING ENERGY-GAP IN RB3C60 OBSERVEDBY MUON SPIN RELAXATION

Muon spin relaxation has been observed in both the normal and supercon ducting states of Rb3C60 (T(c) = 29.3K). The field dependence of the T 1 spin relaxation rate is due to muonium undergoing spin-exchange scat tering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence of T1(-1) shows a Hebe l-Slichter coherence peak just below T(c) which is not seen in C-13 sp in relaxation. The peak can be fit assuming spin relaxation due to int eraction with the quasiparticle excitations of a BCS superconductor pr ovided the density of states is broadened relative to that of BCS. Suc h fits yield a value for the zero temperature energy gap, DELTA0/k(B), of 53(4)K, consistent with weak-coupling BCS.