Hebel-Slichter peak and superconducting energy gap in Rb<sub>3</sub>C<sub>60</sub> observed by muon spin relaxation

Macfarlane, WA, Kiefl RF, Chow KH, Dunsiger S, Duty TL, Johnston TMS, Schneider JW, Sonier J, Brard L, Strongin RM, Fischer JE, Smith AB.  1994.  Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation. Hyperfine Interactions. 86:467-472.


dynamics, state


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