It was noted that the tunnelling probability of a positive muon in high-purity material was significantly enhanced by the onset of superconductivity. It was concluded that this confirmed the predominant effect of low-energy infra-red coupling to conduction electrons during the quantum tunnelling motion of a charged interstitial atom in a metal.

R.Kadono, R.F.Kiefl, J.A.Chakhalian, S.R.Dunsiger, B.Hitti, W.A.MacFarlane, J.Major, L.Schimmele, M.Matsumoto, Y.Ohashi: Physical Review Letters, 1997, 79[1], 107-10