Evidence for local moment formation around a positive muon in graphite

The local-electronic structure of the positive muon has been investigated in the semimetal graphite using muon spin rotation/relaxation. Both the muon Knight shift and spin-relaxation rate in highly oriented pyrolytic graphite are anomalously large compared to simple metals and both have an unusual temperature dependence. These results indicate that a local moment forms around the muon due to the low carrier density. In contrast, measurements on metallic ${\mathrm{LiC}}_6$ reveal a smaller muon Knight shift, which is opposite in sign (negative) and almost temperature independent. We suggest this is due to the core polarization of a ${\mathrm{Mu}}^{-}{\mathrm{Li}}^{+}$ complex.