Significant muon spin-lattice relaxation is found in a number of non-magnet
ic semimetals and metals. Measured in longitudinal magnetic field, the rela
xation rate is independent of field up to several kilogauss and generally i
ncreases monotonically with temperature. This suggests a form of Korringa r
elaxation, originating in the hyperfine interaction between the implanted m
uons and the conduction electrons. Bearing in mind that NMR data on Korring
a relaxation refers chiefly to the host nuclei, the muon offers a probe of
conduction-electron encounter at an interstitial site, linking the topic to
the nature of defect screening in metals, to relaxation by spin-density fl
uctuations in magnetic materials and to spin- and charge-exchange on parama
gnetic muonium centres in semiconductors. Data are presented for C (graphit
e), Bi, Pb and Cd and compared with the Korringa predictions using known va
lues of the muon Knight shift. Control experiments are described on Zn and
Cu, both pure and deliberately doped with magnetic impurity. For graphite,
an interpretation is given in terms of charge-exchange on a molecular radic
al formed by the chemical reaction of interstitial muonium. (C) 2000 Elsevi
er Science B.V. All rights reserved.