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The C 13 nuclear magnetic resonance has been observed in the unenriched carbides of thorium and uranium ThC, TI1C2, UC, U2C3, and UC2. In contrast to nearly all nonmagnetic metals and semimetallic compounds, the Knight shift in the semimetallic thorium carbides is negative. On the other hand, the Knight shifts in all of the uranium carbides are positive, which is consistent with the P 31 shift in UP and PrP. The C 13 Knight shift in all of the carbides except U2C3 was temperature-independent. On the basis of magnetic susceptibility evidence, it is found that the uranium carbides are similar to other semimetallic uranium compounds in that their magnetic properties are best explained at higher temperatures with a localized /-electron model with large crystal-field splitting. The C 13 Knight shift in the uranium carbides is interpreted in terms of an effective hyperfme field resulting from the presence of two 5/ electrons in UC and UC2 and three in U 2 C 3 . Only the lowest crystal-field level of the 3 # 4 configuration (the nonmagnetic level Ti in the case of UC) is appreciably populated in UC and UC2. A comparison of hyperfme fields in a variety of lanthanide and actinide compounds suggests that the fields in semimetallic compounds and insulators have similar origins and that the differences are associated largely with structure, covalency, and overlap, and not with the presence or absence of conduction electrons.

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The excitation spectrum of a free exciton in GaSe has been measured in magnetic fields up to nearly 200 T. The measured spectrum compares favorably with a theoretical calculation done within the effective-mass approximatio'n, consistent with a recent interpretation that the mass anisotropy is smail, p~~/p "=' 1.