2-PHONON NUCLEAR-RELAXATION IN THE GROUP-V SEMIMETAL

Previous work has shown that, for T > 250 K, nuclear spin-lattice rela xation in the semimetals arsenic and antimony involves a phonon mechan ism in addition to the Korringa mechanism. Approximate calculations, b ased on the Kochelaev expression for a cubic structure, suggest that o ptic modes may play a dominant role in phonon relaxation in the semime tals. Theoretical phonon relaxation expressions have been derived for the rhombohedral A(7) structure for both optic and acoustic modes. For both As-75 and Sb-121, the predictions are found to be an order of ma gnitude smaller than the measured values at high temperatures. Possibl e reasons for this discrepancy are briefly discussed.