F7 - MULTIPLE PHONON NUCLEAR SPIN EXCITATION IN GaAs

Abstract

Spin transitions may be induced by interaction of the nuclear quadrupole moment (NQM) with time varying electric field gradients (EFG). These EFG occur when phonons distort the lattice. There are three processes by which phonons of appropriate frequency may excite nuclear spin transitions. (1) Direct interaction of a phonon at the Larmor frequency v0 or at 2v0 (e.g. a phonon at 2v0 is absorbed exciting an upward Δm = +2 spin transition). (2) The indirect interaction with the NQM (Raman process) with two phonons vA and vB such that vA ± vB = v0 or 2v0 (e.g. vA is absorbed with an upward spin transition at 2v0, then phonon vB = vA — 2v0 is emitted). (3) An anharmonic three phonon interaction where vA ± vB → vC + (other phonons). If vC = v0 or 2v0 then through (1) spin transitions can subsequently occur. We have experimentally observed that r.f. voltage applied to 10 mc and 25 mc transducers bonded to opposite faces of a GaAs single crystal reduces the As75 nuclear magnetization (2v0 = 15 mc). The experimental conditions of this observation are predicted within 50 per cent using the Raman process theory. The contribution of the anharmonic three-phonon process to this observation will be discussed in addition to other experimental conditions under which this anharmonic process is observed.