Exchange and Quadrupole Broadening of Nuclear Acoustic Resonance Line Shapes in the III-V Semiconductors

Abstract

Nuclear acoustic resonance (NAR) and nuclear magnetic resonance (NMR) are used to study the line-widths, second moments, and line shapes of nuclear-spin systems of InAs, InSb, GaAs, GaSb, and AlSb. These cw measurements are made at frequencies of 8-10 MHz and at 300°K in single crystals with intrinsic or near intrinsic concentrations of impurities. Different NAR and NMR linewidths for the same nuclear-spin system are explained by the different interaction Hamiltonians for spin-phonon and spin-photon couplings to the nuclear-spin system. When the magnetic field is along $〈001〉$ directions, the resonance line shapes are broadened by dipole-dipole and isotropic nuclear-exchange interactions. When the magnetic field is rotated from $〈001〉$ directions, increased broadening of the resonance line shapes is explained by small anisotropic dipole-dipole and much larger anisotropic quadrupole interactions. The measured like-spin and unlike-spin exchange constants agree with an exchange-constant dependence on the inverse fourth power of the internuclear distance. Like-spin and unlike-spin exchange constants are determined at the nuclear positions in each compound and used with a theory for indirect nuclear-spin exchange to predict the $s$-character electronic wave-function density. The strongly anisotropic quadrupole broadening is explained by electric field gradients produced by the electric fields associated with ionized substitutional impurities. From the measured field gradients, antishielding constants at the In, Sb, Ga, and As nuclear positions are determined relative to each other.