Nuclear acoustic resonance in liquid gallium

Abstract

T o date two relevant interaction mechanisms a r e known which can couple acoustic energy to a nuclear spin system. One is the magnetic dipole interaction with the sound induced rf magnetic field which, in the presence of a dc magnetic field go, may be considerably large in conductors, the other one is the familar quadrupole interaction with the sound induced dynamic electric field gradient (DEFG). However, nuclear acoustic resonance (NAR) in liquid metals, first observed in liquid Ga /1/, only can take place via the magnetic dipole interaction since in liquids with law viscosity it i s impossible /2/ to generate a DEFG whose frequency distribution i s smaller than the width of the Zeeman levels of the nuclei. This is due to the fact that the DEFG depends on the distance R.. This i s a marked difference compared to the sound induced electromagnetic field in conductors which above all depends on the ultrasonic displacement field and not on the distance g..(t) between the particles. As the total displacement field is a superposition of the thermal and the ultrasonic displacement field, in consequence of f i x w e l l ' s equations the resulting electromagnetic field may be written as a s u m of a coherent part (oscillating coherently with the sound wave) and an incoherent part causing motional narrowing. Thus, provided that the diffusion length during the nuclear spin relaxation time i s small compared to the ultrasonic wavelength, it should be possible t o observe NAR in liquid metals. On the other hand, the sound induced rf magnetic field is an internal field and therefore NAR via the coupling to the nuclear magnetic dipole moment can be regarded a s NMR in the interior of a conductor.