Acoustic Excitation of Nuclear Magnetic Energy Levels in the Rotating Frame

Abstract

When crystals are distorted periodically by a vibrating force, transitions take place among nuclear Zeeman levels of Δm=±2 in the rotating frame. This excitation is observed as the reduction in relaxation times of spin-locked magnetization. Unlike ultrasonic excitation in the laboratory frame, the acoustic frequency is low (2_γH₁⁄2π), and this fact, together with a high sensitivity to a strain, enables one to determine the electric field gradient-elastic tensors s accurately. These effects have been demonstrated for Na in NaCl single crystals. Measurements for several orientations of the force and the external magnetic field have yielded S₁₁=(2.8±0.3)×10¹⁵ statcoul/cm³ and S₄₄=−(0.81±0.08)×10¹⁵ statcoul/cm³. These values are explained by a consideration of overlap effects in addition to ionic point-charges.