Theory of Magnetoacoustic Resonance to Probe Multipole Effects Due to a Crystal Field Quartet

Abstract

We present a new method of acoustically driven resonance that probes octupole degrees of freedom as well as a quadrupole usually hidden by the magnetic properties of a crystal field quartet. A characteristic of the quadrupole is reflected in the anisotropic resonance transition rate, which depends on the propagation direction of a surface acoustic wave under an external magnetic field parallel to a typical crystallographic axis. The transition rate is modulated by the anisotropic Zeeman splitting associated with octupoles. We demonstrate how to obtain information about the quartet quadrupole-strain coupling and evaluate the anisotropic octupole effect quantitatively. We also discuss the applicability of our method to identifying a quadrupole order parameter using a multipole-multipole interaction model. For large excitation energy gaps under strong magnetic fields, we propose a photon-assisted magnetoacoustic resonance formulated on the basis of the Floquet theory.