Anisotropic interaction of two-level systems with acoustic waves in disordered crystals

TL;DR

This paper models how two-level systems in disordered crystals interact anisotropically with acoustic waves, comparing theoretical predictions with experimental data to understand the origin of anisotropy.

Contribution

It applies a previously introduced model to cubic crystals, providing a method to distinguish between orientation-driven and lattice-driven anisotropy effects.

Findings

Calculated anisotropic interactions match experimental data.

Provided a way to identify the source of anisotropy in disordered crystals.

Extended the model to different propagation directions of phonons.

Abstract

We apply the model introduced in Phys. Rev. B 75, 064202 (2007), cond-mat/0610469, to calculate the anisotropy effect in the interaction of two level systems with phonons in disordered crystals. We particularize our calculations to cubic crystals and compare them with the available experimental data to extract the parameters of the model. With these parameters we calculate the interaction of the dynamical defects in the disordered crystal with phonons (or sound waves) propagating along other crystalographic directions, providing in this way a method to investigate if the anisotropy comes from the two-level systems being preferably oriented in a certain direction or solely from the lattice anisotropy with the two-level systems being isotropically oriented.