Effective electron coupling to phonon mechanical angular momentum in helical systems

TL;DR

This paper demonstrates that phonon mechanical angular momentum (MAM) can directly couple to electronic degrees of freedom in chiral crystals, expanding understanding of electron-phonon interactions beyond crystal angular momentum (CAM).

Contribution

It introduces a second-order perturbative Hamiltonian showing phonon MAM can influence electronic motion, revealing a new pathway for electron-phonon coupling in helical systems.

Findings

Phonon MAM can be converted to electronic degrees of freedom.

Electronic motion is affected by phonon MAM.

Phonon degrees of freedom influence electronic orbital and spin polarizations.

Abstract

In chiral crystals, two types of phonon angular momenta have been introduced. One is crystal angular momentum (CAM) arising from the rotational or screw-rotational symmetry and the other is mechanical angular momentum (MAM) associated with the circular motion of atomic displacements about equilibrium positions. Recently, the electron--phonon coupling that respects the screw-rotational symmetry is derived, whereby the CAM between electrons and phonons is interconverted. Here, we show that, in addition to CAM, MAM can also be converted to the electronic degrees of freedom by deriving a second-order perturbative Hamiltonian proportional to phonon MAM. This finding highlights that the electronic motion is directly affected by phonon MAM, and consequently, that phonon degrees of freedom can play a crucial role in phenomena related to electronic orbital and spin polarizations.