Abnormal Phonon Angular Momentum due to Off-diagonal Elements in Density Matrix induced by Temperature Gradient

TL;DR

This paper investigates how temperature gradients induce nonzero phonon angular momentum in chiral materials, highlighting the dominant role of off-diagonal density matrix elements and their impact on atomic rotation and spin Seebeck effects.

Contribution

It reveals the significant influence of off-diagonal elements in the phonon density matrix on angular momentum under temperature gradients, advancing understanding of chiral phonon phenomena.

Findings

Off-diagonal terms dominate phonon angular momentum in strong scattering regimes.

Quantum phonon mode transitions are strongly affected by temperature gradients.

The results explain the chiral phonon activated spin Seebeck effect.

Abstract

Nonzero mean value of phonon angular momentum (PAM) in chiral materials can be generated when a temperature gradient is applied. We find that both diagonal and off-diagonal terms of PAM contribute to mean PAM by using the Kubo formula where both diagonal and off-diagonal elements of the heat current operator are considered. The calculation results show that the off-diagonal term is dominant when the phonon scattering is strong enough. This finding reveals that the quantum transition between different phonon modes induced by temperature gradient strongly affects the local atomic rotation. Our discovery provides an explanation of the recently observed chiral phonon activated spin Seebeck effect.