Divergent phonon angular momentum driven by temperature and strain

TL;DR

This study investigates how phonon angular momentum in materials like AlN, GaN, and g-BN varies with temperature and strain, highlighting the importance of phonon lifetime and strain effects on PAM behavior.

Contribution

It introduces a comprehensive first-principles analysis of phonon angular momentum considering phonon lifetime, revealing divergent PAM phenomena under specific conditions.

Findings

Divergent PAM occurs at low temperatures in AlN and GaN.

Strain induces divergent PAM in g-BN beyond a critical compressive threshold.

Phonon lifetime divergence is key to PAM behavior at low temperatures.

Abstract

The phonon angular momentum (PAM) may exhibit exotic temperature dependence as it is sensitive to the phonon lifetime. Constant phonon-lifetime approximation fails to depict such behavior. Here, we study the PAM of AlN, GaN, and graphene-like boron nitride (g-BN) monolayer with full consideration of phonon lifetime using first-principles calculations. We show that wurtzite AlN and GaN acquire divergent PAM at low temperatures from their lowest-lying phonon branches. The g-BN monolayer, on the other hand, does not have finite PAM at equilibrium structure. Rather it shows intriguing strain-dependence in PAM; the compressive strain greater than the critical size generates divergent PAM at low temperatures due to the divergent lifetime of TA phonons. As PAM couples with rotational excitations in solids associated with charge, spin, or electromagnetic fields, our study demonstrates a possibility of mechanical and thermal engineering of such excitations.