Phonon Angular Momentum Induced by Temperature Gradient

TL;DR

Applying a temperature gradient in crystals with specific symmetries induces a non-equilibrium phonon angular momentum, leading to magnetization, crystal rotation, and electron spin effects, revealing novel phonon-related phenomena.

Contribution

This work demonstrates that temperature gradients can generate phonon angular momentum in crystals with certain symmetries, a phenomenon analogous to the Edelstein effect.

Findings

Finite phonon angular momentum is generated by heat current in non-equilibrium conditions.

This phonon angular momentum induces magnetization in the crystal.

The effect can cause the crystal to rotate if free to move.

Abstract

Phonon modes in crystals can have angular momenta in general. It nevertheless cancels in equilibrium when the time-reversal symmetry is preserved. In this paper we show that when a temperature gradient is applied and heat current flows in the crystal, the phonon distribution becomes off-equilibrium, and a finite angular momentum is generated by the heat current. This mechanism is analogous to the Edelstein effect in electronic systems. This effect requires crystals with sufficiently low crystallographic symmetries, such as polar or chiral crystal structures. Because of the positive charges of the nuclei, this phonon angular momentum induces magnetization. In addition, when the crystal can freely rotate, this generated phonon angular momentum is converted to a rigidbody rotation of the crystal, due to the conservation of the total angular momentum. Furthermore, in metallic crystals, the phonon angular momentum will be partially converted into spin angular momentum of electrons.