Magnetic Control of Soft Chiral Phonons in PbTe

TL;DR

This study demonstrates that applying a strong magnetic field can control the properties of soft chiral phonons in PbTe crystals, revealing a novel way to manipulate phonon behavior in ionic lattices.

Contribution

We show that magnetic fields induce splitting and symmetry changes in soft phonons of PbTe, a phenomenon not previously observed in such materials.

Findings

Phonon mode splits into two with opposite handedness under magnetic field

Observed Zeeman splitting and diamagnetic shift of phonon frequencies

Magnetic field induces symmetry changes via Lorentz force on lattice ions

Abstract

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25T, the phonon mode split into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies displayed Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are results of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. This study thus reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.