Magnetic Control of Soft Chiral Phonons in PbTe

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 25 T, 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.

Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet

“…Overall, since the development of the RAMBO system, we have been able to spectroscopically probe a number of new physical phenomena in high magnetic fields, including superfluorescence in InGaAs quantum wells, 18) Faraday and Kerr rotations in a semimetal and topological insulator compound, 37) ultrastrong magnon-magnon coupling in a rareearth orthoferrite, 80) and large magnetic moments of phonons in PbTe. 84) The easy optical access of RAMBO allows one to optically probe various collective excitations in different energy scales in condensed matter systems by using different light sources. Recently, similar table-top high field magnets have been developed by other groups.…”

“…Recently, a strong magnetic response of a transverse optical (TO) phonon mode in a thin film of lead telluride (PbTe) has been observed using THz time-domain magnetospectroscopy. 84) PbTe, one of the most widely used thermoelectric materials, is known to have a soft lattice, hosting anharmonic phonons. The displacements of the TO phonon, schematically shown in Fig.…”

“…The authors concluded that the magnetic phonon moment comes from TO phonon anharmonicity and morphic effects due to high magnetic fields. 84)…”

Lattice dynamics with molecular Berry curvature: chiral optical phonons

“…The narrow distribution of the the molecular Berry curvature in momentum space indicates that, in real space, one atom can be influenced by the velocity of another atom far away. With the molecular Berry curvature, the double degeneracy of the optical phonon at the Brillouin zone center is lifted intrinsically, in contrast to the splitting induced extrinsically by the magnetic field [14][15][16][17][18][19][20][21] . The polarization vectors become left and right-handed, separately, which carry nonzero angular momenta contributing to a nonzero zero-point angular momentum of the lattice vibration 22 .…”