Session F42: Thermal Hall Effect in Quantum Materials

The phonon thermal Hall angle in black phosphorus

The origin of phonon thermal Hall Effect observed in a variety of insulators is yet to be identified. Black phosphorus (BP) is a non-magnetic elemental insulator, displaying a thermal Hall conductivity larger than what has been previously observed in any insulator. Thelongitudinal  and the transverse thermal conductivities peak at the same temperature and at this peak temperature, the thermal Hall angle at ~10 T is of the order of a few parts in thousand.  Both these features are shared by other insulators displaying THE, despite an

absolute amplitude spreading over three orders of magnitude. Thus there is no correlation between  the thermal Hall angle and the phonon mean-free-path. In this particular system,theoretical phon spectrum has a striking feature, which may play a role.  A longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes¹. 

1) Xioakang Li et al., Nature Communications, 14:1027 (2023)   

Phonon Thermal Hall Effect in Magnetic Insulators

Phonons are known to generate a thermal Hall effect in various insulators, including multiferroics [1], cuprate Mott insulators [2], and non-magnetic paraelectrics [3], but the underlying mechanism is still unclear. Theoretical proposals for the phonon thermal Hall effect fall into two categories: intrinsic scenarios based on the coupling of phonons to their environment and extrinsic scenarios based on the skew scattering of phonons by disorders like impurities or defects. 

To shed new light on this question, we studied the thermal Hall effect in a simple cubic material, the antiferromagnetic insulator Cu₃TeO₆, which hosts none of the previously encountered special features. Our observation of a large phonon thermal Hall conductivity in Cu₃TeO₆ indicates that the phonon thermal Hall effect is likely to be a fairly common property of solids [4]. To answer the question of what makes phonons chiral in a magnetic field, we studied the thermal Hall effect in Rh-doped Sr₂IrO₄ and observed a 30-fold enhancement of the thermal Hall angle with just 2% of Rh doping [5]. This work provides systematic evidence to support that the scattering of phonons by impurities embedded within a magnetic environment could possibly be the underlying mechanism in the case of antiferromagnetic insulators. 

 

[1] T. Ideue et al., Nat. Mater. 16, 797-802 (2017).

[2] G. Grissonnanche et al., Nature 571, 376 (2019). 

[3] X. Li et al., Phys. Rev. Lett. 124, 105901 (2020).

[4] L. Chen et al., Proc. Natl. Acad. Sci. U.S.A. 119 (34), e2208016119 (2022).

[5] A. Ataei et al., Nat. Phys. (in press), arXiv:2302.03796.   

Phonon thermal Hall effect in a metallic spin ice

It has been a subject of intense debate whether phonons are responsible for the Hall effect. We have investigated longitudinal kappa_xx and transverse kappa_xy thermal conductivity in a metallic analog of spin ice Pr2Ir2O7. Despite the presence of mobile charge carriers, we find that phonon contribution by far dominates electron contribution both in kappa_xx and kappa_xy. A T/H scaling of kappa_xx unambiguously reveals that resonant scattering of phonons on paramagnetic spins substantially impedes longitudinal heat current and provides glass like longitudinal thermal conductivity without a phonon peak. However, a development of spin ice correlation upon cooling modifies the resonant scattering, leading to the deviation from the scaling. A manifest correlation between kappa_xx and kappa_xy including a success of the T/H scaling and its failure at low temperature points to a definitive role of the resonant phonon scattering by spins for generating Hall signals. Given the ubiquitous nature of phonon scattering on spins, our results will open a new avenue for exploring the phonon thermal Hall effect in magnets in which spins couple to phonons.   

Phonon thermal Hall effect from scattering off two-level systems

With the growing number of experiments observing the phonon thermal Hall effect, it becomes crucial to investigate the underlying theoretical aspects and the correlations probed by this phenomenon. In my talk, I will delve into the exploration of an extrinsic scattering mechanism of phonons called the side-jump mechanism. By focusing on this mechanism, I aim to address the question of how the phonon thermal Hall effect relates to two-point correlations in materials. I will demonstrate that even a simple two-level system, when coupled to phonons, can induce a thermal Hall effect, thereby explaining recent experiments in quantum spin ice Pr2Ir2O7 and cuprates.   

On thermal transport puzzles of strongly correlated quantum materials

Thermal transport is an increasingly important experimental approach to probe extraordinary physics in strongly correlated quantum materials.  In recent years, many unexpected magnetothermal transport signatures have been observed including large thermal Hall effects in a few important insulators (e.g., La₂CuO₄), creating puzzles in the community. In this talk I will show that the key to these puzzles lies on the phonons and how they respond to magnetic field efficiently. I will also discuss possible mechanisms underlying these experimental observations and picture future directions towards probing exotic phases of matter through analyzing thermal transport in a more revealing manner.