Tuning Inelastic Light Scattering via Symmetry Control in 2D Magnet CrI$_3$

The coupling between spin and charge degrees of freedom in a crystal imparts strong optical signatures on scattered electromagnetic waves. This has led to magneto-optical effects with a host of applications, from the sensitive detection of local magnetic order to optical modulation and data storage technologies. Here, we demonstrate a new magneto-optical effect, namely, the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI$_3$). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A$_{1g}$ phonon mode due to the emergence of ferromagnetic order. The linearly polarized, inelastically scattered light rotates by ~40$^o$, more than two orders of magnitude larger than the rotation from MOKE under the same experimental conditions. In CrI$_3$ bilayers, we show that the same A$_{1g}$ phonon mode becomes Davydov-split into two modes of opposite parity, exhibiting divergent selection rules that depend on inversion symmetry and the underlying magnetic order. By switching between the antiferromagnetic states and the fully spin-polarized states with applied magnetic and electric fields, we demonstrate the magnetoelectrical control over their selection rules. Our work underscores the unique opportunities provided by 2D magnets for controlling the combined time-reversal and inversion symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin-phonon coupled physics.

Thermal Hall Effect Induced by Magnon-Phonon Interactions

We propose a new mechanism for the thermal Hall effect in exchange spin-wave systems, which is induced by the magnon-phonon interaction. Using symmetry arguments, we first show that this effect is quite general, and exists whenever the mirror symmetry in the direction of the magnetization is broken. We then demonstrate our result in a collinear ferromagnet on a square lattice, with perpendicular easy-axis anisotropy and Dzyaloshinskii-Moriya interaction from mirror symmetry breaking. We show that the thermal Hall conductivity is controlled by the resonant contribution from the anti-crossing points between the magnon and phonon branches, and estimate its size to be comparable to that of the magnon mediated thermal Hall effect.

Optical Selection Rule of Excitons in Gapped Chiral Fermion Systems

We show that the exciton optical selection rule in gapped chiral fermion systems is governed by their winding number $w$, a topological quantity of the Bloch bands. Specifically, in a $C_N$-invariant chiral fermion system, the angular momentum of bright exciton states is given by $w pm 1 + nN$ with $n$ being an integer. We demonstrate our theory by proposing two chiral fermion systems capable of hosting dark $s$-like excitons: gapped surface states of a topological crystalline insulator with $C_4$ rotational symmetry and biased $3R$-stacked MoS$_2$ bilayers. In the latter case, we show that gating can be used to tune the $s$-like excitons from bright to dark by changing the winding number. Our theory thus provides a pathway to electrical control of optical transitions in two-dimensional material.