Magnon valley Hall effect in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>CrI</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:math>-based van der Waals heterostructures

Abstract

Magnonic excitations in the two-dimensional (2D) van der Waals (vdW) ferromagnet chromium triiodide (${\mathrm{CrI}}_{3}$) are studied. We find that bulk magnons exhibit a nontrivial topological band structure without the need for Dzyaloshinskii-Moriya interaction. This is shown in vdW heterostructures, consisting of single-layer ${\mathrm{CrI}}_{3}$ on different 2D materials such as ${\mathrm{MoTe}}_{2}$, ${\mathrm{HfS}}_{2}$, and ${\mathrm{WSe}}_{2}$. We find numerically that the proposed substrates substantially modify the out-of-plane magnetic anisotropy on each sublattice of the ${\mathrm{CrI}}_{3}$ subsystem. The induced staggered anisotropy, combined with a proper band inversion, leads to the opening of a topological gap of the magnon spectrum. Since the gap is opened nonsymmetrically at the ${\mathbf{K}}^{+}$ and ${\mathbf{K}}^{\ensuremath{-}}$ points of the Brillouin zone, an imbalance in the magnon population between these two valleys can be created under a driving force. This phenomenon has a close analogy to the so-called valley Hall effect and is thus termed the magnon valley Hall effect. In linear response to a temperature gradient, we quantify this effect by the evaluation of the temperature dependence of the magnon thermal Hall effect. These findings open a different avenue by adding the valley degrees of freedom besides the spin in the study of magnons.

Topics & Concepts