Anomalous Second Harmonic Generation from Atomically Thin MnBi2Te4

TL;DR

This study investigates second harmonic generation in MnBi2Te4, revealing surface effects dominate SHG signals regardless of magnetic or layer states, emphasizing surface contributions in van der Waals magnetic materials.

Contribution

It demonstrates that surface SHG, not magnetic order or layer number, primarily causes SHG in MnBi2Te4, highlighting the importance of surface effects in centrosymmetric layered magnets.

Findings

Surface SHG dominates over magnetic or layer effects.

SHG symmetry matches MnBi2Te4-vacuum interface.

Surface contributions are crucial in interpreting SHG in layered magnets.

Abstract

MnBi2Te4 is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBi2Te4 flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi2Te4 is unexpectedly unrelated to both magnetic state and layer number. We suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi2Te4-vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry-breaking in van der Waals centrosymmetric magnets.