180$^\circ$-twisted bilayer ReSe$_2$ as an artificial noncentrosymmetric semiconductor

TL;DR

This study demonstrates that stacking two centrosymmetric ReSe$_2$ monolayers with a 180° twist breaks spatial inversion symmetry, leading to emergent spin-split bands and potential spintronic applications.

Contribution

The paper introduces a novel method of creating a noncentrosymmetric ReSe$_2$ bilayer through twisting, revealing new spin-related phenomena not present in natural monolayers.

Findings

Inversion symmetry is broken in the twisted bilayer ReSe$_2$.

Emergent spin splitting of approximately 50 meV observed.

Spin-momentum locking state can be realized in twisted centrosymmetric layers.

Abstract

We have fabricated a 180$^\circ$-twisted bilayer ReSe$_2$ by stacking two centrosymmetric monolayer ReSe$_2$ flakes in opposite directions, which is expected to lose spatial inversion symmetry. By the second harmonic generation and angle-resolved photoemission spectroscopy, we successfully observed spatial inversion symmetry breaking and emergent band dispersions. The band calculation shows the finite lifting of spin degeneracy (~50 meV) distinct from natural monolayer and bilayer ReSe$_2$. Our results demonstrate that the spin-momentum locked state, which leads to spintronic functions and Berry-curvature-related phenomena, can be realized even with the stacking of centrosymmetric monolayers.