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

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.