180 °-twisted bilayer ReSe2 as an artificial noncentrosymmetric semiconductor

We have fabricated a 180 °-twisted bilayer <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>ReSe</a:mi><a:mn>2</a:mn></a:msub></a:math> by stacking two centrosymmetric monolayer <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>ReSe</b:mi><b:mn>2</b:mn></b:msub></b:math> flakes in opposite directions, which is expected to cause the loss of spatial inversion symmetry. We successfully observed spatial inversion-symmetry breaking, in contrast to the monolayer and natural bilayer <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>ReSe</c:mi><c:mn>2</c:mn></c:msub></c:math> by the second harmonic generation. ARPES measurements further revealed emergent band dispersions in the 180 °-twisted bilayer <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>ReSe</d:mi><d:mn>2</d:mn></d:msub></d:math>, distinct from those of the monolayer <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>ReSe</e:mi><e:mn>2</e:mn></e:msub></e:math> used in its fabrication. The band calculation shows the finite lifting of spin degeneracy (∼50 meV) distinct from natural monolayer and bilayer <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>ReSe</f:mi><f:mn>2</f:mn></f:msub></f:math>, which demonstrates that the spin-momentum locked state leading to Berry curvature related phenomena can be realized even with the stacking of centrosymmetric monolayers.