Switchable Topological Phase Transition and Novel Nonlinear Optical Properties in ReC2H Monolayer

Abstract

Extensive investigations on topological phase transition (TPT) in three-dimensional compounds have been done. Whereas, rare in two-dimensional systems, let alone noncentrosymmetric materials. In this work, based on first-principles calculations, we explore an inversion symmetry broken structural ReC2H monolayer. We reveal that it undergoes two TPTs, namely from normal insulator to Z2 topological insulator and back to normal insulator, at the critical biaxial strain of 2.3% and 7.8%, respectively. The band inversion occurs at the generic momentum in the first TPT, while at high symmetric \(\text{K}\) point in the second one. Usually, band inversion is identified by the exchange in the components or irreducible representations of the wavefunctions. These quantities can be easily obtained in theoretical calculation but hard to be detected in experimental techniques like Angle-resolved photoemission spectroscopy. It is well known that nonlinear optical (NLO) response is very sensitive to the components and symmetries of the engaged bands, which also incorporates information of band topology. Therefore, we study the shift current, one of the widely explored NLO responses in noncentrosymmetric systems, during the two TPTs. We find that in both cases band inversion leads to the sign change of shift vectors around the momenta where the bandgap closes and reopens. Whereas the shift current, as the overall contribution of shift vectors weighted by the absorption rate in the whole Brillouin zone, may keep its direction. This work offers insight that a scrutinized examination is highly demanded in utilizing shift current to detect TPT.

Tailoring Bulk Photovoltaic Effects in Magnetic Sliding Ferroelectric Materials

“…Next, we move to the symmetry constraints for NSC and MIC photoconductivities under LPL. According to eq (see the Methods section), the NSC is defined as the overall contribution of shift vectors weighted by the absorption rate in the whole BZ . As the absorption rate is always positive, the direction of σ a b b is scaled by the shift vector R m n a , which transforms as a polar vector, giving that scriptM x R n m x ( k x , k y ) = prefix− R n m x ( − k x , k y ) and is immune to scriptT .…”

“…( 5) (see Methods), the NSC is defined as the overall contribution of shift vectors weighted by the absorption rate in the whole BZ. 68 As the absorption rate is always positive, the direction of the 𝜎 𝑎𝑏𝑏 is scaled by the shift vector 𝑅 𝑚𝑛 𝑎 , 69 which transforms as a polar vector, giving that ℳ 𝑥 𝑅 𝑛𝑚 𝑥 (𝑘 𝑥 , 𝑘 𝑦 ) = −𝑅 𝑛𝑚 𝑥 (−𝑘 𝑥 , 𝑘 𝑦 ) and is immune to 𝒯. Hence, the 𝜎 𝑥𝑥𝑥 and 𝜎 𝑥𝑦𝑦 are symmetrically forbidden, leaving only 𝜎 𝑦𝑥𝑥 and 𝜎 𝑦𝑦𝑦 to be finite.…”

“…According to eq 5 (see the Methods section), the NSC is defined as the overall contribution of shift vectors weighted by the absorption rate in the whole BZ. 68 As the absorption rate is always positive, the direction of abb is scaled by the shift vector R mn a , 69 which transforms as a polar vector, giving that R k k ( , )…”