arXiv subjects

Zhi-Qiang Bao

Publications and source records attributed to Zhi-Qiang Bao.

3 recordsLinked to original sources

arXiv2025

Machine learning exploration of topological polarization pattern in hexagonal boron nitride moiré superlattice

Twisted moiré supercells, which can be approximated as a combination of sliding bilayers and constitute various topologically nontrivial polarization patterns, attract extensive attention recently. However, because of the excessive size of the moiré supercell, most studies are based on effective models and lack the results of first-principles calculation. In this work, we use machine learning to determine the topological structure of the polarization pattern in twisted and strained bilayer of hexagonal boron nitride (h-BN). We further confirm that the topological pattern can be effectively modulated by the vertical electric field and lattice mismatch. Finally, local polarization also exists in the antiparallel stacked h-BN twisted and strained bilayers. Our work provides a detailed study of the polarization pattern in the moiré superlattice, which we believe can facilitate more research in moiré ferroelectricity, topological physics, and related fields.

cond-mat.mtrl-sci↗

arXiv2024

Dielectric response in twisted MoS2 bilayer facilitated by spin-orbit coupling effect

Twisted van der Waals bilayers offer ideal two-dimensional (2D) platforms for exploring the intricate interplay between the spin and charge degrees of freedom of electrons. By investigating twisted MoS2 bilayer, featuring two distinct stackings but with identical commensurate supercell sizes, we reveal an unusual dielectric response behavior inherent to this system. Our first-principles calculations demonstrate that the application of an out-of-plane electric field gives different responses in electronic polarization. Upon further analysis, it becomes apparent that this dielectric response comes from the planar charge redistribution associated with spin-orbit coupling (SOC) effect. The underlying mechanism lies in the fact that the external electric field tends to modify the internal pseudo-spin texture σ, subsequently generating an out-of-plane (pseudo-) spin current j_s \propto σ\times B_R as response to an in-plane pseudomagnetic field B_R through Rashba SOC. It is found that the generated j_s is opposite for the two distinct stackings, resulting in opposite in-plane electric susceptibility. As a consequence, through magnetoelectric coupling within such nonmagnetic system, there give rise to opposite tendency to redistribute charge, ultimately leading to an amplified or suppressed dielectric response.

cond-mat.mtrl-sci↗

arXiv2024

Quantum valley Hall states in low-buckled counterparts of graphene bilayer

With low-buckled structure for each layer in graphene bilayer system, there breaks inversion symmetry (P-symmetry) for one stacking when both A and B sublattices in top layer are aligned with those in bottom layer. In consideration of spin-orbit coupling (SOC), there opens nontrivial topological gap in each monolayer system to achieve quantum spin Hall effect (QSHE). As long as time-reversal symmetry (T-symmetry) is preserved the gapless edge states is robust in each individual layer even for the bilayer absent of PT symmetry. Based on this platform and through tight-binding (TB) model calculations we find it becomes a typical system that can exhibit quantum valley Hall effect (QVHE) when introduced a layer-resolved Rashba SOC that leads to band inversion at each K valley in the hexagonal Brillion zone (BZ). The topological transition comes from that the valley Chern number Cv = CK - CK' switches from 0 to 2, which characterizes the nontrivial QVHE phase transited from two coupled Z2 topological insulators. We also point that the layer-resolved Rashba SOC can be introduced equivalently by twisting two van der Waals touched layers. And through TB calculations, it is shown that the K bands inverts in its corresponding mini BZ when the two layers twisted by a small angle. Our findings advance potential applications for the devices design in topological valleytronics and twistronics.

cond-mat.mes-hall↗