Chengwang Niu

Functionalized bismuth films: Giant gap quantum spin Hall and valley-polarized quantum anomalous Hall states

The search for new large band gap quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators is critical for their realistic applications at room temperature. Here we predict, based on first-principles calculations, that the band gap of QSH and QAH states can be as large as 1.01 and 0.35 eV in an H-decorated Bi(111) film. The origin of this giant band gap lies in both the large spin-orbit interaction of Bi and the H-mediated exceptional electronic and structural properties. Moreover, we find that the QAH state also possesses the properties of a quantum valley Hall state, thus intrinsically realizing the so-called valley-polarized QAH effect. We further investigate the possibility of large gap QSH and QAH states in an H-decorated Bi(¯ 110) film and X-decorated

Topological crystalline insulator and quantum anomalous Hall states in IV-VI-based monolayers and their quantum wells

Different from the two-dimensional (2D) topological insulator, the 2D topological crystalline insulator (TCI) phase disappears when the mirror symmetry is broken, e.g., upon placing on a substrate. Here, based on a new family of 2D TCIs - SnTe and PbTe monolayers - we theoretically predict the realization of the quantum anomalous Hall effect with Chern number C = 2 even when the mirror symmetry is broken. Remarkably, we also demonstrate that the considered materials retain their large-gap topological properties in quantum well structures obtained by sandwiching the monolayers between NaCl layers. Our results demonstrate that the TCIs can serve as a seed for observing robust topologically non-trivial phases.

Two-Dimensional Topological Crystalline Insulator and Topological Phase Transition in TlSe and TlS Monolayers

The properties that distinguish topological crystalline insulator (TCI) and topological insulator (TI) rely on crystalline symmetry and time-reversal symmetry, respectively, which encodes different bulk and surface/edge properties. Here, we predict theoretically that electron-doped TlM (M = S and Se) (110) monolayers realize a family of two-dimensional (2D) TCIs characterized by mirror Chern number CM = -2. Remarkably, under uniaxial strain (≈ 1%), a topological phase transition between 2D TCI and 2D TI is revealed with the calculated spin Chern number CS = -1 for the 2D TI. Using spin-resolved edge states analysis, we show different edge-state behaviors, especially at the time reversal invariant points. Finally, a TlBiSe2/NaCl quantum well is proposed to realize an undoped 2D TCI with inverted gap as large as 0.37 eV, indicating the high possibility for room-temperature observation.

Mixed Weyl semimetals and low-dissipation magnetization control in insulators by spin–orbit torques

Reliable and energy-efficient magnetization switching by electrically induced spin–orbit torques is of crucial technological relevance for spintronic devices implementing memory and logic functionality. Here we predict that the strength of spin–orbit torques and the Dzyaloshinskii-Moriya interaction in topologically nontrivial magnetic insulators can exceed by far that of conventional metals. In analogy to the quantum anomalous Hall effect, we explain this extraordinary response in the absence of longitudinal currents as hallmark of monopoles in the electronic structure of systems that are interpreted most naturally within the framework of mixed Weyl semimetals. We thereby launch the effect of spin–orbit torque into the field of topology and reveal its crucial role in mediating the topological phase transitions arising from the complex interplay between magnetization direction and momentum-space topology. The presented concepts may be exploited to understand and utilize magnetoelectric coupling phenomena in insulating ferromagnets and antiferromagnets.Electric-field control of magnetization switching is highly promising for low-dissipation spintronics. Here, the authors propose an electrically induced topological phase transition mediated by spin orbit torques as attractive way to control magnetization in absence of longitudinal charge currents.

Two-dimensional topological nodal line semimetal in layered X 2 Y ( X = Ca , Sr, and Ba; Y = As , Sb, and Bi)

In topological semimetals the Dirac points can form zero-dimensional and one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and topological nodal line semimetals, respectively. Here, based on first-principles calculations, we predict a topological nodal line semimetal phase in the two-dimensional compounds

Bi1Te1 is a dual topological insulator

{X}_{2}Y

Two-dimensional topological crystalline insulator phase in quantum wells of trivial insulators

(

Mixed topological semimetals and orbital magnetism in two-dimensional spin-orbit ferromagnets

X

Hybrid quantum anomalous Hall effect at graphene-oxide interfaces

= Ca, Sr, and Ba;

Topology for Magnetization Control in Complex Magnets

Y