Tuning Multiple Landau Quantization in Transition-Metal Dichalcogenide with Strain.

Landau quantization associated with the quantized cyclotron motion of electrons under magnetic field provides the effective way to investigate topologically protected quantum states with entangled degrees of freedom and multiple quantum numbers. Here we report the cascade of Landau quantization in a strained type-II Dirac semimetal NiTe2 with spectroscopic-imaging scanning tunneling microscopy. The uniform-height surfaces exhibit single-sequence Landau levels (LLs) at a magnetic field originating from the quantization of topological surface state (TSS) across the Fermi level. Strikingly, we reveal the multiple sequence of LLs in the strained surface regions where the rotation symmetry is broken. First-principles calculations demonstrate that the multiple LLs attest to the remarkable lifting of the valley degeneracy of TSS by the in-plane uniaxial or shear strains. Our findings pave a pathway to tune multiple degrees of freedom and quantum numbers of TMDs via strain engineering for practical applications such as high-frequency rectifiers, Josephson diode and valleytronics.

Generalization of piezoelectric quantum anomalous Hall insulator based on monolayer Fe2I2: a first-principles study.

To easily synthesize a piezoelectric quantum anomalous Hall insulator (PQAHI), the Janus monolayer Fe2IBr (FeI0.5Br0.5) as a representative PQAHI, is generalized to monolayer FeI1-xBrx (x = 0.25 and 0.75) with α and ß phases. By first-principles calculations, it is proved that monolayer FeI1-xBrx (x = 0.25 and 0.75) are dynamically, mechanically and thermally stable. They are excellent room-temperature PQAHIs with high Curie temperatures, sizable gaps and high Chern number (C = 2). Because the considered crystal structures of α and ß phases possess Mx and My mirror symmetries, the topological properties of monolayer FeI1-xBrx (x = 0.25 and 0.75) are maintained. Namely, if the constructed structures have Mx and My mirror symmetries, the mixing ratio of Br and I atoms can be generalized for other proportions. It is also found that different crystal phases have important effects on the out-of-plane piezoelectric response, and the piezoelectric strain coefficient, d32, of the ß phase is higher than or comparable with those of other known two-dimensional (2D) materials. To further confirm this idea, the physical and chemical properties of monolayer LiFeSe0.75S0.25 with α and ß phases, as a generalization of PQAHI LiFeSe0.5S0.5, is investigated, as it has a similar electronic structure, magnetic and topological properties as LiFeSe0.5S0.5. Our work provides a practical guide to achieve PQAHIs experimentally, and the combination of piezoelectricity, topological and ferromagnetic (FM) orders makes Fe2I2-based monolayers a potential platform for multi-functional spintronics and piezoelectric electronics.

Intrinsic room-temperature piezoelectric quantum anomalous hall insulator in Janus monolayer Fe2IX (X = Cl and Br).

A two-dimensional (2D) material with piezoelectricity, topological and ferromagnetic (FM) properties, namely a 2D piezoelectric quantum anomalous hall insulator (PQAHI), may open new opportunities to realize novel physics and applications. Here, by first-principles calculations, a family of 2D Janus monolayer Fe2IX (X = Cl and Br) with dynamic, mechanical, and thermal stabilities is predicted to be a room-temperature PQAHI. In the absence of spin-orbit coupling (SOC), the monolayer Fe2IX (X = Cl and Br) is in a half Dirac semimetal state. When the SOC is included, these monolayers become quantum anomalous Hall (QAH) states with sizable gaps (more than 200 meV) and two chiral edge modes (Chern number C = 2). It is also found that the monolayer Fe2IX (X = Cl and Br) possesses robust QAH states against the biaxial strain. By symmetry analysis, it is found that only an out-of-plane piezoelectric response can be induced by a uniaxial strain in the basal plane. The calculated out-of-plane d31 of Fe2ICl (Fe2IBr) is 0.467 pm V-1 (0.384 pm V-1), which is higher than or comparable with those of other 2D known materials. Meanwhile, using Monte Carlo (MC) simulations, the Curie temperature TC is estimated to be 429/403 K for the monolayer Fe2ICl/Fe2IBr at the FM ground state, which is above room temperature. Finally, the interplay of electronic correlations with nontrivial band topology is studied to confirm the robustness of the QAH state. The combination of piezoelectricity, topological and FM orders makes the monolayer Fe2IX (X = Cl and Br) become a potential platform for multi-functional spintronic applications with a large gap and high TC. Our work provides the possibility to use the piezotronic effect to control QAH effects, and can stimulate further experimental works.

Freestanding perovskite oxide monolayers as two-dimensional semiconductors.

It is highly desirable to search for promising two-dimensional (2D) monolayer materials for obtaining deep insight of 2D materials and developing device applications. We use first-principles method to investigate tetragonal perovskite oxide monolayers as 2D materials, and find three stable freestanding 2D monolayer materials from important perovskite oxides (ABO3), namely SrTiO3, LaAlO3, and KTaO3, denoting them as STO-ML, LAO-ML, and KTO-ML. Such an oxide monolayer consists of one AO and one BO2 atomic layers. Further study shows that the three monolayers are 2D wide-gap semiconducotors, and there is a large electrostatic potential energy difference between the two sides, reflecting a large out-of-plane dipole, in each of the monolayers. We also investigate optical properties of the three monolayer semiconductors and compare them with graphene and MoS2 monolayer. These make a series of 2D monolayer materials, and should be useful in novel electronic and optoelectronic devices considering emerging phenomena in perovskite oxide heterostructures.