The current focus of valleytronics research lies in how to produce valley polarization. Although many schemes have been broadly studied, spontaneous valley polarization is rarely explored. Here, we report the discovery of a two-dimensional material with the long-pursued spontaneous spin and valley polarizations. Using first-principles calculations, we reveal that single-layer LaBr2 is dynamically and thermally stable, which could be exfoliated from its bulk material. Single-layer LaBr2 is found to be a compelling two-dimensional ferromagnetic semiconductor. More interestingly, we show that single-layer LaBr2 harbors the extremely rare intrinsic valley polarization, owing to the coexistence of inversion symmetry and time-reversal symmetry breakings. Its spontaneous valley polarization reaches 33 meV, sizable enough for operating room-temperature valleytronic physics. Our work thus provides a promising material for experimental studies and practical applications of two-dimensional spintronics and valleytronics.

Van der Waals heterostructures (vdWHs) are attracting a lot of interest for fundamental studies and fabricating novel devices. Currently, most vdWHs exhibit type-I or type-II band alignment, and few systems have been shown to be in the type-III class. Herein, we show first-principles evidence that WTe2/HfS2 vdWH possesses the long-sought type-III band alignment with a broken gap, providing a promising platform for developing tunnel field-effect transistors. Moreover, the electronic features of WTe2/HfS2 vdWH can be effectively modulated via external strain and electric field. Particularly, an interesting transition from type-III to type-II band alignment can be observed in WTe2/HfS2 vdWH upon the application of strain or electric field, which holds great potential for designing multifunctional devices. Our study not only predicts an extraordinary vdWH with type-III band alignment but also provides an outstanding candidate for realizing multiple band alignment transformation.

Two-dimensional multiferroics, harboring antiferromagneticity and ferroelasticity simultaneously, is essential and highly sought for miniaturized device applications, such as high-density data storage, but so far it is rarely explored. Herein, using...

Valleytronics that relies on the valley degree of freedom is attracting growing interest because it provides a new platform for information storage. One obstacle in this field is to realize valley polarization in an efficient route to manipulate the valley physics. Here we propose a strategy to induce valley polarization by nonmetal atom doping in single-layer Tl 2 O. Owing to the intrinsic inversion asymmetry and large spin−orbit coupling, there are a two-fold valley degeneracy and an excellent spin-valley independence in single-layer Tl 2 O. Upon introducing C/N atoms in single-layer Tl 2 O, the intriguing valley polarization successfully appears, and the obtained polarization strengths are considerable. In particular, for N-doped case, the top valence band locates around the Fermi level, and there are no impurity states in the band gap, which is desirable for practical applications. It is predicted that these valley polarizations can be effectively engineered under the magnetic field and external strain, suggesting that the control of valley physics in single-layer Tl 2 O is accessible.

Two-dimensional crystals with coupling of ferroelasticity and attractive electronic properties offer unprecedent opportunities for achieving long-sought controllable devices. But so far, the reported proposals are mainly based on hypothetical structures. Here, using first-principles calculations, we identify single-layer Nb2ATe4 (A = Si, Ge), which could be exfoliated from their layered bulks, are promising candidates. Single-layer Nb2ATe4 are found to be dynamically, thermally and chemically stable. They possess excellent ferroelasticity with high reversible ferroelastic strain and moderate ferroelastic transition energy barrier, beneficial for practical applications. Meanwhile, they harbor outstanding anisotropic electronic properties, including anisotropic carrier mobility and optical properties. More importantly, the anisotropic properties of single-layer Nb2ATe4 can be efficiently controlled through ferroelastic switching. These appealing properties combined with the experimental feasibility render single-layer Nb2ATe4 extraordinary platforms for realizing controllable devices.