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Register a new userIntrinsic room-temperature piezoelectric quantum anomalous hall insulator in Janus monolayer $mathrm{Fe_2IX}$ (X=Cl and Br)
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A two-dimensional (2D) material with piezoelectricity, topological and ferromagnetic (FM) orders, namely 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 $mathrm{Fe_2IX}$ (X=Cl and Br) with dynamic, mechanical and thermal stabilities is predict to be room-temperature PQAHI. At the absence of spin-orbit coupling (SOC), monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) is a half Dirac semimetal state. When the SOC is included, these monolayers become quantum anomalous hall (QAH) states with sizable gaps (more than two hundred meV) and two chiral edge modes (Chern number C=2). It is also found that monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) possesses robust QAH states against biaxial strain. By symmetry analysis, it is found that only out-of-plane piezoelectric response can be induced by a uniaxial strain in the basal plane. The calculated out-of-plane $d_{31}$ of $mathrm{Fe_2ICl}$ ($mathrm{Fe_2IBr}$) is 0.467 pm/V (0.384 pm/V), which is higher than or comparable with ones of other 2D known materials. Meanwhile, using Monte Carlo (MC) simulations, the Curie temperature $T_C$ is estimated to be 429/403 K for monolayer $mathrm{Fe_2ICl}$/$mathrm{Fe_2IBr}$ at 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 QAH state. The combination of piezoelectricity, topological and FM orders makes monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) become a potential platform for multi-functional spintronic applications with large gap and high $T_C$. Our works provide possibility to use the piezotronic effect to control QAH effects, and can stimulate further experimental works.
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The realization of multifunctional two-dimensional (2D) materials is fundamentally intriguing, such as combination of piezoelectricity with topological insulating phase or ferromagnetism. In this work, a Janus monolayer $mathrm{SrAlGaSe_4}$ is built from 2D $mathrm{MA_2Z_4}$ family with dynamic, mechanical and thermal stabilities, which is piezoelectric due to lacking inversion symmetry. The unstrained $mathrm{SrAlGaSe_4}$ monolayer is a narrow gap normal insulator (NI) with spin orbital coupling (SOC). However, the NI to topological insulator (TI) phase transition can be induced by the biaxial strain, and a piezoelectric quantum spin Hall insulator (PQSHI) can be achieved. More excitingly, the phase transformation point is only about 1.01 tensile strain, and nontrivial band topology can hold until considered 1.16 tensile strain. Moreover, a Rashba spin splitting in the conduction bands can exit in PQSHI due to the absence of a horizontal mirror symmetry and the presence of SOC. For monolayer $mathrm{SrAlGaSe_4}$, both in-plane and much weak out-of-plane piezoelectric polarizations can be induced with a uniaxial strain applied. The calculated piezoelectric strain coefficients $d_{11}$ and $d_{31}$ of monolayer $mathrm{SrAlGaSe_4}$ are -1.865 pm/V and -0.068 pm/V at 1.06 tensile strain as a representative TI. In fact, many PQSHIs can be realized from 2D $mathrm{MA_2Z_4}$ family. To confirm that, similar to $mathrm{SrAlGaSe_4}$, the coexistence of piezoelectricity and topological orders can be realized by strain (about 1.04 tensile strain) in the $mathrm{CaAlGaSe_4}$ monolayer. Our works suggest that Janus monolayer $mathrm{SrAlGaSe_4}$ is a pure 2D system for PQSHI, enabling future studies exploring the interplay between piezoelectricity and topological orders, which can lead to novel applications in electronics and spintronics.
A two-dimensional (2D) material system with both piezoelectricity and ferromagnetic (FM) order, referred to as a 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for intriguing physics. Inspired by experimentally synthesized Janus monolayer MoSSe from $mathrm{MoS_2}$, in this work, the Janus monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ with dynamic, mechanical and thermal stabilities is predicted, which is constructed from synthesized ferromagnetic $mathrm{CrI_3}$ monolayer by replacing the top I atomic layer with Br atoms. Calculated results show that monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ is an intrinsic FM half semiconductor with valence and conduction bands being fully spin-polarized in the same spin direction. Furthermore, monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ possesses a sizable magnetic anisotropy energy (MAE). By symmetry analysis, it is found that both in-plane and out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. The calculated in-plane $d_{22}$ value of 0.557 pm/V is small. However, more excitingly, the out-of-plane $d_{31}$ is as high as 1.138 pm/V, which is obviously higher compared with ones of other 2D known materials. The strong out of-plane piezoelectricity is highly desirable for ultrathin piezoelectric devices. Moreover, strain engineering is used to tune piezoelectricity of monolayer $mathrm{CrBr_{1.5}I_{1.5}}$. It is found that compressive strain can improve the $d_{22}$, and tensile strain can enhance the $d_{31}$. A FM order to antiferromagnetic (AFM) order phase transition can be induced by compressive strain, and the critical point is about 0.95 strain. That is to say that a 2D piezoelectric antiferromagnetism (PAFM) can be achieved by compressive strain, and the corresponding $d_{22}$ and $d_{31}$ are 0.677 pm/V and 0.999 pm/V at 0.94 strain, respectively.
Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.
Quantum Spin Hall (QSH) insulators with a large topologically nontrivial bulk gap are crucial for future applications of the QSH effect. Among these, group III-V monolayers and their halides with chair structure (regular hexagonal framework, RHF) were widely studied. Using first-principles calculations, we propose a new structure model for the functionalized group III-V monolayers, which consist of rectangular GaBi-X2 (X=I, Br, Cl) monolayers with a distorted hexagonal framework (DHF). These structures have a much lower energy than the GaBi-X2 monolayers with chair structure. Remarkably, the DHF GaBi-X2 monolayers are all QSH insulators, which exhibit sizeable nontrivial band gaps ranging from 0.17 eV to 0.39 eV. Those band gaps can be widely tuned by applying different spin-orbit coupling (SOC) strengths, resulting in a distorted Dirac cone.
Using Hall photovoltage measurements, we demonstrate that an anomalous Hall-voltage can be induced in few layer WTe2 under circularly polarized light illumination. By applying a bias voltage along different crystal axes, we find that the photo-induced anomalous Hall conductivity coincides with a particular crystal axis. Our results are consistent with the underlying Berry-curvature exhibiting a dipolar distribution due to the breaking of crystal inversion symmetry. Using a time-resolved optoelectronic auto-correlation spectroscopy, we find that the decay time of the anomalous Hall voltage exceeds the electron-phonon scattering time by orders of magnitude but is consistent with the comparatively long spin-lifetime of carriers in the momentum-indirect electron and hole pockets in WTe2. Our observation suggests, that a helical modulation of an otherwise isotropic spin-current is the underlying mechanism of the anomalous Hall effect.
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