arXiv subjects

Bang-Gui Liu

Publications and source records attributed to Bang-Gui Liu.

At least 19 recordsLinked to original sources

arXiv2025

Spin-split flat bands at the band edge and two-dimensional hole gases towards quantum Hall effect in altermagnetic CoF$_2$

Altermagnetic phase is recently found as a new magnetic phase in addition to the conventional collinear spin orders, and great efforts have been made to explore novel effects and potential applications in such materials. Here, we show that there are robust altermagnetic spin-split flat bands near the valence band edge in rutile CoF$_2$ through first-principles investigation. It is uncovered that the magnetic moments can remain in the z axis because of the magnetocrystalline energy due to the spin-orbits coupling and the spin orientation can be made more stable by magnetic field applied in the xy plane. We describe the spin-dependent band structure (including the flat bands) near the Fermi level by a spin-resolved effective low-energy model, and reveal that they can host spin-dependent two-dimensional hole gases. Importantly, we find spin-dependent quantum Hall effects in the two-dimensional hole gases by applying the magnetic field in the xy plane, and then explore the dependence of Hall conductivity and Hall resistance on the Fermi level and the magnetic field (both magnitude and direction) and related longitudinal carrier transport properties.

cond-mat.mtrl-sci↗

arXiv2024

Unconventional Rashba spin splitting and persistent spin helices based on SU(2) symmetry in PtSe$_2$ nanoribbons

2D materials can host interesting physics and have important applications in various fields. Recent experiment shows that monolayer PtSe$_2$ nanoflakes with neutral zigzag edges are stable. Here, we study semiconducting stoichiometric PtSe$_2$ nanoribbons with the stable neutral zigzag edges (with $N$ describing different nanoribbon width) through combining detailed first-principles investigation with low-energy model analysis. Our careful analysis of first-principles conduction and valence bands (with the spin-orbits coupling taken into account) indicates that the low-energy bands assume relativistic energy dispersion in an energy window of [-0.2 eV, 0.2eV] (at least) and have large unconventional Rashba spin splitting (for even $N$). Furthermore, it is demonstrated that the low-energy bands can be well described by an effective one-dimensional electron model and the semiconductor gap will remain finite even for large $N$. Most importantly, it is shown that there exists SU(2) spin symmetry in both of the conduction and valence bands for each edge, which implies persistent spin helices (in the spin xy plane) and spin-conserving carrier transport. When the inter-edge interaction becomes weak ($N$ is large enough), a nearly-perfect Dirac fermion system can be achieved through combining the valence and conduction bands. Thus we realize unconventional Rashba splitting, double SU(2) spin symmetry, persistent spin helices/textures, and pure Dirac fermion systems in stable monolayer PtSe$_2$ nanoribbons.

cond-mat.mes-hall↗

arXiv2024

Dirac-Rashba fermions and quantum valley Hall insulators in graphene-based 2D heterostructures

It is highly desirable to modify and improve the Dirac electron system of graphene for novel electronic properties and promising applications. For this purpose, we study 2D heterostructures consisting of graphene and monolayer TMDs by means of first-principles calculation and effective low-energy hamiltonian model. We determine the model parameters by fitting with the first-prinples bands. MoSe$_2$ and WSe$_2$ are chosen in order to align the Dirac cones of graphene with the intrinsic Fermi levels of the TMDs. It is found that the Dirac energy bands of graphene are modified, but the linear band dispersion near the cones is kept. It is shown that the effective low-energy model hosts Dirac-Rashba feimions in the WSe$_2$/graphene and MoSe$_2$/graphene/WSe$_2$, and there is quantum valley Hall effect in these graphene-based 2D heterostructures. Our further analyses indicate that there are strong interactions between the orbitals and spins especially near the K and K' points. These can be useful in further exploration for novel properties and more functionalities in 2D heterostructures.

cond-mat.mes-hall↗

arXiv2023

Work Statistics and Adiabatic Assumption in Nonequilibrium Many-Body Theory

Keldysh field theory, based on adiabatic assumptions, serves as an widely used framework for addressing nonequilibrium many-body systems. Nonetheless, the validity of such adiabatic assumptions when addressing interacting Gibbs states remains a topic of contention. We use the knowledge of work statistics developed in nonequilibrium thermodynamics to study this problem. Consequently, we deduce a universal theorem delineating the characteristics of evolutions that transition an initial Gibbs state to another. Based on this theorem, we analytically ascertain that adiabatic evolutions fail to transition a non-interacting Gibbs state to its interacting counterpart. However, this adiabatic approach remains a superior approximation relative to its non-adiabatic counterpart. Numerics verifying our theory and predictions are also provided. Furthermore, our findings render insights into the preparation of Gibbs states within the domain of quantum computation.

quant-ph↗

arXiv2022

Piezoelectric quantum spin Hall insulator VCClBr monolayer with pure out-of-plane piezoelectric response

The combination of piezoelectricity with nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulator (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from $\mathrm{VCCl_2}$, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with gap value of 57 meV, which is different from Dirac semimetal $\mathrm{VCCl_2}$. The gap of VCClBr is due to built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by external-electric-field induced gap in $\mathrm{VCCl_2}$. When including SOC, the gap of VCClBr is improved to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by $Z_2$ topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric field. Due to broken horizontal mirror symmetry, only out-of-plane piezoelectric response can be observed, when biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients $d_{31}$ and $d_{32}$ are -0.425 pm/V and -0.219 pm/V, which are higher than or compared with ones of many 2D materials. Finally, another two Janus monolayer VCFBr and VCFCl (dynamically unstable) are constructed, and they are still PQSHIs. Moreover, their $d_{31}$ and $d_{32}$ are higher than ones of VCClBr, and the $d_{31}$ (absolute value) of VCFBr is larger than one.

cond-mat.mtrl-sci↗

arXiv2022

Possible structural and bond reconstruction in 2D ferromagnetic semiconductor VSe2 under uniaxial stress

2D semiconducting transition metal dichalcogenides have been used to make high-performance electronic, spintronic, and optoelectronic devices. Recently, room-temperature ferromagnetism and semiconducting property were found in 2D VSe$_2$ nanoflakes (mechanically exfoliated onto silicon substrates capped with a oxide layer) and are attributed to the stable 2H-phase of VSe$_2$ in the 2D limit. Here, our first-principles investigation show that a metastable semiconducting H' phase can be formed from the H VSe2 monolayer and some other similar when these 2D H-phase materials are under uniaxial stress or uniaxial strain. For the uniaxial stress (uniaxial strain) scheme, the H' phase will become lower in total energy than the H phase at the transition point. The calculated phonon spectra indicate the dynamical stability of the H' structures of VSe$_2$, VS$_2$, and CrS$_2$, and the path of phase switching between the H and H' VSe$_2$ phases is calculated. For VSe$_2$, the H' phase has stronger ferromagnetism and its Currier temperature can be substantially enhanced by applying uniaxial stress or strain. Spin-resolved electronic structures, energy band edges, and effective carrier masses for both of the H and H' phases can be substantially changed by the applied uniaxial stress or strain, leading to huge effective masses near the band edge of the strained H' phase. Analysis indicated that the largest bond length difference between the H' and H phases can reach -19\% for the Se3-Se3' bond, and there is noticeable covalence for the Se3-Se3' bond, which switches the valence of the nearby V atoms, leading to the enhanced ferromagnetism. Therefore, structural and bond reconstruction can be realized by applying uniaxial stress in 2D ferromagnetic H VSe$_2$ and some other similar. These can be useful to seeking more insights and phenomena in such 2D materials for potential applications.

cond-mat.mtrl-sci↗

arXiv2022

Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principle prediction

Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. Electronic correlation effects on physical properties of YBrI are investigated by using generalized gradient approximation plus $U$ (GGA+$U$) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and the valley splitting is larger than 82 meV in considered $U$ range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane with magnetic anisotropy energy (MAE) from 0.309 meV to 0.237 meV ($U$=0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to off-plane direction by external magnetic field, and then lead to the occurrence of valley polarization. Moreover, missing centrosymmetry along with mirror symmetry breaking results in both in-plane and out-of-plane piezoelectricity in YBrI monolayer. At a typical $U$=2.0 eV, the $d_{11}$ is predicted to be -5.61 pm/V, which is higher than or compared with ones of other two-dimensional (2D) known materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying a biaxial strain. For example, tensile strain can enhance valley splitting and $d_{11}$ (absolute value). The predicted Curie temperature of YBrI is higher than those of experimentally synthesized 2D ferromagnetic materials $\mathrm{CrI_3}$ and $\mathrm{Cr_2Ge_2Te_6}$.

cond-mat.mtrl-sci↗

arXiv2022

Creating quantum spin chains through edge reconstruction in pure graphene armchair nanoribbons towards ballistic spin transport

It is well-known that ferromagnetism can be realized along the zigzag graphene nanoribbon edges, but the armchair graphene nanoribbon edges (AGNEs) are nonmagnetic. Here, we achieve Heisenberg antiferromagnetic spin chains through edge reconstruction along the AGNEs. The reconstructed edge consists of pentagonal carbon rings or a hybrid of pentagonal and hexagonal carbon rings. The resultant nanoribbons are narrow-gap semiconductors and the band edge states are either spin-degenerate edge states or nonmagnetic bulk states. The spin is located on the outermost carbon of the pentagonal ring, and the inter-spin exchange is the nearest-neighbor antiferromagnetic interaction. For finite chain lengthes or nonzero magnetization, there are nonzero spin Drude weights and thus ballistic quantum spin transport can be achieved along the reconstructed edges, These could be used for quantum spin information transfer and spintronic applications.

cond-mat.mes-hall↗

arXiv2022

Correlation-driven threefold topological phase transition in monolayer $\mathrm{OsBr_2}$

Spin-orbit coupling (SOC) combined with electronic correlation can induce topological phase transition, producing novel electronic states. Here, we investigate the impact of SOC combined with correlation effects on physical properties of monolayer $\mathrm{OsBr_2}$, based on first-principles calculations with generalized gradient approximation plus $U$ (GGA+$U$) approach. With intrinsic out-of-plane magnetic anisotropy, $\mathrm{OsBr_2}$ undergoes threefold topological phase transition with increasing $U$, and valley-polarized quantum anomalous Hall insulator (VQAHI) to half-valley-metal (HVM) to ferrovalley insulator (FVI) to HVM to VQAHI to HVM to FVI transitions can be induced. These topological phase transitions are connected with sign-reversible Berry curvature and band inversion between $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$ orbitals. Due to $\bar{6}m2$ symmetry, piezoelectric polarization of $\mathrm{OsBr_2}$ is confined along the in-plane armchair direction, and only one $d_{11}$ is independent. For a given material, the correlation strength should be fixed, and $\mathrm{OsBr_2}$ may be a piezoelectric VQAHI (PVQAHI), piezoelectric HVM (PHVM) or piezoelectric FVI (PFVI). The valley polarization can be flipped by reversing the magnetization of Os atoms, and the ferrovalley (FV) and nontrivial topological properties will be suppressed by manipulating out-of-plane magnetization to in-plane one. In considered reasonable $U$ range, the estimated Curie temperatures all are higher than room temperature. Our findings provide a comprehensive understanding on possible electronic states of $\mathrm{OsBr_2}$, and confirm that strong SOC combined with electronic correlation can induce multiple quantum phase transition.

cond-mat.mtrl-sci↗

arXiv2022

Correlation-enhanced spin-orbit coupling and quantum anomalous Hall insulator with large band gap and stable ferromagnetism in monolayer $\mathrm{Fe_2Br_2}$

Nontrivial band topology combined with magnetic ordering can produce quantum anomalous Hall insulator (QAHI), which may lead to advances in device concepts. Here, through first-principles calculations, stable monolayer $\mathrm{Fe_2Br_2}$ is predicted as a room-temperature large-gap high-Chern-number QAHI by using generalized gradient approximation plus $U$ (GGA+$U$) approach. The large gap is due to correlation-enhanced spin-orbit coupling (SOC) effect of Fe atoms, which equates with artificially increasing the strength of SOC without electronic correlation. Out-of-plane magnetic anisotropy is very key to produce quantum anomalous Hall (QAH) state because in-plane magneitization will destroy nontrivial band topology. In the absence of SOC, $\mathrm{Fe_2Br_2}$ is a half Dirac semimetal state protected by mirror symmetry, and the electronic correlation along with SOC effect creates QAH state with a sizable gap and two chiral edge modes. It is found that the QAH state is robust against biaxial strain ($a/a_0$: 0.96 to 1.04) in monolayer $\mathrm{Fe_2Br_2}$ with stable ferromagnetic (FM) ordering and out-of-plane magnetic anisotropy. Calculated results show that Curie temperature is sensitive to correlation strength and strain. The reduced correlation and compressive strain are in favour of high Curie temperature. These analysis and results can be readily extended to other monolayer $\mathrm{Fe_2XY}$ (X/Y=Cl, Br and I), which possesses the same Fe-dominated low-energy states with a $\mathrm{Fe_2Br_2}$ monolayer. These findings open new opportunities to design new high-temperature topological quantum devices.

cond-mat.mtrl-sci↗

arXiv2022

Valley-polarized quantum anomalous Hall insulator in monolayer $\mathrm{RuBr_2}$

Coexistence of intrinsic ferrovalley (FV) and nontrivial band topology attracts intensive interest both for its fundamental physics and for its potential applications, namely valley-polarized quantum anomalous Hall insulator (VQAHI). Here, based on first-principles calculations by using generalized gradient approximation plus $U$ (GGA+$U$) approach, the VQAHI induced by electronic correlation or strain can occur in monolayer $\mathrm{RuBr_2}$. For perpendicular magnetic anisotropy (PMA), the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions can be driven by increasing electron correlation $U$. However, there are no special QAH states and valley polarization for in-plane magnetic anisotropy. By calculating actual magnetic anisotropy energy (MAE), the VQAHI indeed can exist between two HVM states due to PMA, a unit Chern number/a chiral edge state and spontaneous valley polarization. The increasing $U$ can induce VQAHI, which can be explained by sign-reversible Berry curvature or band inversion between $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$ orbitals. Even though the real $U$ falls outside the range, the VQAHI can be achieved by strain. Taking $U$$=$2.25 eV as a concrete case, the monolayer $\mathrm{RuBr_2}$ can change from a common ferromagentic (FM) semiconductor to VQAHI under about 0.985 compressive strain. It is noted that the edge states of VQAHI are chiral-spin-valley locking, which can achieve complete spin and valley polarizations for low-dissipation electronics devices. Both energy band gap and valley splitting of VQAHI in monolayer $\mathrm{RuBr_2}$ are higher than the thermal energy of room temperature (25 meV), which is key at room temperature for device applications.

cond-mat.mtrl-sci↗

arXiv2022

Coexisting two-dimensional electron and hole gases highly confined at the interfaces of undoped KTaO3-sandwiching heterostructures

Two-dimensional electron gas (2DEG) in interfaces and surfaces based on perovskite SrTiO$_3$ (STO) has exhibited various interesting phenomena and is used to develop oxide electronics. Recently, KTaO$_3$ (KTO) shows great potential and is believed to host more exciting effects and phenomena toward novel devices. Here, through first-principles investigation and analysis, we find two types of coexisting 2DEG and 2D hole gas (2DHG) highly confined at the interfaces in undoped STO/KTO/BaTiO$_3$ heterostructures, when the KTO thickness $m$ reaches a crititcal value. The two interfaces are made by (SrO)$^0$/(TaO$_2$)$^+$ and (KO)$^-$/(TiO$_2$)$^0$ for the A-type, and by (TiO$_2$)$^0$/(KO)$^-$ and (TaO$_2$)$^+$/(BO)$^0$ for the B-type. The 2D electron carriers originate from Ta-$5 d_{xy}$ states at the interface including TaO$_2$ atomic layer, and the hole carriers from O-$2 p_x/p_y$ orbitals at the other interface. The electron and hole effective masses are 0.3$m_0$ and $1.06\sim 1.12 m_0$, respectively, where $m_0$ is mass of free electron, and the 2D carrier concentrations are in the order of $10 ^{13}$ cm$^{-2}$. Our analysis indicates that the interfacial 2DEG and 2DHG are simultaneously formed because of the band bending due to the polar discontinuity at the interfaces and the stress-induced polarization within the KTO layer. These could stimulate more exploration for new phenomena and novel devices.

cond-mat.mtrl-sci↗

arXiv2021

Sensitive electronic correlation effects on electronic properties in ferrovalley material Janus FeClF monolayer

The electronic correlation may have essential influence on electronic structures in some materials with special structure and localized orbital distribution. In this work, taking Janus monolayer FeClF as a concrete example, the correlation effects on its electronic structures are investigated by using generalized gradient approximation plus $U$ (GGA+$U$) approach. For perpendicular magnetic anisotropy (PMA), the increasing electron correlation effect can induce the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions. For QAH state, there are a unit Chern number and a chiral edge state connecting the conduction and valence bands. The HVM state is at the boundary of the QAH phase, whose carriers are intrinsically 100\% valley polarized. With the in-plane magnetic anisotropy, no special QAH states and prominent valley polarization are observed. However, for both out-of-plane and in-plane magnetic anisotropy, sign-reversible Berry curvature can be observed with increasing $U$. It is found that these phenomenons are related with the change of $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$ orbital distributions and different magnetocrystalline directions. It is also found that the magnetic anisotropy energy (MAE) and Curie temperature strongly depend on the $U$. With PMA, taking typical $U=$2.5 eV, the electron valley polarization can be observed with valley splitting of 109 meV, which can be switched by reversing the magnetization direction. The analysis and results can be readily extended to other nine members of monolayer FeXY (X/Y=F, Cl, Br and I) due to sharing the same Fe-dominated low-energy states and electronic correlations with FeClF monolayer.

cond-mat.mtrl-sci↗

arXiv2021

Valley polarization transition driven by biaxial strain in Janus $\mathrm{GdClF}$ monolayer

The valley degrees of freedom of carriers in crystals is useful to process information and perform logic operations, and it is a key factor for valley application to realize the valley polarization. Here, we propose a model that the valley polarization transition at different valley points (-K and K points) is produced by biaxial strain. By the first-principle calculations, we illustrate our idea with a concrete example of Janus $\mathrm{GdClF}$ monolayer. The predicted $\mathrm{GdClF}$ monolayer is dynamically, mechanically and thermally stable, and is a ferromagnetic (FM) semiconductor with perpendicular magnetic anisotropy (PMA), valence band maximum (VBM) at valley points and high Curie temperature ($T_C$). Due to its intrinsic ferromagnetism and spin orbital coupling (SOC), a spontaneous valley polarization will be induced, but the valley splitting is only -3.1 meV, which provides an opportunity to achieve valley polarization transition at different valley points by strain. In considered strain range ($a/a_0$: 0.94$\sim$1.06), the strained GdClF monolayer has always energy bandgap, strong FM coupling and PMA. The compressive strain is in favour of -K valley polarization, while the tensile strain makes for K valley polarization. The corresponding valley splitting at 0.96 and 1.04 strain are -44.5 meV and 29.4 meV, which are higher than the thermal energy of room temperature (25 meV). Due to special Janus structure, both in-plane and out-of-plane piezoelectric polarizations can be observed. It is found that the direction of in-plane piezoelectric polarizations can be overturned by strain, and the $d_{11}$ at 0.96 and 1.04 strain are -1.37 pm/V and 2.05 pm/V. Our works pave the way to design the ferrovalley material as multifunctional valleytronics and piezoelectric devices by strain.

cond-mat.mtrl-sci↗

arXiv2021

A possible way to achieve anomalous valley Hall effect by piezoelectric effect in $\mathrm{GdCl_2}$ monolayer

Ferrovalley materials can achieve manipulation of the valley degree of freedom with intrinsic spontaneous valley polarization introduced by their intrinsic ferromagnetism. A good ferrovalley material should possess perpendicular magnetic anisotropy (PMA), valence band maximum (VBM)/conduction band minimum (CBM) at valley points, strong ferromagnetic (FM) coupling and proper valley splitting. In this work, the monolayer $\mathrm{GdCl_2}$ is proposed as a potential candidate material for valleytronic applications by the first-principles calculations. It is proved that monolayer $\mathrm{GdCl_2}$ is a FM semiconductor with the easy axis along out of plane direction and strong FM coupling. A spontaneous valley polarization with a valley splitting of 42.3 meV is produced due to its intrinsic ferromagnetism and spin orbital coupling (SOC). Although the VBM of unstrained monolayer $\mathrm{GdCl_2}$ is away from valley points, a very small compressive strain (about 1\%) can make VBM move to valley points. We propose a possible way to realize anomalous valley Hall effect in monolayer $\mathrm{GdCl_2}$ by piezoelectric effect, not an external electric field, namely piezoelectric anomalous valley Hall effect (PAVHE). This phenomenon could be classified as piezo-valleytronics, being similar to piezotronics and piezophototronics. The only independent piezoelectric strain coefficient $d_{11}$ is -2.708 pm/V, which is comparable to one of classical bulk piezoelectric material $α$-quartz ($d_{11}$=2.3 pm/V). The biaxial in-plane strain and electronic correlation effects are considered to confirm the reliability of our results. Finally, the monolayer $\mathrm{GdF_2}$ is predicted to be a ferrovalley material with dynamic and mechanical stabilities, PMA, VBM at valley points, strong FM coupling, valley splitting of 47.6 meV, and $d_{11}$ of 0.584 pm/V.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic room-temperature piezoelectric quantum anomalous hall insulator in Janus monolayer $\mathrm{Fe_2IX}$ (X=Cl and Br)

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.

cond-mat.mtrl-sci↗

arXiv2020

Giant Rashba spin splitting in strained KTaO3 ultrathin films for circular photogalvanic currents

Strong Rashba effects at surfaces and interfaces have attracted great attention for basic scientific exploration and practical applications. Here, the first-principles investigation shows that giant and tunable Rashba effects can be achieved in KTaO$_3$ (KTO) ultrathin films by applying biaxial stress. When increasing the in-plane compressive strain nearly to -5\%, the Rashba spin splitting energy reaches $E_{R}=140$ meV, approximately corresponding to the Rashba coupling constant $α_{R}=1.3$ eV Å. We investigate its strain-dependent crystal structures, energy bands, and related properties, and thereby elucidate the mechanism for the giant Rashba effects. Furthermore, we show that giant Rashba spin splitting can be kept in the presence of SrTiO$_3$ capping layer and/or Si substrate, and strong circular photogalvanic effect can be achieved to generate spin-polarized currents in the KTO thin films or related heterostructures, which are promising for future spintronic and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Uniaxial stress controlled anisotropic Rashba effects and carriers-based currents in BiTeI monolayer semiconductor

Manipulation of Rashba effects in two-dimensional (2D) electron systems is highly desirable for controllable applications in spintronics and optoelectronics. Here, by combining first-principles investigation and model analysis, we use uniaxial stress to control BiTeI monolayer as a Rashba 2D semiconductor for useful spin and transport properties. We find that the stress-driven electron system can be described by an effective anisotropic Rashba model including all the three Pauli matrixes, and uniaxial stress allows an out-of-plane spin component. When appropriate electron carriers are introduced into the monolayer, an in-plane electric field can induce a charge current and three spin current components (including that based on the out-of-plane spin) because of the reduced symmetry. Therefore, uniaxial stress can be used to control such Rashba 2D electron systems as the BiTeI monolayer for seeking promising devices.

cond-mat.mes-hall↗