arXiv subjects

Xiao-Shu Guo

Publications and source records attributed to Xiao-Shu Guo.

14 recordsLinked to original sources

arXiv2025

Weak valley-layer coupling and valley polarization in centrosymmetric $\mathrm{FeCl_2}$ monolayer

Using the valley degree of freedom as a carrier of information for storage and processing, valley polarization plays a crucial role. A variety of mechanisms for valley polarization have been proposed, among which the valley-layer coupling mechanism involves the induction of valley polarization by an out-of-plane electric field. Here, through first-principles calculations, it is found that the weak valley-layer coupling can exist in centrosymmetric $\mathrm{FeCl_2}$ monolayer. It is crucial to note that valley-layer coupling only occurs with out-of-plane magnetization and vanishes with in-plane magnetization. Compared to monolayers with strong valley-layer coupling, $\mathrm{FeCl_2}$ requires an extremely strong electric field to achieve the same magnitude of valley splitting. Valley polarization switching can be achieved by manipulating the directions of magnetization and electric field. Reversing only one of these directions switches the valley polarization, whereas reversing both simultaneously leaves it unchanged. Moreover, the simply stacked bilayer $\mathrm{FeCl_2}$, as a $PT$-antiferromagnet, can spontaneously achieve valley polarization without an external electric field, highlighting its potential for miniaturization, ultradensity, and ultrafast performance. Our work provides guidelines for identifying materials with weak valley-layer coupling, and further enables the regulation of valley polarization through electric field and stacking engineering.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry-breaking induced transition among net-zero-magnetization magnets

Net-zero-magnetization magnets have garnered intensive research attention due to their ultradense and ultrafast potential. In terms of the symmetric classification of connecting magnetic atoms with opposite spin polarization, the net-zero-magnetization magnets mainly include $PT$-antiferromagnet (the joint symmetry ($PT$) of space inversion symmetry ($P$) and time-reversal symmetry ($T$)), altermagnet and fully-compensated ferrimagnet. Studying transitions among net-zero-magnetization magnets is essentially the research on symmetry breaking, which can also clearly reveal the transformation of spin-splitting symmetry. Symmetry breaking can be achieved through methods such as Janus engineering, isovalent alloying, and external electric field. Here, we start from a parent $PT$-antiferromagnet that simultaneously possesses both $P$ and rotational/mirror symmetries to induce altermagnet and fully-compensated ferrimagnet. Based on first-principles calculations, the proposed transitions can be verified in $PT$-antiferromagnet $\mathrm{CrC_2S_6}$ monolayer. By Janus engineering and isovalent alloying, $\mathrm{CrC_2S_6}$ can change into altermagnetic $\mathrm{CrC_2S_3Se_3}$ and fully-compensated ferrimagnetic $\mathrm{CrMoC_2S_6}$. The $\mathrm{CrC_2S_3Se_3}$ can also become fully-compensated ferrimagnetic $\mathrm{CrMoC_2S_3Se_3}$ by isovalent alloying. Our work provides a clear and intuitive example to explain the transitions among net-zero-magnetization magnets, which can inspire more research on net-zero-magnetization magnets.

cond-mat.mtrl-sci↗

arXiv2024

Isovalent alloying assisted anomalous valley Hall effect in hexagonal antiferromagnetic monolayer

Exploring combination of antiferromagnetic (AFM) spintronics and anomalous valley Hall effect (AVHE) is one of the most important questions for valleytronic applications. The key to address this issue is to achieve spin splitting around the valleys in AFM systems. Here, we propose a possible way for achieving AVHE in hexagonal AFM monolayer, which involves the isovalent alloying. This can break the combined symmetry ($PT$ symmetry) of spatial inversion ($P$) and time reversal ($T$), giving rise to spin splitting. More specifically, the large spin splitting around the Fermi energy level owes to $d$ orbital mismatch among these different transition metal ions. Based on first-principles calculations, the proposed way can be verified in out-of-plane AFM $\mathrm{CrMoC_2S_6}$ monolayer, which possesses spontaneous valley polarization and spitting splitting, providing possibility to realize AVHE. It is also proved that tensile strain can strengthen the valley splitting and maintain the out-of-plane AFM ordering. Our works provide an experimentally feasible way for developing AFM valleytronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Piezoelectric altermagnetism and spin-valley polarization in Janus monolayer $\mathrm{Cr_2SO}$

The altermagnetism can achieve spin-split bands in collinear symmetry-compensated antiferromagnets. Here, we predict altermagnetic order in Janus monolayer $\mathrm{Cr_2SO}$ with eliminated inversion symmetry, which can realize the combination of piezoelectricity and altermagnetism in a two-dimensional material, namely 2D piezoelectric altermagnetism. It is found that $\mathrm{Cr_2SO}$ is an altermagnetic semiconductor, and the spin-split bands of both valence and conduction bands are near the Fermi level. The $\mathrm{Cr_2SO}$ has large out-of-plane piezoelectricity ($|d_{31}|$$=$0.97 pm/V), which is highly desirable for ultrathin piezoelectric device application. Due to spin-valley locking, both spin and valley can be polarized by simply breaking the corresponding crystal symmetry with uniaxial strain. Our findings provide a platform to integrate spin, piezoelectricity and valley in a single material, which is useful for multi-functional device applications.

cond-mat.mtrl-sci↗

arXiv2023

Janus monolayer ScXY (X$\neq$Y=Cl, Br and I) for piezoelectric and valleytronic application: a first-principle prediction

Coexistence of ferromagnetism, piezoelectricity and valley in two-dimensional (2D) materials is crucial to advance multifunctional electronic technologies. Here, Janus ScXY (X$\neq$Y=Cl, Br and I) monolayers are predicted to be in-plane piezoelectric ferromagnetic (FM) semiconductors with dynamical, mechanical and thermal stabilities. The predicted piezoelectric strain coefficients $d_{11}$ and $d_{31}$ (absolute values) are higher than ones of most 2D materials. Moreover, the $d_{31}$ (absolute value) of ScClI reaches up to 1.14 pm/V, which is highly desirable for ultrathin piezoelectric device application. To obtain spontaneous valley polarization, charge doping are explored to tune the direction of magnetization of ScXY. By appropriate hole doping, their easy magnetization axis can change from in-plane to out-of-plane, resulting in spontaneous valley polarization. Taking ScBrI with 0.20 holes per f.u. as a example, under the action of an in-plane electric field, the hole carriers of K valley turn towards one edge of the sample, which will produce anomalous valley Hall effect (AVHE), and the hole carriers of $Γ$ valley move in a straight line. These findings could pave the way for designing piezoelectric and valleytronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Electric-field induced magnetic-anisotropy transformation to achieve spontaneous valley polarization

Valleytronics has been widely investigated for providing new degrees of freedom to future information coding and processing. Here, it is proposed that valley polarization can be achieved by electric field induced magnetic anisotropy (MA) transformation. Through the first-principle calculations, our idea is illustrated by a concrete example of $\mathrm{VSi_2P_4}$ monolayer. The increasing electric field can induce a transition of MA from in-plane to out-of-plane by changing magnetic anisotropy energy (MAE) from negative to positive value, which is mainly due to increasing magnetocrystalline anisotropy (MCA) energy. The out-of-plane magnetization is in favour of spontaneous valley polarization in $\mathrm{VSi_2P_4}$. Within considered electric field range, $\mathrm{VSi_2P_4}$ is always ferromagnetic (FM) ground state. In a certain range of electric field, the coexistence of semiconductor and out-of-plane magnetization makes $\mathrm{VSi_2P_4}$ become a true ferrovalley (FV) material. The anomalous valley Hall effect (AVHE) can be observed under in-plane and out-of-plane electrical field in $\mathrm{VSi_2P_4}$. Our works pave the way to design the ferrovalley material by electric field.

cond-mat.mtrl-sci↗

arXiv2022

Out-of-plane high-temperature ferromagnetic monolayer CrSCl with large vertical piezoelectric response

For two-dimensional (2D) material, piezoelectric ferromagnetism (PFM) with large out-of-plane piezoresponse is highly desirable for multifunctional ultrathin piezoelectric device application. Here, we predict that Janus monolayer CrSCl is an out-of-plane ferromagnetic (FM) semiconductor with large vertical piezoelectric response and high Curie temperature. The predicted out-of-plane piezoelectric strain coefficient $d_{31}$ is -1.58 pm/V, which is higher than ones of most 2D materials (compare absolute values of $d_{31}$). The large out-of-plane piezoelectricity is robust against electronic correlation and biaxial strain, confirming reliability of large $d_{31}$. Calculated results show that tensile strain is conducive to high Curie temperature, large magnetic anisotropy energy (MAE) and large $d_{31}$. Finally, by comparing $d_{31}$ of CrYX (Y=S; X=Cl, Br I) and CrYX (Y=O; X=F, Cl, Br), we conclude that the size of $d_{31}$ is positively related to electronegativity difference of X and Y atoms. Such findings can provide valuable guidelines for designing 2D piezoelectric materials with large vertical piezoelectric response.

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

Intrinsic piezoelectric ferromagnetism with large out-of-plane piezoelectric response in Janus monolayer $\mathrm{CrBr_{1.5}I_{1.5}}$

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.

cond-mat.mtrl-sci↗

arXiv2021

Tuning transport coefficients of monolayer $\mathrm{MoSi_2N_4}$ with biaxial strain

Experimentally synthesized $\mathrm{MoSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020)}) is a piezoelectric semiconductor. Here, we systematically study the large biaxial (isotropic) strain effects (0.90 to 1.10) on electronic structures and transport coefficients of monolayer $\mathrm{MoSi_2N_4}$ by density functional theory (DFT). With $a/a_0$ from 0.90 to 1.10, the energy band gap firstly increases, and then decreases, which is due to transformation of conduction band minimum (CBM). Calculated results show that the $\mathrm{MoSi_2N_4}$ monolayer is mechanically stable in considered strain range. It is found that the spin-orbital coupling (SOC) effects on Seebeck coefficient depend on the strain. In unstrained $\mathrm{MoSi_2N_4}$, the SOC has neglected influence on Seebeck coefficient. However, the SOC can produce important influence on Seebeck coefficient, when the strain is applied, for example 0.96 strain. The compressive strain can change relative position and numbers of conduction band extrema (CBE), and then the strength of conduction bands convergence can be enhanced, to the benefit of n-type $ZT_e$. Only about 0.96 strain can effectively improve n-type $ZT_e$. Our works imply that strain can effectively tune the electronic structures and transport coefficients of monolayer $\mathrm{MoSi_2N_4}$, and can motivate farther experimental exploration.

cond-mat.mtrl-sci↗

arXiv2019

Large piezoelectric coefficients combined with high electron mobilities in Janus monolayer XTeI (X=Sb and Bi): a first-principle study

The absence of both the inversion symmetry and out-of-plane mirror symmetry together with spin-orbit coupling (SOC) can induce novel electronic and piezoelectric properties. In this work, the piezoelectric properties along with carrier mobilities of Janus monolayer XTeI (X=Sb and Bi) are studied by density functional theory (DFT). By using generalized gradient approximation (GGA) plus SOC, they are found to be indirect gap semiconductors with the Rashba spin splitting. The piezoelectric tensors of Janus monolayer XTeI (X=Sb and Bi) are reported by using density functional perturbation theory (DFPT). Due to lacking both the inversion symmetry and out-of-plane mirror symmetry for Janus monolayer XTeI (X=Sb and Bi), both in-plane and out-of-plane piezoelectric effects can be observed, and the large piezoelectric coefficients are predicted (e.g. $d_{11}$=12.95 pm/V for SbTeI and $d_{11}$=8.20 pm/V for BiTeI), which are comparable and even higher than ones of many other two-dimensional (2D) materials and other well-known bulk piezoelectric materials, especially for out-of-plane piezoelectric coefficients. With GGA+SOC, the high electron carrier mobilities are obtained, and the electron mobility of BiTeI along armchair direction reaches up to about 1319 $\mathrm{cm^2V^{-1}s^{-1}}$. The carrier mobility shows a rather pronounced anisotropy between electron and hole/armchair and zigzag directions. It is found that tensile strain can improve the piezoelectric coefficients $d_{11}$ of Janus monolayer XTeI (X=Sb and Bi). For example, at 4\% strain, the $d_{11}$ of SbTeI (BiTeI) is up to 20.12 pm/V (11.48 pm/V), compared with unstrained 12.95 pm/V (8.20 pm/V). Our works imply Janus monolayer XTeI (X=Sb and Bi) have potential applications in flexible electronics and piezoelectric devices, and can stimulate further experimental works.

cond-mat.mtrl-sci↗

arXiv2019

Small strain induced large piezoelectric coefficient in $α$-AsP monolayer

Strain engineering can effectively tune the electronic, topological and piezoelectric properties of materials. In this work, the small strain (-4\% to 4\%) effects on piezoelectric properties of $α$-AsP monolayer are studied by density functional theory (DFT). The piezoelectric stress tensors $e_{ij}$ and elastic stiffness tensors $C_{ij}$ are reported by using density functional perturbation theory (DFPT) and finite difference method (FDM). It is found that the Young's modulus of $α$-AsP monolayer shows very strong anisotropy, and the armchair direction is very softer than zigzag direction, which provides possibility for tuning easily piezoelectric coefficients along the armchair direction. In considered strain range, uniaxial compressive (tensile) strain along the armchair (zigzag) direction is found to raise observably both the $e_{22}$ and $d_{22}$ (absolute value). In fact, both compressive strain along the armchair direction and tensile strain along the zigzag direction essentially reduce the lattice constants along the armchair direction, which can enhance the piezoelectric coefficients. The $e_{ij}$ of $β$-AsP monolayer as a function of strain is also studied to illustrate the importance of particular puckered structure of $α$-AsP in enhancing the piezoelectric coefficients. A classic SnSe monolayer with puckered structure is used to further declare that small strain along the armchair direction can effectively improve the piezoelectric coefficients. For example, the $d_{22}$ of SnSe monolayer at -3.5\% strain is up to 628.8 pm/V from unstrained 175.3 pm/V. For SnSe monolayer, a large peak is observed for $e_{22}$, which is due to a structural phase transition. For $e_{16}$ of SnSe monolayer, a large peak is also observed due to the cross of lattice constants $a$ along the zigzag direction and $b$ along the armchair direction.

cond-mat.mtrl-sci↗

arXiv2019

Predicted Janus SnSSe monolayer: a comprehensive first-principle study

The Janus structure, by combining properties of different transition metal dichalcogenide (TMD) monolayers in a single polar material, has attracted increasing research interest because of their particular structure and potential application in electronics, optoelectronics and piezoelectronics. In this work, Janus SnSSe monolayer is predicted by means of first-principles calculations, which exhibits dynamic and mechanical stability. By using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC), the Janus SnSSe monolayer is found to be an indirect band-gap semiconductor, whose gap can easily be tuned by strain. High carrier mobilities are obtained for SnSSe monolayer, and the hole mobility is higher than the electron mobility. For SnSSe monolayer, a uniaxial strain in the basal plane can induce both strong in-plane and much weaker out-of-plane piezoelectric polarizations, which reveals the potential as a piezoelectric two-dimensional (2D) material. The high absorption coefficients in the visible light region are observed, suggesting a potential photocatalytic application. Calculated results show that SnSSe monolayer has very high power factor, making it a promising candidate for thermoelectric applications. Our works reveal that the Janus SnSSe structure can be fabricated with unique electronic, optical, piezoelectric and transport properties, and can motivate related experimental works.

cond-mat.mtrl-sci↗

arXiv2019

Tuning the electronic structures and transport coefficients of Janus PtSSe monolayer with biaxial strain

Due to their great potential in electronics, optoelectronics and piezoelectronics, Janus transition metal dichalcogenide (TMD) monolayers have attracted increasing research interest, the MoSSe of which with sandwiched S-Mo-Se structure has been synthesized experimentally. In this work, the biaxial strain dependence of electronic structures and transport properties of Janus PtSSe monolayer is systematically investigated by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). Calculated results show that SOC has a detrimental effect on power factor of PtSSe monolayer, which can be understood by considering SOC effects on energy bands near the Fermi level. With $a/a_0$ from 0.94 to 1.06, the energy band gap firstly increases, and then decreases, which is due to the position change of conduction band minimum (CBM). It is found that compressive strain can increase the strength of conduction bands convergence by changing relative position of conduction band extrema (CBE), which can enhance n-type $ZT_e$ values. Calculated results show that compressive strain can also induce the flat valence bands around the $Γ$ point near the Fermi level, which can lead to high Seebeck coefficient due to large effective masses, giving rise to better p-type $ZT_e$ values. The calculated elastic constants with $a/a_0$ from 0.94 to 1.06 all satisfy the mechanical stability criteria, which proves that the PtSSe monolayer is mechanically stable in the considered strain range. Our works further enrich studies of Janus TMD monolayers, and can motivate farther experimental works.

cond-mat.mtrl-sci↗