arXiv subjects

Yu-Tong Zhu

Publications and source records attributed to Yu-Tong Zhu.

11 recordsLinked to original sources

arXiv2022

Two-dimensional Janus Si dichalcogenides: A first-principles study

Strong structural asymmetry is actively explored in two-dimensional (2D) materials, because it can give rise to many interesting physical properties. Motivated by the recent synthesis of monolayer $\mathrm{Si_2Te_2}$, we explore a family of 2D materials, termed as the Janus Si dichalcogenides (JSD), which parallel the Janus transition metal dichalcogenides and exhibit even stronger inversion asymmetry. Using first-principles calculations, we demonstrate excellent stability of these materials. We show that their strong structural asymmetry leads to pronounced intrinsic polar field, sizable spin splitting due to spin-orbit coupling, and large piezoelectric response. The spin splitting involves an out-of-plane component, which is beyond the linear Rashba model. The piezoelectric tensor has large value in both in-plane $d_{11}$ coefficient and out-of-plane $d_{31}$ coefficient, making the monolayer JSDs distinct among the existing 2D piezoelectrics. In addition, we find interesting strain-induced phase transitions in these materials. Particularly, there are multiple valleys in the conduction band that compete for the conduction band minimum, which will lead to notable changes in optical and transport properties under strain. Our work reveals a new family of Si based 2D materials, which could find promising applications in spintronic and piezoelectric devices.

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↗

arXiv2022

Huge out-of-plane piezoelectric response in ferromagnetic monolayer NiClI

The combination of piezoelectricity and ferromagnetic (FM) order in a two-dimensional (2D) material, namely 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for novel device applications. Here, we predict an in-plane FM semiconductor Janus monolayer NiClI with considerably large magnetic anisotropy energy (MAE) of 1.439 meV, exhibiting dynamic, mechanical and thermal stabilities. The NiClI monolayer possesses larger in-plane piezoelectricity ($d_{11}$$=$5.21 pm/V) comparable to that of $\mathrm{MoS_2}$. Furthermore, NiClI has huge out-of-plane piezoelectricity ($d_{31}$$=$1.89 pm/V), which is highly desirable for ultrathin piezoelectric device application. It is proved that huge out-of-plane piezoelectricity is robust against electronic correlation, which confirms reliability of huge $d_{31}$. Finally, being analogous to NiClI, PFM with large out-of-plane piezoelectricity can also be achieved in the Janus monolayers of NiClBr and NiBrI, with the predicted $d_{31}$ of 0.73 pm/V and 1.15 pm/V, respectively. The predicted huge out-of-plane piezoelectric response makes Janus monolayer NiClI a good platform for multifunctional semiconductor spintronic applications, which is also compatible with the bottom/top gate technologies of conventional semiconductor nanoelectronic devices.

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↗

arXiv2021

Piezoelectric quantum spin Hall insulator with Rashba spin splitting in Janus monolayer $\mathrm{SrAlGaSe_4}$

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.

cond-mat.mtrl-sci↗

arXiv2021

Coexistence of intrinsic piezoelectricity and nontrivial band topology in monolayer InXO (X=Se and Te)

The combination of piezoelectricity with other unique properties (like topological insulating phase and intrinsic ferromagnetism) in two-dimensional (2D) materials is much worthy of intensive study. In this work, the piezoelectric properties of 2D topological insulators InXO (X=Se and Te) from monolayer InX (X=Se and Te) with double-side oxygen functionalization are studied by density functional theory (DFT). The large piezoelectric strain coefficients (e.g. $d_{11}$=-13.02 pm/V for InSeO and $d_{11}$=-9.64 pm/V for InTeO) are predicted, which are comparable and even higher than ones of many other familiar 2D materials. Moreover, we propose two strategies to enhance piezoelectric response of monolayer InXO (X=Se and Te). Firstly, the biaxial strain (0.94-1.06) is applied, and the $d_{11}$ (absolute value) is increased by 53\%/56\% for monolayer InSeO/InTeO at 1.06 strain, which is due to increased $e_{11}$ (absolute value) and reduced $C_{11}-C_{12}$. In considered strain range, InXO (X=Se and Te) monolayers are always 2D topological insulators, which confirm the coexistence of piezoelectricity and nontrivial band topology. Secondly, a Janus monolayer $\mathrm{In_2SeTeO_2}$ is designed by replacing the top Se/Te atomic layer in monolayer InSeO/InTeO with Te/Se atoms, which is dynamically and mechanically stable. More excitingly, Janus monolayer $\mathrm{In_2SeTeO_2}$ is also a 2D topological insulator with sizeable bulk gap up to 0.158 eV, confirming the coexistence of intrinsic piezoelectricity and topological nature. The calculated $d_{11}$ is -9.9 pm/V, which is in the middle of ones of InSeO and InTeO monolayers.

cond-mat.mtrl-sci↗

arXiv2020

Structure effect on intrinsic piezoelectricity in septuple-atomic-layer $\mathrm{MSi_2N_4}$ (M=Mo and W)

The recently experimentally synthesized monolayer $\mathrm{MoSi_2N_4}$ and $\mathrm{WSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020})) lack inversion symmetry, which allows them to become piezoelectric. In this work, based on ab initio calculations, we report structure effect on intrinsic piezoelectricity in septuple-atomic-layer $\mathrm{MSi_2N_4}$ (M=Mo and W), and six structures ($α_i$ ($i$=1 to 6)) are considered with the same space group.It is found that $\mathrm{MSi_2N_4}$ (M=Mo and W) with $α_i$ ($i$=1 to 6) all are indirect band gap semiconductors. Calculated results show that $\mathrm{MoSi_2N_4}$ and $\mathrm{WSi_2N_4}$ monolayers have the same structural dependence on piezoelectric strain and stress coefficients ($d_{11}$ and $e_{11}$), together with the ionic and electronic contributions to $e_{11}$.Finally, we investigate the intrinsic piezoelectricity of monolayer $\mathrm{MA_2Z_4}$ (M=Cr, Mo and W; A=Si and Ge; Z=N and P) with $α_1$ and $α_2$ phases expect $\mathrm{CrGe_2N_4}$, because they all are semiconductors and their enthalpies of formation between $α_1$ and $α_2$ phases are very close. The most important result is that monolayer $\mathrm{MA_2Z_4}$ containing P atom have more stronger piezoelectric polarization than one including N atom. The largest $d_{11}$ among $\mathrm{MA_2N_4}$ materials is 1.85 pm/V, which is close to the smallest $d_{11}$ of 1.65 pm/V in $\mathrm{MA_2P_4}$ monolayers. For $\mathrm{MA_2P_4}$, the largest $d_{11}$ is up to 6.12 pm/V. Among the 22 monolayers, $α_1$-$\mathrm{CrSi_2P_4}$, $α_1$-$\mathrm{MoSi_2P_4}$, $α_1$-$\mathrm{CrGe_2P_4}$, $α_1$-$\mathrm{MoGe_2P_4}$ and $α_2$-$\mathrm{CrGe_2P_4}$ have large $d_{11}$, which are greater than or close to 5 pm/V, a typical value for bulk piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2020

Predicted septuple-atomic-layer Janus $\mathrm{MSiGeN_4}$ (M=Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities

Janus two-dimensional (2D) materials have attracted much attention due to possessing unique properties caused by their out-of-plane asymmetry, which have been achieved in many 2D families. In this work, the Janus monolayers are predicted in new 2D $\mathrm{MA_2Z_4}$ family by means of first-principles calculations, $\mathrm{MoSi_2N_4}$ and $\mathrm{WSi_2N_4}$ of which have been synthesized in experiment(\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020)}). The predicted $\mathrm{MSiGeN_4}$ (M=Mo and W) monolayers exhibit dynamic, thermodynamical and mechanical stability, and they are indirect band-gap semiconductors. The inclusion of spin-orbit coupling (SOC) gives rise to the Rashba-type spin splitting, which is observed in the valence bands, being different from common conduction bands. Calculated results show valley polarization at the edge of the conduction bands due to SOC together with inversion symmetry breaking. It is found that $\mathrm{MSiGeN_4}$ (M=Mo and W) monolayers have high electron mobilities. Both in-plane and much weak out-of-plane piezoelectric polarizations can be observed, when a uniaxial strain in the basal plane is applied. The values of piezoelectric strain coefficient $d_{11}$ of the Janus $\mathrm{MSiGeN_4}$ (M=Mo and W) monolayers fall between those of the $\mathrm{MSi_2N_4}$ (M=Mo and W) and $\mathrm{MGe_2N_4}$ (M=Mo and W) monolayers, as expected. It is proved that strain can tune the positions of valence band maximum (VBM) and conduction band minimum (CBM), and enhance the the strength of conduction bands convergence caused by compressive strain. It is also found that tensile biaxial strain can enhance $d_{11}$ of $\mathrm{MSiGeN_4}$ (M=Mo and W) monolayers, and the compressive strain can improve the $d_{31}$ (absolute values).

cond-mat.mtrl-sci↗

arXiv2020

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer $\mathrm{VSi_2P_4}$

The septuple-atomic-layer $\mathrm{VSi_2P_4}$ with the same structure of experimentally synthesized $\mathrm{MoSi_2N_4}$ is predicted to be a spin-gapless semiconductor (SGS). In this work, the biaxial strain is applied to tune electronic properties of $\mathrm{VSi_2P_4}$, and it spans a wide range of properties upon the increasing strain from ferromagnetic metal (FMM) to SGS to ferromagnetic semiconductor (FMS) to SGS to ferromagnetic half-metal (FMHM). Due to broken inversion symmetry, the coexistence of ferromagnetism and piezoelectricity can be achieved in FMS $\mathrm{VSi_2P_4}$ with strain range of 0\% to 4\%. The calculated piezoelectric strain coefficients $d_{11}$ for 1\%, 2\% and 3\% strains are 4.61 pm/V, 4.94 pm/V and 5.27 pm/V, respectively, which are greater than or close to a typical value of 5 pm/V for bulk piezoelectric materials. Finally, similar to $\mathrm{VSi_2P_4}$, the coexistence of piezoelectricity and ferromagnetism can be realized by strain in the $\mathrm{VSi_2N_4}$ monolayer. Our works show that $\mathrm{VSi_2P_4}$ in FMS phase with intrinsic piezoelectric properties can have potential applications in spin electronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Biaxial strain enhanced piezoelectric properties in monolayer g-$\mathrm{C_3N_4}$

Graphite-like carbon nitride (g-$\mathrm{C_3N_4}$) is considered as a promising candidate for energy materials. In this work, the biaxial strain (-4\%-4\%) effects on piezoelectric properties of g-$\mathrm{C_3N_4}$ monolayer are studied by density functional theory (DFT). It is found that the increasing strain can reduce the elastic coefficient $C_{11}$-$C_{12}$, and increases piezoelectric stress coefficient $e_{11}$, which lead to the enhanced piezoelectric strain coefficient $d_{11}$. Compared to unstrained one, strain of 4\% can raise the $d_{11}$ by about 330\%. From -4\% to 4\%, strain can induce the improved ionic contribution to $e_{11}$ of g-$\mathrm{C_3N_4}$, and almost unchanged electronic contribution, which is different from $\mathrm{MoS_2}$ monolayer (the enhanced electronic contribution and reduced ionic contribution). To prohibit current leakage, a piezoelectric material should be a semiconductor, and g-$\mathrm{C_3N_4}$ monolayer is always a semiconductor in considered strain range. Calculated results show that the gap increases from compressive strain to tensile one. At 4\% strain, the first and second valence bands cross, which has important effect on transition dipole moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric effect of g-$\mathrm{C_3N_4}$ monolayer, which gives a useful guidence for developing efficient energy conversion devices.

cond-mat.mtrl-sci↗

arXiv2020

Intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W)

Motived by experimentally synthesized $\mathrm{MoSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020})), the intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W) are studied by density functional theory (DFT). Among the six monolayers, the $\mathrm{CrSi_2N_4}$ has the best piezoelectric strain coefficient $d_{11}$ of 1.24 pm/V, and the second is 1.15 pm/V for $\mathrm{MoSi_2N_4}$. Taking $\mathrm{MoSi_2N_4}$ as a example, strain engineering is applied to improve $d_{11}$. It is found that tensile biaxial strain can enhance $d_{11}$ of $\mathrm{MoSi_2N_4}$, and the $d_{11}$ at 4\% can improve by 107\% with respect to unstrained one. By replacing the N by P or As in $\mathrm{MoSi_2N_4}$, the $d_{11}$ can be raise substantially. For $\mathrm{MoSi_2P_4}$ and $\mathrm{MoSi_2As_4}$, the $d_{11}$ is as high as 4.93 pm/V and 6.23 pm/V, which is mainly due to smaller $C_{11}-C_{12}$ and very small minus or positive ionic contribution to piezoelectric stress coefficient $e_{11}$ with respect to $\mathrm{MoSi_2N_4}$. The discovery of this piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ enables active sensing, actuating and new electronic components for nanoscale devices, and is recommended for experimental exploration.

cond-mat.mtrl-sci↗