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Register a new userPredicted septuple-atomic-layer Janus $mathrm{MSiGeN_4}$ (M=Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities
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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).
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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 ($alpha_i$ ($i$=1 to 6)) are considered with the same space group.It is found that $mathrm{MSi_2N_4}$ (M=Mo and W) with $alpha_i$ ($i$=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 $alpha_1$ and $alpha_2$ phases expect $mathrm{CrGe_2N_4}$, because they all are semiconductors and their enthalpies of formation between $alpha_1$ and $alpha_2$ 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, $alpha_1$-$mathrm{CrSi_2P_4}$, $alpha_1$-$mathrm{MoSi_2P_4}$, $alpha_1$-$mathrm{CrGe_2P_4}$, $alpha_1$-$mathrm{MoGe_2P_4}$ and $alpha_2$-$mathrm{CrGe_2P_4}$ have large $d_{11}$, which are greater than or close to 5 pm/V, a typical value for bulk piezoelectric materials.
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.
Two-dimensional topological insulators and two-dimensional materials with ferroelastic characteristics are intriguing materials and many examples have been reported both experimentally and theoretically. Here, we present the combination of both features - a two-dimensional ferroelastic topological insulator that simultaneously possesses ferroelastic and quantum spin Hall characteristics. Using first-principles calculations, we demonstrate Janus single-layer MSSe (M=Mo, W) stable two-dimensional crystals that show the long-sought ferroelastic topological insulator properties. The material features low switching barriers and strong ferroelastic signals, beneficial for applications in nonvolatile memory devices. Moreover, their topological phases harbor sizeable nontrivial band gaps, which supports the quantum spin Hall effect. The unique coexistence of excellent ferroelastic and quantum spin Hall phases in single-layer MSSe provides extraordinary platforms for realizing multi-purpose and controllable devices.
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.
The strong spin-orbit interaction in the organic-inorganic perovskites tied to the incorporation of heavy elements (textit{e.g.} Pb, I) makes these materials interesting for applications in spintronics. Due to a lack of inversion symmetry associated with distortions of the metal-halide octahedra, the Rashba effect (used textit{e.g.} in spin field-effect transistors and spin filters) has been predicted to be much larger in these materials than in traditional III-V semiconductors such as GaAs, supported by the recent observation of a near record Rashba spin splitting in CH$_3$NH$_3$PbBr$_3$ using angle-resolved photoemission spectroscopy (ARPES). More experimental studies are needed to confirm and quantify the presence of Rashba effects in the organic-inorganic perovskite family of materials. Here we apply time-resolved circular dichroism techniques to the study of carrier spin dynamics in a 2D perovskite thin film [(BA)$_2$MAPb$_2$I$_7$; BA = CH$_3$(CH$_2$)$_3$NH$_3$, MA = CH$_3$NH$_3$]. Our findings confirm the presence of a Rashba spin splitting via the dominance of precessional spin relaxation induced by the Rashba effective magnetic field. The size of the Rashba spin splitting in our system was extracted from simulations of the measured spin dynamics incorporating LO-phonon and electron-electron scattering, yielding a value of 10 meV at an electron energy of 50 meV above the band gap, representing a 20 times larger value than in GaAs quantum wells.
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