Nonlinear Rashba spin splitting in transition metal dichalcogenide monolayers

The tunable anisotropic Rashba spin-orbit coupling effect in Pb-adsorbed Janus monolayer WSeTe

The spin-splitting properties of Pb-adsorbed monolayer Janus WSeTe are investigated based on first-principles calculations. The adsorbed system shows large Rashba splitting (the Rashba parameter is up to 0.75 eV Å), and we find that different adsorption layers (Te/Se adsorption layers) exhibit different significant features under spin-orbit coupling. Zeeman splitting and Rashba splitting co-exist at the high symmetry Γ point of the Te adsorption layer, while the Se adsorption layer exhibits anisotropic Rashba spin-orbit coupling. It was determined using k·p perturbation theory that Pb atom adsorption reduces the initial symmetry of the 2H-WSeTe monolayer and induces a strong spin-orbit coupling effect, so as to induce the anisotropic Rashba effect. Furthermore, the tunability of Rashba splitting was demonstrated by varying the adsorption concentration, adjusting the adsorption distance, and applying biaxial strain. This predicted adsorption system has potential value in spintronic devices.

Insights into selected 2D piezo Rashba semiconductors for self-powered flexible piezo spintronics: material to contact properties

The new paradigm in electronics consists in realizing the seamless integration of many properties latent in nanomaterials, such as mechanical flexibility, strong spin-orbit coupling (Rashba spin splitting-RSS), and piezoelectricity. Taking cues from the pointers given on 1D ZnO nanowires (ACS Nano2018121811-20), the concept can be extended to multifunctional two-dimensional (2D) materials, which can serve as an ideal platform in next-generation electronics such as self-powered flexible piezo-spintronic device. However, a microscopically clear understanding reachable from the state-of-the-art density functional theory-based approaches is a prerequisite to advancing this research domain. Atomic-scale insights gained from meticulously performed scientific computations can firmly anchor the growth of this important research field, and that is of undeniable relevance from scientific and technological outlooks. This article reviews the scientific advance in understanding 2D materials hosting all the essential properties, i.e. flexibility, piezoelectricity, and RSS. Important 2D semiconducting monolayers that deserve a special mention, include monolayers of buckled MgX (X = S, Se, Te), CdTe, ZnTe, Janus structures of transition metal trichalcogenides, Janus tellurene and 2D perovskites. van Der Waals multilayers are also built to design multifunctional materials via modulation of the stacking sequence and interlayer coupling between the constituent layers. External electric field, strain engineering and charge doping are perturbations mainly used to tune the spintronic properties. Finally, the contact properties of these monolayers are also crucial for their actual implementation in electronic devices. The nature of the contacts, Schottky/Ohmic, needs to be carefully examined first as it controls the device's performance. In this regard, the rare occurrence of Ohmic contact in graphene/MgS van der Waals hetero bilayer has been presented in this review article.

Coupled Spin-Valley, Rashba Effect, and Hidden Spin Polarization in WSi2N4 Family

Using first-principles calculations, we report the electronic properties with a special focus on the band splitting in the WSi₂N₄ class of materials. Due to the broken inversion symmetry and strong spin-orbit coupling, we detect coupled spin-valley effects at the corners of the first Brillouin zone (BZ). Additionally, we observe cubically and linearly split bands around the Γ and M points, respectively. The in-plane mirror symmetry (σ_h) and reduced symmetry of the arbitrary k-point, enforce the persistent spin textures (PST) to occur in full BZ. We induce the Rashba splitting by breaking the σ_h through an out-of-plane external electric field (EEF). The inversion asymmetric site point group of the W atom introduces the hidden spin polarization in centrosymmetric layered bulk counterparts. Low energy k.p models demonstrate that the PST along the M-K line is robust to EEF and layer thickness, making them suitable for applications in spintronics and valleytronics.

Visualizing Giant Ferroelectric Gating Effects in Large-Scale WSe2/BiFeO3 Heterostructures

Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc.) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO₃, a room-temperature multiferroic oxide, and WSe₂, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe₂ is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO₃ films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe₂ and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO₃. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2xalloy

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS_1.4Se_0.6alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of thek-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 ± 0.7 eV Å. The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS_1.4Se_0.6alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this 2D material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

MX family: an efficient platform for topological spintronics based on Rashba and Zeeman-like spin splittings

Taking various combinations of M = (Mo, W) and X = (C, S, Se) as examples, we propose that MX (M = transition metals, X = IV,V or VI elements) family can establish an excellent platform for both conventional and topological spintronics applications based on anisotropic Rashba-like and non-magnetic Zeeman-type spin splittings with electrically tunable nature. In particular, we observe sizeable Zeeman-like and Rashba-like spin splittings with an anisotropic nature. Meanwhile, they exhibit Rashba-like and topologically robust helical edge states when grown in ferroelectric and paraelectric phases, respectively. These MX monolayers are realized to be quantum valley Hall insulators due to valley contrasting Berry curvatures. The carriers in these MX monolayers can be selectively excited from opposite valleys depending on the polarity of circularly polarized light. The amplitude of the spin splitting can be further tuned by applying external means such as strain, electric field or alloy engineering. Furthermore, considering graphene sheet over the WC monolayer as a prototype example, we show that these MX monolayers can boost the relativistic effect by coupling with the systems exhibiting extremely weak spin-orbit coupling (SOC). Depending on the surface of WC monolayer in contact with the graphene sheet, graphene over WC monolayer passes through the transformation from the semiconducting junction to the Shotcky barrier-free contact. Finally, we reveal that these MX monolayers could also be grown on the substrates such as WS2(001)and GaTe (001) with type-II band alignment, where electron and hole become layer splitted across the interface. Our analysis should be fairly applied to other systems with strong SOC and an equivalent geometrical structure to the MX monolayers.

Giant tunable Rashba spin splitting in two-dimensional polar perovskites TlSnX3 (X = Cl, Br, I)

The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX₃ (X = Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effects are found around the Γ point. In particular, TlSnI₃ has the largest Rashba constant of 1.072 eV Å^-1. Additionally, TlSnBr₃ and TlSnI₃ respond strongly to the applied electric field, and the electric field responsivity of TlSnI₃ can reach 0.790 e Ų. Therefore, due to the large Rashba constants and strong electric field responses, these 2D polar perovskites are promising semiconductor materials with short channel lengths. The nano-scale short spin coherence length can keep the spin coherence of spin FETs, which is superior to the traditional 3D micron spin FETs, and will show a broad application prospect in the Rashba semiconductor field.

Electric field and strain engineering tuning Rashba spin splitting in quasi-one-dimensional organic-inorganic hybrid perovskites (MV)AI3Cl2 (MV = methylviologen, A = Bi, Sb)

We systematically study the Rashba spin texture of lead-free quasi-one-dimensional organic-inorganic hybrid perovskites (OIHP), (MV)AI₃Cl₂ (MV = methylviologen, A = Bi, Sb) with first-principles calculations. The k_x-k_y plane Rashba spin splitting was found to depend on the composition of Bi (Sb) and I atoms at band edges. Importantly, increasing ferroelectric polarization and the stretch along the z-direction can effectively enhance the amplitude of the Rashba spin splitting. This work provides an avenue for electric field and strain-controlled spin splitting and highlights the potential of quasi-one-dimensional OIHP for further applications in spin field effect transistors and photovoltaic cells.

Evidence for highly p-type doping and type II band alignment in large scale monolayer WSe2/Se-terminated GaAs heterojunction grown by molecular beam epitaxy

Two-dimensional materials (2D) arranged in hybrid van der Waals (vdW) heterostructures provide a route toward the assembly of 2D and conventional III-V semiconductors. Here, we report the structural and electronic properties of single layer WSe₂ grown by molecular beam epitaxy on Se-terminated GaAs(111)B. Reflection high-energy electron diffraction images exhibit sharp streaky features indicative of a high-quality WSe₂ layer produced via vdW epitaxy. This is confirmed by in-plane X-ray diffraction. The single layer of WSe₂ and the absence of interdiffusion at the interface are confirmed by high resolution X-ray photoemission spectroscopy and high-resolution transmission microscopy. Angle-resolved photoemission investigation revealed a well-defined WSe₂ band dispersion and a high p-doping coming from the charge transfer between the WSe₂ monolayer and the Se-terminated GaAs substrate. By comparing our results with local and hybrid functionals theoretical calculation, we find that the top of the valence band of the experimental heterostructure is close to the calculations for free standing single layer WSe₂. Our experiments demonstrate that the proximity of the Se-terminated GaAs substrate can significantly tune the electronic properties of WSe₂. The valence band maximum (VBM, located at the K point of the Brillouin zone) presents an upshift of about 0.56 eV toward the Fermi level with respect to the VBM of the WSe₂ on graphene layer, which is indicative of high p-type doping and a key feature for applications in nanoelectronics and optoelectronics.

Electrostatic Modulation and Mechanism of the Electronic Properties of Monolayer MoS2 via Ferroelectric BiAlO3(0001) Polar Surfaces

In the present work, first-principles density functional theory calculations were carried out to explore the intrinsic interface coupling and electrostatic modulation as well as the effect of ferroelectric polarization reversal in the MoS₂/BiAlO₃(0001) [MoS₂/BAO(0001)] hybrid system. In addition to the interaction mechanism of the large ionic-van der Waals (vdW) coupling, our results indicate that the electronic properties of monolayer MoS₂ on the BAO(0001) polar surface can be effectively modulated by reversing the ferroelectric polarization and/or engineering the domain structures of the substrate. Due to the unusual charge transfer between the MoS₂ overlayer and the down-polarized ferroelectric BAO(0001) substrate, in the final analysis, the physical mechanism determining the interfacial charge transfer in the MoS₂/BAO(0001) hybrid system is attributed to the specific band alignment between the clean BAO(0001) surface and the freestanding monolayer MoS₂. Furthermore, our study predicts that MoS₂-based ferroelectric field-effect transistors and various types of seamless p-i, n-i, p-n, p⁺-p, and n⁺-n homojunctions possessing an extremely steep built-in electric field can be fabricated by reversing the ferroelectric polarization and/or patterning the domain structure of the BAO(0001) substrate.

Tunable Rashba Spin Splitting in Two-Dimensional Polar Perovskites

Two-dimensional (2D) Rashba semiconductors with structure inversion asymmetry and a spin-orbit coupling (SOC) effect show promising applications in nanospintronics, such as spin field effect transistors (FETs). Here, we systematically investigate the electronic structures and Rashba effect of 2D polar perovskites ABX₃ (A = Cs⁺ or Rb⁺; B = Pb²⁺ or Sn²⁺; X = Cl, Br, or I) by first-principles density functional theory calculations. We demonstrate that, except for the cubic case, 2D polar perovskites from tetragonal and orthorhombic three-dimensional (3D) bulks exhibit a strong intrinsic Rashba effect around the Γ point, due to their structure inversion asymmetry and the strong SOC effect of heavy atoms. In particular, 2D orthorhombic RbSnI₃ shows the largest Rashba constant of 1.176 eV Å among these polar perovskites, which is comparable to that of 3D bulk perovskites previously reported in experiments and theory. Furthermore, several 2D polar perovskites also show a strong electric field response. In particular, 2D tetragonal RbPbI₃ and tetragonal CsPbI₃ have strong electric field responses of >0.5 e Ų. Therefore, 2D polar perovskites as promising Rashba semiconductors possess large Rashba constants and strong electric field responses, resulting in a short spin channel length of tens of nanometers to preserve the spin coherence in spin FETs, superior to conventional 3D micrometer spin FETs.

The electrical and spin properties of monolayer and bilayer Janus HfSSe under vertical electrical field

In this paper, the electrical and spin properties of mono- and bilayer HfSSe in the presence of a vertical electric field are studied. The density functional theory is used to investigate their properties. Fifteen different stacking orders of bilayer HfSSe are considered. The mono- and bilayer demonstrate an indirect bandgap, whereas the bandgap of bilayer can be effectively controlled by the electric field. While the bandgap of bilayer closes at large electric fields and a semiconductor to metal transition occurs, the effect of a normal electric field on the bandgap of the monolayer HfSSe is quite weak. Spin-orbit coupling causes band splitting in the valence band and Rashba spin splitting in the conduction band of both mono- and bilayer structures. The band splitting in the valence band of the bilayer is smaller than a monolayer, however, the vertical electric field increases the band splitting in bilayer one. The stacking configurations without mirror symmetry exhibit Rashba spin splitting which is enhanced with the electric field.

Computational Study of Janus Transition Metal Dichalcogenide Monolayers for Acetone Gas Sensing

Recently, Janus two-dimensional (2D) transition metal dichalcogenides (TMDs) have been widely investigated and have provided exciting prospects in many fields such as photoelectric materials, photocatalysis, and gas sensors. In this study, we performed density functional theory (DFT) calculations to study the sensitivity of four volatile organic compounds (VOCs), including acetone, methanol, ethanol, and formyl aldehyde, over pristine 2D TMDs and 2D Janus TMD monolayers. We found that MoS2, Janus MoSSe, and Janus MoSTe demonstrated greater sensitivity toward acetone than other VOCs. Furthermore, the band gap values of the Janus MoSSe and Janus MoSTe monolayers dramatically changed after acetone adsorption on their sulfur layers, which was quite larger than the band gap change after acetone adsorption on the MoS2 monolayer. This result also leads to the extremely large conductivity change of Janus MoSSe and Janus MoSTe after sensing acetone. Hence, Janus MoSSe and Janus MoSTe monolayers show much higher sensitivity toward acetone in comparison with the pristine MoS2 monolayer. Finally, our finding indicates that Janus MoSSe and Janus MoSTe monolayers can be proposed as ultrahigh-sensitivity 2D TMD materials for acetone sensors.

Dipole control of Rashba spin splitting in a type-II Sb/InSe van der Waals heterostructure

InSe monolayer, belonging to group III-VI chalcogenide family, has shown promising performance in the realm of spintronic. Nevertheless, the out-of-plane mirror symmetry in InSe monolayer constrains the electrons' degrees of freedom, and this will confine its spin-related applications. Herein, we construct Sb/InSe van der Waals heterostructure to extend the electronic and spintronic properties of InSe. The density functional theory is utilized to verify the tunable electronic properties and Rashba spin splitting (RSS) of Sb/InSe heterostructure. According to the obtained results, the Sb/InSe heterostructure can be considered as a direct band gap semiconductor with typical type-II band alignment, where the electrons and holes are localized in the InSe and Sb layers, respectively. The RSS is recognized at conduction band minimum around Γ point in Sb/InSe, which is induced by the spontaneous internal electric field with electric dipole moment of 0.016 e Å from Sb to InSe. The vertical strain, in-plane strain, and external electric field are employed to modulate the strength of RSS. The Rashba coefficient and dipole moment exhibit the similar variation tendency, suggesting the strength of RSS depends on the magnitude of dipole moment. The controllable RSS makes Sb/InSe heterostructure become an appropriate candidate material for spintronic devices.

Enhancement of van der Waals Interlayer Coupling through Polar Janus MoSSe

Interlayer coupling plays essential roles in the quantum transport, polaritonic, and electrochemical properties of stacked van der Waals (vdW) materials. In this work, we report the unconventional interlayer coupling in vdW heterostructures (HSs) by utilizing an emerging 2D material, Janus transition metal dichalcogenides (TMDs). In contrast to conventional TMDs, monolayer Janus TMDs have two different chalcogen layers sandwiching the transition metal and thus exhibit broken mirror symmetry and an intrinsic vertical dipole moment. Such a broken symmetry is found to strongly enhance the vdW interlayer coupling by as much as 13.2% when forming MoSSe/MoS₂ HS as compared to the pristine MoS₂ counterparts. Our noncontact ultralow-frequency Raman probe, linear chain model, and density functional theory calculations confirm the enhancement and reveal the origins as charge redistribution in Janus MoSSe and reduced interlayer distance. Our results uncover the potential of tuning interlayer coupling strength through Janus heterostacking.

Remarkable Rashba spin splitting induced by an asymmetrical internal electric field in polar III-VI chalcogenides

Herein, the Rashba spin orbit coupling (SOC) of polar group III-VI chalcogenide XABY (A, B = Ga, In; X ≠ Y = S, Se, Te) monolayers is investigated based on density functional theory. The different electronegativities of X and Y atoms lead to an asymmetrical internal electric field in the XABY monolayer; this implies that the internal electric field between A and X is not equal to that between B and Y. Mirror symmetry breaking in the XABY monolayer induces a remarkable Rashba spin splitting (RSS) at the conduction band minimum (CBM). Moreover, it is demonstrated that an external electric field and an in-plane biaxial strain can affect the internal electric field by varying the charge distribution, and this further manipulates the RSS. Under a positive external electric field and tensile strain, the RSS at the CBM exhibits a near-linear increasing behavior, whereas under a negative external electric field and compressive strain, the RSS displays a monotonous decreasing pattern. In addition, we explored the influence of interlayer coupling and substrate on the RSS. The stacking pattern of bilayer structures has a significant impact on the RSS. The investigation of SInGaSe on the Si(111) substrate suggests that the Rashba band is situated inside the large band gap of the substrate. Overall, our investigations suggest that the polar group III-VI chalcogenides are promising candidates for future spintronic applications.

Electric field control of Rashba spin splitting in 2D NIIIXVI (N = Ga, In; X = S, Se, Te) monolayer

Spin splitting of the nonmagnetic two dimensional (2D) layered N^IIIX^VI (N  =  Ga, In; X  =  S, Se, Te) monolayer is investigated based on the density functional theory. Due to the mirror symmetry, there is no Rashba spin splitting (RSS) in the freestanding NX plane. It is found that applying the external electric field perpendicular to the NX plane can result in sizable RSS around the Γ point due to the mirror symmetry breaking. The induced RSS is mainly influenced by the anions X and gradually strengthens with the increase of external electric field. The considerable RSS is observed in NTe systems. Moreover, the influence of in-plane biaxial strain on RSS is explored, and the tensile strain can enhance the RSS, especially for those bands around the Fermi level. Our theoretical investigation provides a deep insight in spin splitting behaviors in NX monolayers and agrees well with the experimental report.

Janus TiXY Monolayers with Tunable Berry Curvature

Up to now, two-dimensional (2D) materials with both valley polarization and the Rashba effect are still rare. In this work, a new kind of Janus monolayers TiXY (X ≠ Y, X/Y = Cl, Br, I) is demonstrated to have physical properties of benefit for spintronics and valleytronics. In particular, Janus TiBrI shows Zeeman-type spin splitting of 70 meV, large Berry curvature of 106.22 bohr², and, at the same time, a large Rashba parameter of 147.95 meV Å. On the basis of k·p perturbation theory, we proposed that the Berry curvature can be adjusted by changing the lattice parameter, which will greatly improve the transverse velocities of carriers and promote the efficiency of the valley Hall device. Biaxial strain from -2.5 to 2.5% was applied on Janus TiBrI to verify the theory mentioned above, and a general relationship between the Berry curvature and lattice constant was obtained.

Investigation of atomic and electronic properties of 2D-MoS2/3D-GaN mixed-dimensional heterostructures

We have performed density functional theory calculations to study the effects caused by the interfacial structure between 2D-MoS₂ and 3D-GaN. Two different surface terminations of GaN are considered: Ga-terminated (0001) (Ga-GaN) and N-terminated ([Formula: see text]) (N-GaN) configurations. We confirm that Rashba spin splitting occurs in band structure of MoS₂ on GaN. We also find that the surface states of GaN move to the deep position in band structure in the MoS₂/Ga-GaN case, while the surface states of GaN are hybridized with MoS₂ near the Fermi level for the MoS₂/N-GaN case. Furthermore, we investigate the variation in electronic structure of MoS₂/GaN heterostructures depending on the number of MoS₂ layers. Especially, the top layer MoS₂ of the 2L-MoS₂/GaN structures shows both n-type and p-type properties depending on the GaN surface termination. As a result, we suggest that the electrical characteristics of the 2D/3D heterostructures could be controlled by the surface terminations of substrate materials.

Evidence of Topological Edge States in Buckled Antimonene Monolayers

Two-dimensional topological materials have attracted intense research efforts owing to their promise in applications for low-energy, high-efficiency quantum computations. Group-VA elemental thin films with strong spin-orbit coupling have been predicted to host topologically nontrivial states as excellent two-dimensional topological materials. Herein, we experimentally demonstrated for the first time that the epitaxially grown high-quality antimonene monolayer islands with buckled configurations exhibit significantly robust one-dimensional topological edge states above the Fermi level. We further demonstrated that these topologically nontrivial edge states arise from a single p-orbital manifold as a general consequence of atomic spin-orbit coupling. Thus, our findings establish monolayer antimonene as a new class of topological monolayer materials hosting the topological edge states for future low-power electronic nanodevices and quantum computations.

Spin-Orientation-Dependent Topological States in Two-Dimensional Antiferromagnetic NiTl2S4 Monolayers

The topological states of matter arising from the nontrivial magnetic configuration provide a better understanding of physical properties and functionalities of solid materials. Such studies benefit from the active control of spin orientation in any solid, which is known to take place rarely in the two-dimensional (2D) limit. Here we demonstrate by the first-principles calculations that spin-orientation-dependent topological states can appear in the geometrically frustrated monolayer antiferromagnet (AFM). Different topological states including the quantum anomalous Hall (QAH) effect and time-reversal-symmetry (TRS) broken quantum spin Hall (QSH) effect can be obtained by changing the spin orientation in the NiTl₂S₄ monolayer. Remarkably, the dilated nc-AFM NiTl₂S₄ monolayer gives birth to the QAH effect with the hitherto reported largest number of quantized conducting channels (Chern number [Formula: see text] = -4) in 2D materials. Interestingly, under tunable chemical potential, the nc-AFM NiTl₂S₄ monolayer hosts a novel state supporting the coexistence of QAH and TRS broken QSH effects with a Chern number of [Formula: see text] = 3 and a spin Chern number of [Formula: see text] = 1. This work manifests a promising concept and material realization of topological spintronics in 2D antiferromagnets by manipulating their spin degree of freedom.

Anomalous Hall Effect in 2D Dirac Materials

We present a unified theory of charge carrier transport in 2D Dirac systems with broken mirror inversion and time-reversal symmetries (e.g., as realized in ferromagnetic graphene). We find that the entanglement between spin and pseudospin SU(2) degrees of freedom stemming from spin-orbit effects leads to a distinctive gate voltage dependence (change of sign) of the anomalous Hall conductivity approaching the topological gap, which remains robust against impurity scattering and thus is a smoking gun for magnetized 2D Dirac fermions. Furthermore, we unveil a robust skew scattering mechanism, modulated by the spin texture of the energy bands, which causes a net spin accumulation at the sample boundaries even for spin-transparent disorder. The newly unveiled extrinsic spin Hall effect is readily tunable by a gate voltage and opens novel opportunities for the control of spin currents in 2D ferromagnetic materials.

Substrate modified thermal stability of mono- and few-layer MoS2

Two-dimensional semiconducting transition metal dichalcogenides have been employed as key components in various electronic devices. The thermal stability of these ultrathin materials must be carefully considered in device applications because the heating caused by current flow, light absorption, or other harsh environmental conditions is usually unavoidable. In this work, we found that the substrate plays a role in modifying the thermal stability of mono- and few-layer MoS₂. Triangular etching holes, which are considered to initiate from defect sites, form on MoS₂ when the temperature exceeds a threshold. On Al₂O₃ and SiO₂, monolayer MoS₂ is found to be more stable in thermal annealing than few-layer MoS₂ either in atmospheric-pressure air or under vacuum; while on mica, the absolute opposite behavior exists. However, this difference due to substrates appears to vanish when using defective, chemical-vapor-deposited MoS₂ samples. The substrate modification of the thermal stability of MoS₂ with various thicknesses is attributed to the competition between MoS₂-substrate interface interaction and MoS₂-MoS₂ interlayer interaction. Our findings provide important design rules for MoS₂-based devices, and also potentially point to a route of controlled patterning of MoS₂ with substrate engineering.

Hidden spin polarization in the 1T-phase layered transition-metal dichalcogenides MX2 (M = Zr, Hf; X = S, Se, Te)

The recent discovery of hidden spin polarization emerging in layered materials of specific nonmagnetic crystal is a fascinating phenomenon, though hardly explored yet. Here, we have studied hidden spin textures in layered nonmagnetic 1T-phase transition-metal dichalcogenides MX₂ (M = Zr, Hf; X = S, Se, Te) by using first-principles calculations. Spin-layer locking effect, namely, energy-degenerate opposite spins spatially separated in the top and bottom layer respectively, has been identified. In particular, the hidden spin polarization of β-band can be easily probed, which is strongly affected by the strength of spin-orbit coupling. The hidden spin polarization of ξ-band locating at high symmetry M point (conduction band minimum) has a strong anisotropy. In the bilayer, the hidden spin polarization is preserved at the upmost Se layer, while being suppressed if the ZrSe₂ layer is taken as the symmetry partner. Our results on hidden spin polarization in 1T-phase dichalcogenides, verifiable by spin-resolved and angle-resolved photoemission spectroscopy (ARPES), enrich our understanding of spin physics and provide important clues to search for specific spin polarization in two dimensional materials for spintronic and quantum information applications.

Ferroelectric control of Rashba spin orbit coupling at the GeTe(111)/InP(111) interface

GeTe is a prototypical compound of a new class of multifunctional materials, i.e., ferroelectric Rashba semiconductors (FRS). In the present work, by combining the first-principles calculations and Rashba model analysis, we reexamine Rashba spin-orbit coupling (SOC) in a GeTe(111) crystal and clarify its linear Rashba SOC strength. We further investigate Rashba SOC at the interface of a GeTe(111)/InP(111) superlattice and demonstrate the ferroelectric manipulation of Rashba SOC in detail. A large modulation of Rashba SOC is obtained, and surprisingly, we find that Rashba SOC does not monotonically increase with the increase of ferroelectric displacement, due to the parabola opening reversal of Rashba splitting bands. In addition, a reversal of the spin texture is realized by tuning the ferroelectric polarization. Our investigation provides a deep insight into the ferroelectric control of Rashba SOC, which is of great importance in FRS spin field effect transistors.