In a multilayer comprising ferromagnet and heavy metal, in-plane carrier spin is induced by applied electric current owing to Rashba spin-orbit coupling, while the out-of-plane component is absent. We propose the out-of-plane carrier spin can emerge in ferromagnetic transition-metal dichalcogenides monolayer, by symmetry arguments and first-principles calculations. An intrinsic spin-orbit coupling in the monolayer provides valley-contrasting Zeeman-type spin splitting for generating the vertical induced spin. The current direction can be exploited to tune the induced spin, accompanied with valley polarization. The exotic spin accumulation paves an accessible way for perpendicular magnetization switching and electric control of valleys.
High Intrinsic Mobility and Ultrafast Carrier Dynamics in Multilayer Metal Dichalcogenide MoS2
在金属二硫属化物中载流子迁移率的最终限制，以及与载流子弛豫相关的动力学，尚不清楚。我们提出了测量的多层dichalcogenide二硫化钼的频率依赖性的电导率的光泵太赫兹探测光谱。我们发现这种材料在低温下的迁移率接近4200 cm 2/Vs。散射的温度依赖性表明，迁移率，一个数量级大于以前报道的二硫化钼，是固有的限制在太赫兹频率的声学声子散射。我们的测量载流子弛豫揭示皮秒冷却时间，然后再复合持续数十纳秒，并占主导地位的俄歇散射到缺陷。我们的研究结果提供了一个有用的背景下，了解和评估的二硫化钼为基础的电子和光电子器件的性能。
Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using \textit{ab-initio} methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence ba…
The problem of screening the Coulomb interaction between charge carriers in type II heterostructures based on transition metal dichalcogenides is Analytically solved. At a sufficiently high density of charge carriers, the density dependence of the interlayer exciton energy is obtained. The energy of the interlayer exciton tends to zero in the metal--insulator transition point. The presented scheme of calculations makes it possible to find the temperature dependence of the density of this transition.
Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are a new class of materials with unique physical properties. However, the carrier mobility of most Janus TMDs calculated by deformation potential theory (DPT) is not reliable due to the unconsidered part of lattice scattering. In this work, we propose a new method of Born effective charge (BEC) to calculate the carrier mobility of Janus TMDs by including the important factors that neglected in the DPT. The BEC could be used in the calculation of both pure and defective Janus TMDs by employing density functional perturbation th…
Ultrafast Spin Injection in Graphene via Dynamical Carrier Filtering at Transition Metal Dichalcogenide Interfaces
Kansai Institute for Photon Science, National Institutes for Quantum Science and Technology (QST)（光子科学关西研究所，量子科学技术研究开发机构）；Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo（东京大学大学院工学系研究科核工程与管理学专业）
Electron valleys in transition-metal dichalcogenide monolayers drive novel physics and allow designing multifunctional architectures for applications. We propose to manipulate the electron valleys in these systems for spin/valley filter and valve devices through band engineering. Instead of the magnetic proximity effect that has been extensively used in previous studies, in our strategy, the electron valleys are directly coupled to the spin-polarized states of the two-dimensional ferromagnets. We find that this coupling results in a valley-selective gap opening due to the spin-momentum locking…
Two-dimensional transition metal dichalcogenide (2D-TMD) monolayers, which reveal remarkable semiconductor properties, are the subject of active experimental research.Recently it has been shown experimentally that quantum yield in MoS2 and WSe2 monoatomic layers can reach values close to unity when electrostatic doping makes them intrinsic semiconductors. However, the available theoretical description does not give an understanding of the physical mechanisms underlying in the gate voltage control of quantum yield.This work is an attempt to propose a consistent semi-phenomenological theory of p…
We discuss the effect of the dielectric environment (insulators and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (`…
The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS$_2$ and WS$_2$ on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9-2.3 eV bet…
First-principles theory of field-effect doping in transition-metal dichalcogenides: Structural properties, electronic structure, Hall coefficient, and electrical conductivity
The objective of this work is to study the effects of charge redistribution, applied layer-normal electric fields, applied strain, and layer engineering on the band alignment of Black Phosphorus (BP)/Molybdenum disulphide (MoS2) heterostructure through Density Functional Theory (DFT) simulations. Black phosphorus works as a p-type material with high mobility, mechanical flexibility, and sensitivity to number of layers. Combining it with the more electronegative material, MoS2 results in strong carrier confinement and a Type II heterostructure. Charge redistribution among the layers shifts the …
Exploring novel two-dimensional multiferroic materials that can realize electric-field control of two-dimensional magnetism has become an emerging topic in spintronics. Using first-principles calculations, we demonstrate that non-metallic bilayer transition metal dichalcogenides (TMDs) can be an ideal platform for building multiferroics by intercalated magnetic atoms. Moreover, we unveil that with Co intercalated bilayer MoS2, Co(MoS2)2, two energetic degenerate states with opposite chirality of Dzyaloshinskii-Moriya interaction (DMI) are the ground states, indicating electric-field control of…
Electronic interactions associated with atomic adsorbates on transition metal dichalcogenides such as MoS2 can induce massive electronic reconstruction that results in the formation of a new type of heavily doped transition metal dichalcogenide that exhibits characteristics of a two-dimensional electron gas. The impact of quantum confinement and reduced dimensionality on the carrier dynamics in such two-dimensional systems is at present not known, but is of paramount importance if they are to find application in optoelectronic devices. Here we show by a combination of angle-resolved photoemiss…
Excitons versus electron-hole plasma in monolayer transition metal dichalcogenide semiconductors
当电子-空穴对在半导体中被激发时，先验地不清楚它们是否形成未结合粒子的费米子等离子体或玻色子激子气体。通常，激子相与低温相关联。在原子薄的过渡金属二硫属化物半导体中，激子由于强库仑相互作用和大的激子态密度而即使在室温下也是特别重要的。使用国家的最先进的多体理论，包括动态屏蔽，我们表明，激子与等离子体的比例可以有效地调整介电基板屏蔽以及载流子掺杂。此外，我们预测莫特过渡从激子占主导地位的制度完全电离的电子-空穴等离子体之间的激发密度$3\times10^{12}$ cm $^{-2}$和$1\times10^{13}$ cm $^{-2}$取决于温度，载流子掺杂和介电环境。我们建议通过研究激子卫星光电子能谱和扫描隧道显微镜观察这些影响。
We examine the full static non-linear optical response of uniaxially strained transition metal dichalcogenide monolayers doped with a finite carrier density in the conduction band, in the presence of disorder. We find that the customary shift current is suppressed, yet we identify a strong, valley-dependent non-reciprocal response, which we term a \textit{unidirectional valley-contrasting photo-current} (UVCP). This DC current originates from the combined effect of strain and Kramers symmetry breaking by trigonal warping, while the contributions due to individual valleys can be separated by in…
Over the last few years, $ab~initio$ methods have become an increasingly popular tool to evaluate intrinsic carrier transport properties in 2D materials. The lack of experimental information, and the progress made in the development of DFT tools to evaluate electronic band structures, phonon dispersions, and electron-phonon scattering matrix-elements, have made them a favored choice. However, a large discrepancy is observed in the literature among the $ab~initio$ calculated carrier mobility in 2D materials. Some of the discrepancies are a result of the physical approximations made in calculati…
Recently, signatures of nonlinear Hall effects induced by Berry-curvature dipoles have been found in atomically thin 1T'/Td-WTe$_2$. In this work, we show that in strained polar transition-metal dichalcogenides(TMDs) with 2H-structures, Berry-curvature dipoles created by spin degrees of freedom lead to strong nonlinear Hall effects. Under an easily accessible uniaxial strain of order 0.2%, strong nonlinear Hall signals, characterized by a Berry-curvature dipole on the order of 1Å, arise in electron-doped polar TMDs such as MoSSe, and this is easily detectable experimentally. Moreover, the magn…
The transport and optical properties of semiconducting transition metal dichalcogenides around room temperature are dictated by electron-phonon scattering mechanisms within a complex, spin-textured and multi-valley electronic landscape. The relative positions of the valleys are critical, yet they are sensitive to external parameters and very difficult to determine directly. We propose a first-principle model as a function valley positions to calculate carrier mobility and Kerr rotation angles. The model brings valuable insights, as well as quantitative predictions of macroscopic properties for…
Here, we describe synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than in TMDs grown by a single-step flux technique. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/Vs and low-temperature disorder-limited mobility above 44,000 cm2/Vs. Electrical transport measurements of graphene-WSe2 heterostru…
Ionic liquid gated field-effect transistors (FETs) based on semiconducting transition metal dichalcogenides (TMDs) are used to study a rich variety of extremely interesting physical phenomena, but important aspects of how charge carriers are accumulated in these systems are not understood. We address these issues by means of a systematic experimental study of transport in monolayer MoSe$_2$ and WSe$_2$ as a function of magnetic field and gate voltage, exploring accumulated densities of carriers ranging from approximately 10$^{14}$ cm$^{-2}$ holes in the valence band to 4x10$^{14}$ cm$^{-2}$ el…
Generation of photocurrents in semiconducting materials requires dissociation of excitons into free charge carriers. While thermal agitation is sufficient to induce dissociation in most bulk materials, an additional push is required to induce efficient dissociation of the strongly bound excitons in monolayer transition-metal dichalcogenides (TMDs). Recently, static in-plane electric fields have proven to be a promising candidate. In the present paper, we introduce a numerical procedure, based on exterior complex scaling, capable of computing field-induced exciton dissociation rates for a wider…
Van der Waals (vdW) materials have greatly expanded our design space of heterostructures by allowing individual layers to be stacked at non-equilibrium configurations, for example via control of the twist angle. Such heterostructures not only combine characteristics of the individual building blocks, but can also exhibit emergent physical properties absent in the parent compounds through interlayer interactions. Here we report on a new family of emergent, nanometer-thick, semiconductor 2D ferroelectrics, where the individual constituents are well-studied non-ferroelectric monolayer transition …
Electron-phonon scatterings in solid-state systems are pivotal processes in determining many key physical quantities such as charge carrier mobilities and thermal conductivities. Here, we report on the direct probing of phonon mode specific electron-phonon scatterings in layered semiconducting transition metal dichalcogenides WSe2, MoSe2, WS2, and MoS2 through inelastic electron tunneling spectroscopy measurements, quantum transport simulations, and density functional calculation. We experimentally and theoretically characterize momentum-conserving single- and two-phonon electron-phonon scatte…
Motivated by recent observations of unconventional out-of-plane dampinglike torque in \ch{WTe2}/Permalloy bilayer systems, we calculate the spin-orbit torque generated in two-dimensional transition metal dichalcogenide (TMD)-ferromagnet heterostructures using first-principles methods and linear response theory. Our numerical calculation of spin-orbit torques in \ch{WTe2}/Co and \ch{MoTe2}/Co heterostructures shows both conventional and novel dampinglike torkances (torque per electric field) with comparable magnitude, around $100~\hbar/2e~(\rm Ω\cdot cm)^{-1}$, for an electric field applied per…
The development of nanomaterials with a large nonlinear susceptibility is essential for nonlinear nanophotonics. We show that transition-metal-dichalcogenide (TMD) nanotriangles have a large effective second-order susceptibility [$χ^{(2)}$] at mid-infrared to near-infrared frequencies owing to their broken centrosymmetry. $χ^{(2)}$ is calculated within the density-matrix formalism that accounts for dissipation and screening. $χ^{(2)}$ peaks in the vicinity of both two-photon resonances (specified by the geometry) and plasmon resonances (tunable via the carrier density). Aligning the resonances…
TMDs have recently been spotlighted due to their original features. Firstly, they have layered structures with Van der Waals interactions. Secondly, they have different phases, affecting a degree of anisotropy. Finally, they have excellent carrier mobilities. In this review, based on these characteristics, this journal shows how TMDs can be applied in a wide variety of industries. The main applications that will be treated is biomedical and electronic applications.
Layered transition metal dichalcogenides (TMDs) offer many attractive features for next-generation low-dimensional device geometries. Due to the practical and fabrication challenges related to in situ methods, the atomistic dynamics that give rise to realizable macroscopic device properties are often unclear. In this study, in situ transmission electron microscopy techniques are utilized in order to understand the structural dynamics at play, especially at interfaces and defects, in the prototypical film of monolayer MoS2 under electrical bias. Through our sample fabrication process, we clearl…
We developed a Landau type theory for the description of polar phenomena in low-dimensional transition metal dichalcogenides (TMDs), specifically exploring flexoelectric origin of the polarization induced by a spontaneous bending and by inversion symmetry breaking due to the interactions with substrate. We consider the appearance of the spontaneous out-of-plane polarization due to the flexoelectric coupling with the strain gradient of the spontaneous surface rippling and surface-induced piezoelectricity. Performed calculations proved that the out-of-plane spontaneous polarization, originated f…
