Transition metal dichalcogenides (TMDs) are an exciting family of 2D materials; a member of this family, MoS$_2$, became the first measured monolayer semiconductor. In this article, a generalized phenomenological continuum model for the optical vibrations of the monolayer TMDs valid in the long-wavelength limit is developed. Non-polar oscillations involve differential equations for the phonon displacement vector that describe phonon dispersion up to a quadratic approximation. On the other hand, the polar modes satisfy coupled differential equations for the displacement vectors and the inner electric field. The two-dimensional phonon dispersion curves for in-plane and out-of-plane oscillations are thoroughly analyzed. This model provides an efficient approach to obtain the phonon dispersion curves at the $Γ$-point of the Brillouin zone of the whole family of TMD monolayers. The model parameters are fitted from density functional perturbation theory calculations. A detailed evaluation of the intravalley Pekar-Fröhlich (P-F) and the $A_1$-homopolar mode deformation potential (Dp) coupling mechanisms is performed. The effects of metal ions and chalcogen atoms on polaron mass and binding energy are studied, considering these two contributions, the short-range Dp and P-F. It is argued that both mechanisms must be considered for a correct analysis of the polaron properties.
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 materials and their heterostructures have opened up new possibilities for magnetism at the nanoscale. In this study, we utilize first-principles simulations to investigate the structural, electronic, and magnetic properties of $\textrm{Fe}/\textrm{WSe}_2/\textrm{Pt}$ systems containing pristine, defective, or doped $\textrm{WSe}_2$ monolayers. The proximity effects of the ferromagnetic Fe layer are studied by considering defective and vanadium-doped $\textrm{WSe}_2$ monolayers. All heterostructures are found to be ferromagnetic, and the insertion of the transition-metal dichalc…
We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit-torque memories. Using first principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling …
Tunneling light-emitting devices (LEDs) based on transition metal dichalcogenides (TMDs) and other 2D materials are a new platform for on-chip optoelectronic integration. Some of the physical processes underlying this LED architecture are not fully understood, especially the emission at photon energies higher than the applied electrostatic potential, so-called overbias emission. Here we report overbias emission for potentials that are near half of the optical bandgap energy in TMD-based tunneling LEDs. We show that this emission is not thermal in nature, but consistent with exciton generation …
We investigate the spin-nonconserving relaxation channel of excitons by their couplings with phonons in two-dimensional transition metal dichalcogenides using $\textit{ab initio}$ approaches. Combining $\text{GW}$-Bethe-Salpeter equation method and density functional perturbation theory, we calculate the electron-phonon and exciton-phonon coupling matrix elements for the spin-flip scattering in monolayer WSe$_{\text{2}}$, and further analyze the microscopic mechanisms influencing these scattering strengths. We find that phonons could produce effective in-plane magnetic fields which flip spin o…
We present a comprehensive theoretical analysis of composite excitonic states in doped transition-metal dichalcogenide monolayers. Making use of the pair distribution function, we introduce a method to include the effect of screening in the interaction between the excitonic state and Fermi-sea electrons. Employing the screened potential, we study tetrons and pexcitons in which a trion is bound to one and two holes in the conduction band, respectively. A conduction-band hole denotes the lack of electrons with certain quantum numbers at the vicinity of the trion. We then analyze the hexciton com…
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…
The multivalley band structure of monolayer transition metal dichalcogenides (TMDs) gives rise to intravalley and intervalley excitons. Much knowledge of these excitons has been gained, but fundamental questions remain, such as how to describe them all in a unified picture with their correlations, how are those from different valleys coupled to form the intervalley biexciton? To address the issues, we derive an exciton Hamiltonian from interpair correlations between the constituent carriers-fermions of two excitons. Identifying excitons by irreducible representations of their point symmetry gr…
Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolv…
We study theoretically effects of an anisotropic elastic strain on the exciton energy spectrum fine structure and optical selection rules in atom-thin crystals based on transition-metal dichalcogenides. The presence of strain breaks the chiral selection rules at the $\bm K$-points of the Brillouin zone and makes optical transitions linearly polarized. The orientation of the induced linear polarization is related to the main axes of the strain tensor. Elastic strain provides an additive contribution to the exciton fine structure splitting in agreement with experimental evidence obtained from un…
Nonlinear Optical Properties of Transition Metal Dichalcogenide MX$_2$ (M = Mo, W; X = S, Se) Monolayers and Trilayers from First-principles Calculations
The electron valley and spin degree of freedom in monolayer transition-metal dichalcogenides can be manipulated in optical and transport measurements performed in magnetic fields. The key parameter for determining the Zeeman splitting, namely the separate contribution of the electron and hole g-factor, is inaccessible in most measurements. Here we present an original method that gives access to the respective contribution of the conduction and valence band to the measured Zeeman splitting. It exploits the optical selection rules of exciton complexes, in particular the ones involving inter-vall…
Charged excitons (trions) are essential for the optical spectra in low dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of the three-body Hamiltonian, we explore the low-lying trion states in four types of TMDC MLs. We show that the trions fine structure results from the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction between the composing particles. We demonstrate that by variations of the doping and dielectric environment, trion energy fine structure can be tuned, leading …
We have developed an ab-initio-based fully-quantum numerically-accurate methodology for the simulation of the exciton dynamics and time- and frequency-resolved fluorescence spectra of the cavity-controlled two-dimensional materials at finite temperature and applied this methodology to the single-layer WSe2 system. This allowed us to establish dynamical and spectroscopic signatures of the polaronic and polaritonic effects as well as uncover their characteristic timescales in the relevant range of temperatures.
Magnetism of Ta Dichalcogenide Monolayers Tuned by Strain and Hydrogenation
本文用密度泛函理论研究了应变和氢化对Ta基二硫属化合物（TaX_2; X = S，Se，Te）单分子膜的电磁性质的影响。我们预测了一个复杂的情况下，应变相关的磁相变涉及顺磁，铁磁，和调制反铁磁状态。用氢覆盖两个硫族化物表面之一，将反铁磁/反铁磁TaX 2单层转换为半导体。我们的研究为未来的光电子学和自旋电子学应用开辟了操纵磁特性的新途径。
Doped transition-metal dichalcogenides monolayers exhibit exciting magnetic properties for the benefit of two-dimensional spintronic devices. Using density functional theory (DFT) incorporating Hubbard-type of correction (DFT$+U$) to account for the electronic correlation, we study the magnetocrystalline anisotropy energy (MAE) characterizing Mn-doped MS$_2$ (M=Mo, W) monolayers. A single isolated Mn dopant exhibits a large perpendicular magnetic anisotropy of 35 meV (8 meV) in the case of Mn-doped WS$_2$ (MoS$_2$) monolayer. This value originates from the Mn in-plane orbitals degeneracy lifti…
We investigate the magnetic phase diagram of 1T-vanadium dichalcogenide monolayers in Janus configuration (VSeTe, VSSe, and VSTe) from first principles. The magnetic exchange, magnetocrystalline anisotropy and Dzyaloshinskii-Moriya interaction (DMI) are computed using density functional theory calculations, while the temperature- and field-dependent magnetic phase diagram is simulated using large-scale atomistic spin modeling in the presence of thermal fluctuations. The boundaries between magnetic ordered phases and paramagnetic phases are determined by cross-analyzing the average topological …
