Tunable Spin-Polarized Edge Currents in Proximitized Transition Metal Dichalcogenides

Strain-Dependent Band Splitting and Spin-Flip Dynamics in Monolayer WS2

Triggered by the expanding demands of semiconductor devices, strain engineering of two-dimensional transition metal dichalcogenides (TMDs) has garnered considerable research interest. Through steady-state measurements, strain has been proved in terms of its modulation of electronic energy bands and optoelectronic properties in TMDs. However, the influence of strain on the spin-orbit coupling as well as its related valley excitonic dynamics remains elusive. Here, we demonstrate the effect of strain on the excitonic dynamics of monolayer WS₂ via steady-state fluorescence and transient absorption spectroscopy. Combined with theoretical calculations, we found that tensile strain can reduce the spin-splitting value of the conduction band and lead to transitions between different exciton states via spin-flip mechanism. Our findings suggest that the spin-flip process is strain-dependent, provides a reference for application of valleytronic devices, where tensile strain is usually existing during their design and fabrication.

Realization of quasi-1D topological magnetism at the V-alloyed MoS2 zigzag edge

Topological magnetism in quasi-1D systems can be interesting because of the significant quantum confinement. However, the realization is missing. In this letter, we propose the use of 3× periodicities related edge reconstructions of MoS₂ zigzag edges to construct a topological quasi-1D spin chain. Specifically, a trimer Su-Schrieffer-Heeger model can be applied to illustrate the topological and spin order when the inter-cell hopping integral is larger than the intra-cell ones. As a result, topological ferromagnetic order is achieved for S-oriented edge states magnetized by V atoms and confirmed by first-principles calculations and Wannier functions analysis. Finally, gap opening and spin-polarized end states are realized.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS2

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Strain-Modulated Magnetism in MoS2

Since the experiments found that two-dimensional (2D) materials such as single-layer MoS2 can withstand up to 20% strain, strain-modulated magnetism has gradually become an emerging research field. However, applying strain alone is difficult to modulate the magnetism of single-layer pristine MoS2, but applying strain combined with other tuning techniques such as introducing defects makes it easier to produce and alter the magnetism in MoS2. Here, we summarize the recent progress of strain-dependent magnetism in MoS2. First, we review the progress in theoretical study. Then, we compare the experimental methods of applying strain and their effects on magnetism. Specifically, we emphasize the roles played by web buckles, which induce biaxial tensile strain conveniently. Despite some progress, the study of strain-dependent MoS2 magnetism is still in its infancy, and a few potential directions for future research are discussed at the end. Overall, a broad and in-depth understanding of strain-tunable magnetism is very necessary, which will further drive the development of spintronics, straintronics, and flexible electronics.

Strain modulation of the spin-valley polarization in monolayer manganese chalcogenophosphates alloys

Inspired by the profound physical connotations and potential applications of the spintronics and valleytronics, two-dimensional (2D) monolayer manganese chalcogenophosphates alloys are constructed, and the strain modulated spin-valley characteristics are investigated through the first principles calculations. For both the MnFePS₃and MnFePSe₃, the conductivity can be tuned reversibly between semiconductive and half-metallic, while and magnetic stability is controllable between ferromagnetism and antiferromagnetism. Large valley splitting of up to 1000 meV is achieved in MnFePS₃under a -4% strain. Simultaneous spin splitting of 219 meV and valley splitting of 160 meV are acquired in MnFePS₃under a 4% strain. Strain tunable magnetic moment and interaction between Mn, Fe and S/Se atoms are revealed as the internal mechanisms of controlling the magnetic stability, spin and valley polarizations in the two structures. All the findings in this work provide a strategy for the manipulation of spin and valley degrees of freedom in 2D magnetic materials.

Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS₂ is demonstrated over large area (cm²), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS₂ layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS₂ layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS₂ film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS₂ metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies.

Swapping Exchange and Spin-Orbit Coupling in 2D van der Waals Heterostructures

The concept of swapping the two most important spin interactions-exchange and spin-orbit coupling-is proposed based on two-dimensional multilayer van der Waals heterostructures. Specifically, we show by performing realistic ab initio simulations, that a single device consisting of a bilayer graphene sandwiched by a 2D ferromagnet Cr_{2}Ge_{2}Te_{6} (CGT) and a monolayer WS_{2}, is able not only to generate, but also to swap the two interactions. The highly efficient swapping is enabled by the interplay of gate-dependent layer polarization in bilayer graphene and short-range spin-orbit and exchange proximity effects affecting only the layers in contact with the sandwiching materials. We call these structures ex-so-tic, for supplying either exchange (ex) or spin-orbit (so) coupling in a single device, by gating. Such bifunctional devices demonstrate the potential of van der Waals spintronics engineering using 2D crystal multilayers.

Optically Probing Tunable Band Topology in Atomic Monolayers

In many atomically thin materials, their optical absorption is dominated by excitonic transitions. It was recently found that optical selection rules in these materials are influenced by the band topology near the valleys. We propose that gate-controlled band ordering in a single atomic monolayer, through changes in the valley winding number and excitonic transitions, can be probed in helicity-resolved absorption and photoluminescence. This predicted tunable band topology is confirmed by combining an effective Hamiltonian and a Bethe-Salpeter equation for an accurate description of excitons, with first-principles calculations suggesting its realization in Sb-based monolayers.

Signatures of nontrivial Rashba metal states in a transition metal dichalcogenides Josephson junction

Nontrivial Rashba metal states in conventional semiconductor materials generated by both Rashba spin-orbit coupling and ferromagnetic exchange coupling coexisting were recently predicted and exploited. Single layered transition metal dichalcogenides (TMDC) featuring those states and their potential applications have been less focused. We find that, in the materials with Rashba spin-orbit coupling only, nontrivial Rashba metallic states can be manipulated by an external gate voltage. Based on extensive numerical simulations, the relationships between the supercurrent and nontrivial Rashba metallic states in the TMDC Josephson junction have been investigated. It is shown that, in the absence of the Rashba spin-orbit coupling, the critical supercurrent exhibits a stark difference between normal Rashba metal state and anomalous Rashba metal state in the finite junction as compared to the case of the short junction. While in the case of the finite Rashba spin-orbit coupling, the critical supercurrent demonstrates a reentrant behavior when Fermi level sweeps from anomalous Rashba metal state to Rashba ring metal state. Intriguingly, not only at the conversion of the nontrivial Rashba metallic states but also in the Rashba ring metal state the reentrant behavior exhibits again, which could be well explained by the mixing of spin-triplet Cooper pairs with spin-singlet Cooper pairs in Ising superconductor. Such a reentrant effect offers a new way to detect Ising superconductivity based on the TMDC systems. Meanwhile our study also clarified that the nontrivial Rashba metallic state plays an important role in controlling the supercurrent in the TMDC Josephson junction, which is useful for designing future superconducting devices.

Scanning gate microscopy mapping of edge current and branched electron flow in a transition metal dichalcogenide nanoribbon and quantum point contact

We study scanning gate microscopy conductance mapping of a [Formula: see text] zigzag ribbon exploiting tight-binding and continuum models. We show that, even though the edge modes of a pristine nanoribbon are robust to backscattering on the potential induced by the tip, the conductance mapping reveals presence of both the edge modes and the quantized spin- and valley-current carrying modes. By inspecting the electron flow from a split gate quantum point contact (QPC) we find that the mapped current flow allows to determine the nature of the quantization in the QPC as spin-orbit coupling strength affects the number of branches in which the current exits the constriction. The radial conductance oscillation fringes found in the conductance mapping reveal the presence of two possible wavevectors for the charge carriers that correspond to spin and valley opposite modes. Finally, we show that disorder induced valley mixing leads to a beating pattern in the radial fringes.

Valley-Dependent Spin Transport in Monolayer Transition-Metal Dichalcogenides

We study valley-dependent spin transport theoretically in monolayer transition-metal dichalcogenides in which a variety of spin and valley physics are expected because of spin-valley coupling. The results show that the spins are valley-selectively excited with appropriate carrier doping and valley polarized spin current (VPSC) is generated. The VPSC leads to the spin-current Hall effect, transverse spin accumulation originating from the Berry curvature in momentum space. The results indicate that spin excitations with spin-valley coupling lead to both valley and spin transport, which is promising for future low-consumption nanodevice applications.