Valley Degree Of Freedom Research Articles

Electronic transmission in the lateral heterostructure of semiconducting and metallic transition-metal dichalcogenide monolayers

We investigate the electronic transport property of lateral heterojunctions of semiconducting and metallic transition-metal dichalcogenide monolayers, MoSe2 and NbSe2, respectively. We calculate the electronic transmission probability by using a multiorbital tight-binding model based on the first-principles band structure. The transmission probability depends on the spin and valley degrees of freedom. This dependence qualitatively changes by the interface structure. The heterostructure with a zigzag interface preserves the spin and the valley of electrons in the transmission process. On the other hand, the armchair interface enables conduction electrons to transmit with changing the valley and increases the conductance in the hole-doped junctions due to the valley-flip transmission. We also discuss the spin and valley polarizations of electronic current in the heterojunctions.

Long valley lifetime of dark excitons in single-layer WSe2

Single-layer transition metal dichalcogenides provide a promising material system to explore the electron’s valley degree of freedom as a quantum information carrier. The valley degree of freedom can be directly accessed by means of optical excitation. However, rapid valley relaxation of optically excited electron-hole pairs (excitons) through the exchange interaction has been a major roadblock. Theoretically such valley relaxation is suppressed in dark excitons, suggesting a potential route for long valley lifetimes. Here we develop a waveguide-based method to detect time-resolved and energy-resolved dark exciton emission in single-layer WSe2, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for the dark charged excitons.

Nonmetal-Atom-Doping-Induced Valley Polarization in Single-Layer Tl2O

Valleytronics that relies on the valley degree of freedom is attracting growing interest because it provides a new platform for information storage. One obstacle in this field is to realize valley polarization in an efficient route to manipulate the valley physics. Here we propose a strategy to induce valley polarization by nonmetal atom doping in single-layer Tl2O. Owing to the intrinsic inversion asymmetry and large spin-orbit coupling, there are a two-fold valley degeneracy and an excellent spin-valley independence in single-layer Tl2O. Upon introducing C/N atoms in single-layer Tl2O, the intriguing valley polarization successfully appears, and the obtained polarization strengths are considerable. In particular, for N-doped case, the top valence band locates around the Fermi level, and there are no impurity states in the band gap, which is desirable for practical applications. It is predicted that these valley polarizations can be effectively engineered under the magnetic field and external strain, suggesting that the control of valley physics in single-layer Tl2O is accessible.

Inversion Symmetry Breaking Induced Valley Hall Effect in Multilayer WSe2.

Two-dimensional transition metal dichalcogenides possess the K (K') valley degree of freedom (DOF). Based on that, the research on valleytronics draws considerable attention. In this report, by breaking the spatial-inversion symmetry by an out-of-plane electric field, the valley Hall effect (VHE) is observed in multilayer tungsten diselenide (WSe2) at room temperature. The non-zero Berry curvature emerges, leading to the carriers at K (K') valley being deflected to the opposite sides of the channel, giving rise to a spatial polarization of carriers at K (K') valleys in multilayer WSe2. This observation of the VHE illustrates that the K (K') valley DOF can be generated in multilayer WSe2, which makes it an alternative candidate for valleytronics.

Chiral light-matter interactions using spin-valley states in transition metal dichalcogenides.

Chiral light-matter interactions can enable polarization to control the direction of light emission in a photonic device. Most realizations of chiral light-matter interactions require external magnetic fields to break time-reversal symmetry of the emitter. One way to eliminate this requirement is to utilize strong spin-orbit coupling present in transition metal dichalcogenides that exhibit a valley-dependent polarized emission. Such interactions were previously reported using plasmonic waveguides, but these structures exhibit short propagation lengths due to loss. Chiral dielectric structures exhibit much lower loss levels and could therefore solve this problem. We demonstrate chiral light-matter interactions using spin-valley states of transition metal dichalcogenide monolayers coupled to a dielectric waveguide. We use a photonic crystal glide-plane waveguide that exhibits chiral modes with high field intensity, coupled to monolayer WSe2. We show that the circularly polarized emission of the monolayer preferentially couples to one direction of the waveguide, with a directionality as high as 0.35, limited by the polarization purity of the bare monolayer emission. This system enables on-chip directional control of light and could provide new ways to control spin and valley degrees of freedom in a scalable photonic platform.

Excited States in Bilayer Graphene Quantum Dots.

We report ground- and excited-state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different from quantum dots in carbon nanotubes, where the two-particle ground state is a spin-singlet state. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35meV and measure a valley and spin g factor of 36 and 2, respectively.

Valley switch effect based on monolayer WSe2 modulated by circularly polarized light and valley Zeeman field

The high-quality monolayer (ML) WSe2 leads to the possibility of an efficient manipulation of the spin and valley degrees of freedom. Here considering valley Zeeman field, which refers to the strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field, we investigate the spin and valley transport properties in the ML WSe2 quantum structure with circularly polarized light. Without valley Zeeman field, it is obviously found that the valley-dependent transmission can be well modulated by the orientation of circularly polarized light. The valley-dependent transmission can be filtered by the right () circularly polarized light, while the perfect filtering effect of K valley-dependent transmission occurs when the left () circularly polarized light irradiates. In the presence of valley Zeeman field, the valley-dependent transmission presents more interesting phenomena by controlling the orientation of circularly polarized light. When the circularly polarized light is applied, the valley polarization can be turned 100% to −100% with the enhancement of the effective energy about circularly polarized light. However, irradiating the circularly polarized light only 100% valley polarization can be obtained, which is independent of the effective energy of circularly polarized light and the height of electric barrier. Moreover, the results show that the valley polarization can also be from 100% to −100% in this device by adjusting the valley Zeeman field coupled with electrical field. Thus a two-way switch is formed in this quantum structure. These intriguing features indicate that the ML WSe2 structure is a promising candidate for future valleytronics devices, for valley switch, valley filter.

Control of spin diffusion and suppression of the Hanle oscillation by the coexistence of spin and valley Hall effects in Dirac materials

In addition to spin, electrons in many materials possess an additional pseudo-spin degree of freedom known as 'valley'. In materials where the spin and valley degrees of freedom are weakly coupled, they can be both excited and controlled independently. In this work, we study a model describing the interplay of the spin and valley Hall effects in such two-dimensional materials. We demonstrate the emergence of an additional longitudinal neutral current that is both spin and valley polarized. The additional neutral current allows to control the spin density by tuning the magnitude of the valley Hall effect. In addition, the interplay of the two effects can suppress the Hanle effect, that is, the oscillation of the nonlocal resistance of a Hall bar device with in-plane magnetic field. The latter observation provides a possible explanation for the absence of the Hanle effect in a number of recent experiments. Our work opens also the possibility to engineer the conversion between the valley and spin degrees of freedom in two-dimensional materials.

Topological superconductivity in Dirac honeycomb systems

We predict two topological superconducting phases in microscopic models arising from the Berry phase associated with the valley degree of freedom in gapped Dirac honeycomb systems. The first one is a topological helical spin-triplet superconductor with a nonzero center-of-mass momentum that does not break time-reversal symmetry. We also find a topological chiral-triplet superconductor with Chern number $\pm 1$ with equal-spin-pairing in one valley and opposite-spin-triplet pairing in the other valley. Our results are obtained for the Kane-Mele model in which we have explored the effect of three different interactions, onsite attraction $U$, nearest-neighbor density-density attraction $V$, and nearest-neighbor antiferromagnetic exchange $J$, within self-consistent Bogoliubov--de Gennes theory. Transition metal dichalcogenides and cold atom experiments are promising platforms to explore these phases.

Strain engineering of valley polarized currents in topological crystalline insulators

Although the existence of four valley degrees of freedom in the (0 0 1) surface of IV–VI semiconductor topological crystalline insulators (TCIs) provides the opportunity to multiply the valleytronic functionality, it makes the generation of highly polarized valley currents less plausible. We investigate quantum adiabatic valley pumping in (0 0 1) surface of these TCIs and show that applying shear strains and exchange field gives the possibility of control and manipulation of the valley resolved currents with high polarizations. Interchange of polarizations, simply by turning the tensile strain into compressive mode and vice versa, highlights the potential application for valleytronic switching process. Furthermore, since the surface states are robust against disorders, we can increase the lengths of driving regions and pump significantly larger currents without breaking the coherency of the quantum transport regime.

Valley‐Hall Photonic Topological Insulators with Dual‐Band Kink States

AbstractExtensive researches have revealed that valley, a binary degree of freedom (DOF), can be an excellent candidate of information carrier. Recently, valley DOF is introduced into photonic systems, and several valley‐Hall photonic topological insulators (PTIs) are experimentally demonstrated. However, in the previous valley‐Hall PTIs, topological kink states only work at a single frequency band, which limits potential applications in multiband waveguides, filters, communications, and so on. To overcome this challenge, here a valley‐Hall PTI, where the topological kink states exist at two separated frequency bands, is experimentally demonstrated in a microwave substrate‐integrated circuitry. Both the simulated and experimental results demonstrate the dual‐band valley‐Hall topological kink states are robust against the sharp bends of the internal domain wall with negligible intervalley scattering. This work may pave the way for multichannel substrate‐integrated photonic devices with high efficiency and high capacity for information communications and processing.

Room-temperature valley coherence in a polaritonic system

The emerging field of valleytronics aims to coherently manipulate an electron and/or hole’s valley pseudospin as an information bearing degree of freedom (DOF). Monolayer transition metal dichalcogenides, due to their strongly bound excitons, their degenerate valleys and their seamless interfacing with photons are a promising candidate for room temperature valleytronics. Although the exciton binding energy suggests room temperature valley coherence should be possible, it has been elusive to-date. A potential solution involves the formation of half-light, half-matter cavity polaritons based on 2D material excitons. It has recently been discovered that cavity polaritons can inherit the valley DOF. Here, we demonstrate the room temperature valley coherence of valley-polaritons by embedding a monolayer of tungsten diselenide in a monolithic dielectric cavity. The extra decay path introduced by the exciton-cavity coupling, which is free from decoherence, is the key to room temperature valley coherence preservation. These observations paves the way for practical valleytronic devices.

Valley Vortex States and Degeneracy Lifting via Photonic Higher-Band Excitation.

We demonstrate valley-dependent vortex generation in photonic graphene. Without breaking inversion symmetry, the excitation of two valleys leads to the formation of an optical vortex upon Bragg reflection to the third equivalent valley, with its chirality determined by the valley degree of freedom. Vortex-antivortex pairs with valley-dependent topological charge flipping are also observed and corroborated by numerical simulations. Furthermore, we develop a three-band effective Hamiltonian model to describe the dynamics of the coupled valleys and find that the commonly used two-band model is not sufficient to explain the observed vortex degeneracy lifting. Such valley-polarized vortex states arise from high-band excitation without a synthetic-field-induced gap opening. Our results from a photonic setting may provide insight for the study of valley contrasting and Berry-phase-mediated topological phenomena in other systems.

Probing the exciton k-space dynamics in monolayer tungsten diselenides

Monolayer transition metal dichalcogenides (TMDs) provide 2D semiconductors with a rich interplay of electron spin and valley degrees of freedom. Along with strong optical coupling strength, it provides an ideal platform to study exciton-related physics in the 2D limit. Intensive experimental approaches on exciton dynamics in monolayer TMDs have been focusing on the lifetime of the exciton, exciton valley/spin polarization, and coherence, whereas the ‘hot’ exciton relaxation is rarely addressed. Here we probe the dynamics of bright excitons in monolayer tungsten diselenides, by pumping at exciton ground state and probing at higher energy, where excitonic excited states can be identified from the pump-probe spectroscopy. Remarkably, the rise time of the differential reflection signal increases significantly with the index of the excited states, which we attribute to the phase-space filling effect that probes the momentum distribution of ground state exciton. Our results reveal fast exciton momentum relaxation as a signature of the strong Coulomb interaction in 2D TMDs.

Distinctive Signatures of the Spin- and Momentum-Forbidden Dark Exciton States in the Photoluminescence of Strained WSe2 Monolayers under Thermalization.

With both spin and valley degrees of freedom, the low-lying excitonic spectra of photoexcited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intravalley bright exciton states as well as various intra- and intervalley dark ones. The latter states can be classified as those of the spin- and momentum-forbidden dark excitons according to the violated optical selection rules. Because of their optical invisibility, these two types of the dark states are in principle hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photoluminescence spectra of strained tungsten diselenide monolayers (WSe2-MLs) where the intravalley spin-forbidden dark exciton lies in the lowest exciton states and other momentum-forbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe2-ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin- and momentum-forbidden exciton states of strained WSe2-MLs in the temperature-dependent photoluminescences and present the guiding principle to infer the relative energetic locations of the two types of dark excitons.

Quantum valley Hall effects and spin-valley locking in topological Kane-Mele circuit networks

The Kane-Mele model of time-reversal symmetry, which simultaneously contains spin and valley degrees of freedom, is simulated in a topological circuit network. By varying capacitances and inductances, we are able to demonstrate the quantum valley Hall states, in addition to the quantum spin Hall states, as well as the topological phase transition between them. We also study the rich topological interface states between nontrivial phases with different spin and valley Chern numbers via tuning capacitances and inductances so as to tune the couplings and gauge fields. Finally, we demonstrate the spin-valley locking in the Kane-Mele circuit network. The topological circuit network provides us with a platform to simultaneously control spin and valley degrees of freedom so that a spin splitter and a valley splitter can be realized.

Giant Seebeck magnetoresistance triggered by electric field and assisted by a valley through a ferromagnetic/antiferromagnetic junction in heavy group-IV monolayers

Electrons in heavy group-IV monolayers, including silicene, germanene, and stanene, have the ability to exhibit rich physics due to the compatibility of the spin and valley degrees of freedom. We propose here that a valley-mediated giant Seebeck magnetoresistance (MR) effect, triggered and controlled by an interlayer electric field ${E}_{z}$, can be engineered near room temperature in a ferromagnetic/antiferromagnetic (FM/AFM) junction based on heavy group-IV monolayers, where the FM and AFM fields can be induced by the proximity effect, and ${E}_{z}$ is locally applied in the AFM region. Attributed to the specific tunneling mechanism of spin-valley matching, the high thermal MR state is dominated by a Seebeck state with nearly pure spin current (no charge current) from one valley under ${E}_{z}=0$, while the low thermal MR state is dominated by a spin filter state from the other valley under ${E}_{z}\ensuremath{\ne}0$. We also demonstrate that such a giant Seebeck MR effect is robust against the small perturbation of the Fermi level, and is sensitive to ${E}_{z}$ by changing the electron-hole transport symmetry and tuning the bands. Further calculations indicate that the Seebeck MR effect is relatively too weak in other magnetic junctions, typically the FM/${E}_{z}$ and AFM/${E}_{z}$ junctions, even when the pure-spin-current state is present. These findings may pave the way for heavy group-IV monolayers in developing valley-assisted thermomagnetic storing and reading technologies in future spin caloritronic devices.

Giant magnetoresistance and spin-valley polarization of Dirac fermions modulated by magnetic fields

In the presence of magnetic fields, we studied the giant magnetoresistance (MR) effect of Dirac fermions in graphene and silicene, which is related to the spin and valley degrees of freedom. It is found that the electron could resonantly tunnel through the parallel magnetization configuration, but it is blocked in the antiparallel situation. Such a transmission difference leads to a giant MR ratio, which can be controllable electrically in silicene. Furthermore, for the asymmetric structure in silicene, the transport depends strongly on the spin and valley degrees of freedom. Accordingly, this MR device achieves a remarkable spin-valley polarized current which can be effectively controlled by the magnetic fields and electric fields.

Growth and structure of singly oriented single-layer tungsten disulfide on Au(111)

We present a complete characterisation at the nanoscale of the growth and structure of single-layer tungsten disulfide (WS$_2$) epitaxially grown on Au(111). Following the growth process in real time with fast x-ray photoelectron spectroscopy, we obtain a singly-oriented layer by choosing the proper W evaporation rate and substrate temperature during the growth. Information about the morphology, size and layer stacking of the WS$_2$ layer were achieved by employing x-ray photoelectron diffraction and low-energy electron microscopy. The strong spin splitting in the valence band of WS$_2$ coupled with the single-orientation character of the layer make this material the ideal candidate for the exploitation of the spin and valley degrees of freedom.

Electrical Control of Magnetic Behavior and Valley Polarization of Monolayer Antiferromagnetic MnPSe3 on an Insulating Ferroelectric Substrate from First Principles

The valley degree of freedom in a compound's electronic structure has enormous potential for use in information storage and processing. Electrical switching of the valley index is most practical for real-world applications, but has not yet emerged. This work develops a route toward electrically controlled valley polarization by utilizing a multiferroic substrate as an intermediate. The proposed ferroelectricity-valley coupling and tunable magnetic behavior are both achieved in antiferromagnetic monolayer MnPSe${}_{3}$ on insulating ferroelectric YMnO${}_{3}$ substrate, raising the prospects of electrically controlled valleytronics and boosting the development of high-performance memory devices.