Generation of pure bulk valley current in graphene

Non-Hermitian Moiré Valley Filter

A valley filter capable of generating a valley-polarized current is a crucial element in valleytronics, yet its implementation remains challenging. Here, we propose a valley filter made of a graphene bilayer which exhibits a 1D moiré pattern in the overlapping region of the two layers controlled by heterostrain. In the presence of a lattice modulation between layers, electrons propagating in one layer can have valley-dependent dissipation due to valley asymmetric interlayer coupling, thus giving rise to a valley-polarized current. Such a process can be described by an effective non-Hermitian theory, in which the valley filter is driven by a valley-resolved non-Hermitian skin effect. Nearly 100% valley polarization can be achieved within a wide parameter range and the functionality of the valley filter is electrically tunable. The non-Hermitian topological scenario of the valley filter ensures high tolerance against imperfections such as disorder and edge defects. Our work opens a new route for efficient and robust valley filters while significantly relaxing the stringent implementation requirements.

Tunneling Valley Hall Effect Driven by Tilted Dirac Fermions

Valleytronics is a research field utilizing a valley degree of freedom of electrons for information processing and storage. A strong valley polarization is critical for realistic valleytronic applications. Here, we predict a tunneling valley Hall effect (TVHE) driven by tilted Dirac fermions in all-in-one tunnel junctions based on a two-dimensional (2D) valley material. Different doping of the electrode and spacer regions in these tunnel junctions results in momentum filtering of the tunneling Dirac fermions, generating a strong transverse valley Hall current dependent on the Dirac-cone tilting. Using the parameters of an existing 2D valley material, we demonstrate that such a strong TVHE can host a giant valley Hall angle even in the absence of the Berry curvature. Finally, we predict that resonant tunneling can occur in a tunnel junction with properly engineered device parameters such as the spacer width and transport direction, providing significant enhancement of the valley Hall angle. Our work opens a new approach to generate valley polarization in realistic valleytronic systems.

Valley-polarized and enhanced transmission in graphene with a smooth strain profile

We explore the influence of strain on the valley-polarized transmission of graphene by employing the wave-function matching and the non-equilibrium Green's function technique. When the transmission is along the armchair direction, we show that the valley polarization and transmission can be improved by increasing the width of the strained region and increasing (decreasing) the extensional strain in the armchair (zigzag) direction. It is noted that the shear strain does not affect transmission and valley polarization. Furthermore, when we consider the smooth strain barrier, the valley-polarized transmission can be enhanced by increasing the smoothness of the strain barrier. We hope that our finding can shed new light on constructing graphene-based valleytronic and quantum computing devices by solely employing strain.

Theory of quantized photonic spin Hall effect in strained graphene under a sub-Tesla external magnetic field

The quantized photonic spin Hall effect (PSHE) in the strained graphene-substrate system is predicted under a sub-Tesla external magnetic field, which is two orders of magnitude smaller than required to produce the quantized effect in the conventional graphene-substrate system. It is found that in-plane and transverse spin-dependent splittings in the PSHE, exhibit different quantized behaviors and are closely related to the reflection coefficients. Unlike the quantized PSHE in the conventional graphene-substrate system formed by the splitting of real Landau levels, the quantized PSHE in the strained graphene-substrate system is attributed to the splitting of pseudo-Landau levels caused by the pseudo-magnetic field and the lifting of valley degeneracy of the n ≠ 0 pseudo-Landau levels induced by the sub-Tesla external magnetic field. At the same time, the pseudo-Brewster angles of the system are also quantized with the change of Fermi energy. The sub-Tesla external magnetic field and the PSHE appear as quantized peak values near these angles. The giant quantized PSHE is expected to be used for direct optical measurements of the quantized conductivities and pseudo-Landau levels in the monolayer strained graphene.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Weyl Semimetal Path to Valley Filtering in Graphene

We propose a device in which a sheet of graphene is coupled to a Weyl semimetal, allowing for the physical access to the study of tunneling from two- to three-dimensional massless Dirac fermions. Because of the reconstructed band structure, we find that this device acts as a robust valley filter for electrons in the graphene sheet. We show that, by appropriate alignment, the Weyl semimetal draws away current in one of the two graphene valleys, while allowing current in the other to pass unimpeded. In contrast to other proposed valley filters, the mechanism of our proposed device occurs in the bulk of the graphene sheet, obviating the need for carefully shaped edges or dimensions.

Fully spin-valley-polarized current induced by electric field in zigzag stanene and germanene nanoribbons

We theoretically investigated the spin-dependent valleytronics in stanene and germanene nanoribbons, considering the electron-electron interaction and the external electric field without any magnetic exchange element. Our results showed that applying an electric field in these two-dimensional materials with a large intrinsic spin-orbit coupling can provide a versatile platform to create spin-valley currents by exploiting the edge magnetism at their zigzag edges. Generally, manipulating the electric field can generate a fully spin-valley-polarized current with a large magnitude even at room temperature. The rich and tunable spin and valley degrees of freedom can turn these structures into ideal candidates for spin-valley applications.

Spontaneous Valley Spirals in Magnetically Encapsulated Twisted Bilayer Graphene

Van der Waals heterostructures provide a rich platform for emergent physics due to their tunable hybridization of layers, orbitals, and spin. Here, we find that twisted bilayer graphene stacked between antialigned ferromagnetic insulators can feature flat electronic bands due to the interplay between twist, exchange proximity, and spin-orbit coupling. These flat bands are nearly degenerate in valley only and are effectively described by a triangular superlattice model. At half filling, we find that interactions induce spontaneous valley correlations that favor spiral order and derive a low-energy valley-Heisenberg model with symmetric and antisymmetric exchange couplings. We also show how electric interlayer bias broadens the bands and tunes these couplings. Our results put forward magnetic van der Waals heterostructures as a platform to explore valley-correlated states.

Valley-Layer Coupling: A New Design Principle for Valleytronics

The current valleytronics research is based on the paradigm of time-reversal-connected valleys in two-dimensional (2D) hexagonal materials, which forbids the fully electric generation of valley polarization by a gate field. Here, we go beyond the existing paradigm to explore 2D systems with a novel valley-layer coupling (VLC) mechanism, where the electronic states in the emergent valleys have a valley-contrasted layer polarization. The VLC enables a direct coupling between a valley and a gate electric field. We analyze the symmetry requirements for a system to host VLC, demonstrate our idea via first-principles calculations and model analysis of a concrete 2D material example, and show that an electric, continuous, wide-range, and switchable control of valley polarization can be achieved by VLC. Furthermore, we find that systems with VLC can exhibit other interesting physics, such as valley-contrasting linear dichroism and optical selection of the valley and the electric polarization of interlayer excitons. Our finding opens a new direction for valleytronics and 2D materials research.

Quantum Hall Response to Time-Dependent Strain Gradients in Graphene

Mechanical deformations of graphene induce a term in the Dirac Hamiltonian that is reminiscent of an electromagnetic vector potential. Strain gradients along particular lattice directions induce local pseudomagnetic fields and substantial energy gaps as indeed observed experimentally. Expanding this analogy, we propose to complement the pseudomagnetic field by a pseudoelectric field, generated by a time-dependent oscillating stress applied to a graphene ribbon. The joint Hall-like response to these crossed fields results in a strain-induced charge current along the ribbon. We analyze in detail a particular experimental implementation in the (pseudo)quantum Hall regime with weak intervalley scattering. This allows us to predict an (approximately) quantized Hall current that is unaffected by screening due to diffusion currents.

Polarization-Dependent Optical Properties and Optoelectronic Devices of 2D Materials

The development of optoelectronic devices requires breakthroughs in new material systems and novel device mechanisms, and the demand recently changes from the detection of signal intensity and responsivity to the exploration of sensitivity of polarized state information. Two-dimensional (2D) materials are a rich family exhibiting diverse physical and electronic properties for polarization device applications, including anisotropic materials, valleytronic materials, and other hybrid heterostructures. In this review, we first review the polarized-light-dependent physical mechanism in 2D materials, then present detailed descriptions in optical and optoelectronic properties, involving Raman shift, optical absorption, and light emission and functional optoelectronic devices. Finally, a comment is made on future developments and challenges. The plethora of 2D materials and their heterostructures offers the promise of polarization-dependent scientific discovery and optoelectronic device application.

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.

Bandgap engineering of PbTe ultra-thin layers by surface passivations

We calculate the electronic structures of the PbTe (1 1 1) ultra-thin films by performing the first-principles calculations. The PbTe (1 1 1) ultra-thin films possess direct or indirect band gaps depending sensitively on surface passivations with hydrogen or halogen atoms, and the band gaps depend sensitively on the passivation elements. The bandgaps of PbTe (1 1 1) ultra-thin films with hydrogen passivations can be tuned from 15 meV to 65 meV by applying external strains, making PbTe ultra-thin films promising candidates for optoelectronic device applications in terahertz regime.

Multiple harmonics control of edge pseudomagnetoplasmons in strained grapheme

Valley-resolved edge plasmons are relevant to nano-optics at subwavelength scales. However, less attention has been paid to their tunable properties in time domain. In this work we investigate edge pseudomagnetoplasmons in a strained graphene modulated by multiple harmonics with frequency in the THz regime. The edge plasmon is described by a set of nonlinear hydrodynamic equations, which are self-consistently solved by the flux-corrected transport method. Without the applied voltage, there exist two unidirectional-propagating edge-plasmon modes with weak valley polarization P. It is demonstrated that by varying the amplitude of multiple harmonics one can alter both the amplitude and the polarity of the valley polarization in the edge plasmon. One can achieve a full valley polarization P=1 at the instant of half cycle of the multiple harmonics and P=-1 at the instant of one cycle. The edge-plasmon density and the transverse velocity vanish for the frozen valley.

Controlling electron flow in anisotropic Dirac materials heterojunctions: a super-diverging lens

Ballistic heterojunctions of Dirac materials offer the opportunity of exploring optics-like phenomena in electronic systems. In this paper, a new perfect lens through special positive refraction is predicted with omnidirectional Klein tunneling of massless Dirac fermions. The novel optics component called a super-diverging lens (SDL) is the counterpart of a Veselago lens (VL). The use of SDL and VL creates a device that simulates the ocular vision. This atypical refraction is due to electrons obeying different Snell's laws of pseudo-spin and group velocity in heterojunctions with elliptical Dirac cones. These findings pave the way for an electron elliptical Dirac optics and open up new possibilities for the guiding of electrons.

Quantum anomalous/valley Hall effect and tunable quantum state in hydrogenated arsenene decorated with a transition metal

The quantum anomalous Hall (QAH) effect is superior to the quantum spin Hall (QSH) effect, which can avoid the inelastic scattering of two edge electrons located on one side of a topological nontrivial material, and thus it has attracted both theoretical and experimental interest. Here, we systematically investigate the lattice structures, and electronic and magnetic properties of hydrogenated arsenene decorated with certain transition metals (Cr, Mo and Cu) based on density-functional theory. A unique QAH effect in Mo@AsH is predicted, whose Chern number (C = 1) indicates only one chiral edge channel located on its one side. Then, we prove that this QAH effect realization is closely related with band inversion, which is the competitive result between its spin-orbit coupling (SOC) strength and exchange field. The quantum state of Mo@AsH can also be tuned by an external strain, similar to SOC, and it is noted that its increased topological gap of about 35 meV under 5.0% tensile strain, is large enough to realize the QAH effect at room-temperature. Additionally, the quantum valley Hall effect in Cu@AsH contributed by the inequality of AB sublattices is also found. Our results reveal the physical mechanism to realize the QAH effect in TM@AsH and provide a platform for electrically controllable topological states, which are highly desirable for nanoelectronics and spintronics.

Spin- and Valley-Dependent Electronic Structure in Silicene Under Periodic Potentials

We study the spin- and valley-dependent energy band and transport property of silicene under a periodic potential, where both spin and valley degeneracies are lifted. It is found that the Dirac point, miniband, band gap, anisotropic velocity, and conductance strongly depend on the spin and valley indices. The extra Dirac points appear as the voltage potential increases, the critical values of which are different for electron with different spins and valleys. Interestingly, the velocity is greatly suppressed due to the electric field and exchange field, other than the gapless graphene. It is possible to achieve an excellent collimation effect for a specific spin near a specific valley. The spin- and valley-dependent band structure can be used to adjust the transport, and perfect transmissions are observed at Dirac points. Therefore, a remarkable spin and valley polarization is achieved which can be switched effectively by the structural parameters. Importantly, the spin and valley polarizations are greatly enhanced by the disorder of the periodic potential.

Chiral Response of Twisted Bilayer Graphene

We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electromagnetic coupling without exhibiting a topological gap. As an example, we study the response of doped twisted bilayers, unveiling unusual phenomena in the zero frequency limit. An in-plane magnetic field induces a huge paramagnetic response at the neutrality point and, upon doping, also gives rise to a substantial longitudinal Hall response. The system also accommodates nontrivial longitudinal plasmonic modes that are associated with a longitudinal magnetic moment, thus endowing them with a chiral character. Finally, we note that the optical activity can be considerably enhanced upon doping and our general approach would enable systematic exploration of 2D material heterostructures with optical activity.

The role of the strain induced population imbalance in Valley polarization of graphene: Berry curvature perspective

Real magnetic and lattice deformation gauge fields have been investigated in honeycomb lattice of graphene. The coexistence of these two gauges will induce a gap difference between two valley points (K and K') of system. This gap difference allows us to study the possible topological valley Hall current and valley polarization in the graphene sheet. In the absence of magnetic field, the strain alone could not generate a valley polarization when the Fermi energy coincides exactly with the Dirac points. Since in this case there is not any imbalance between the population of the valley points. In other words each of these gauges alone could not induce any topological valley-polarized current in the system at zero Fermi energy. Meanwhile at non-zero Fermi energies population imbalance can be generated as a result of the external strain even at zero magnetic field. In the context of Berry curvature within the linear response regime the valley polarization (both magnetic free polarization, Π0, and field dependent response function, χ α ) in different values of gauge fields of lattice deformation has been obtained.

Random Gauge Field Scattering in Monolayer Graphene

Strain-induced lattice deformation affects electron hopping between the atoms. This effectively gives rise to a gauge field which impacts on the charge transport. In graphene, such gauge field is associated with a vector potential which mimics that of a magnetic field. Understanding the impact of the gauge field on charge transport is of essential importance for emerging topics including straintronics and valleytronics in two-dimensional materials. While extensive theoretical works have been carried out over the past decade, experimental progress has been largely limited to local probe and optical studies. Experimental charge transport study has been baffled by the challenge in creating an effective and independent tuning knob of strain without compromising the quality of graphene. Here we studied high quality suspended graphene field effect transistors fabricated on flexible Polyimide substrates. Applying uniaxial strain by bending the substrate, we observed a strain-induced resistivity with power-law carrier density dependence. The power factor is found to be correlated with the surface fractal dimension of the rippled graphene, in good agreement with the random gauge field scattering theory. Both phase coherent transport and magnetotransport properties are found to be strain-dependent, which can be understood in terms of a strain-tunable disorder.

Valley precession and valley polarization in graphene with inter-valley coupling

We theoretically investigate the valley precession and valley polarization in graphene under inter-valley coupling. Our results show that the inter-valley coupling can induce valley polarization in graphene and also precess valleys in real space in a manner similar to the Rashba spin-orbit interaction rotating spins. Moreover, using strain modulation, we can achieve high valley polarization with large valley-polarized currents. These findings provide a new way to create and manipulate valley polarization in graphene.

Spatial valley separation in strained graphene pn junction

Valleytronics in analogy to spintronics aims to use the electron valley degree of freedom to carry and manipulate information, and one of urgent tasks in this field is to generate valley-polarized electrons. In this work, we propose using the electron focusing effect in a strained graphene pn junction to separate valleys spatially through a beam of valley-unpolarized electrons, since the strain-induced pseudo-gauge potentials are opposite for K and [Formula: see text] valleys and severely affect the trajectories of K and [Formula: see text] electron propagation. We numerically simulate this valley-separated Veselago lens effect in a lattice model and demonstrate that pseudo-gauge potentials can efficiently control valley separation patterns.

Valley filter and valve effect by strong electrostatic potentials in graphene

We report a theoretical study on the valley-filter and valley-valve effects in the monolayer graphene system by using electrostatic potentials, which are assumed to be electrically controllable. Based on a lattice model, we find that a single extremely strong electrostatic-potential barrier, with its strength exceeding the hopping energy of electrons, will significantly block one valley but allow the opposite valley flowing in the system, and this is dependent on the sign of the potential barrier as well as the flowing direction of electrons. In a valley-valve device composed of two independent potential barriers, the valley-valve efficiency can even amount to 100% that the electronic current is entirely prohibited or allowed by reversing the sign of one of potential barriers. The physics origin is attributed to the valley mixing effect in the strong potential barrier region. Our findings provide a simple electric way of controlling the valley transport in the monolayer graphene system.

Valley filtering in graphene due to substrate-induced mass potential

The interaction of monolayer graphene with specific substrates may break its sublattice symmetry and results in unidirectional chiral states with opposite group velocities in the different Dirac cones (Zarenia et al 2012 Phys. Rev. B 86 085451). Taking advantage of this feature, we propose a valley filter based on a transversal mass kink for low energy electrons in graphene, which is obtained by assuming a defect region in the substrate that provides a change in the sign of the substrate-induced mass and thus creates a non-biased channel, perpendicular to the kink, for electron motion. By solving the time-dependent Schrödinger equation for the tight-binding Hamiltonian, we investigate the time evolution of a Gaussian wave packet propagating through such a system and obtain the transport properties of this graphene-based substrate-induced quantum point contact. Our results demonstrate that efficient valley filtering can be obtained, provided: (i) the electron energy is sufficiently low, i.e. with electrons belonging mostly to the lowest sub-band of the channel, and (ii) the channel length (width) is sufficiently long (narrow). Moreover, even though the transmission probabilities for each valley are significantly affected by impurities and defects in the channel region, the valley polarization in this system is shown to be robust against their presence.

Generation of large spin and valley currents in a quantum pump based on molybdenum disulfide

Generation of large currents, versatile functionality, and simple structures are of fundamental importance in the development of adiabatic quantum pump devices with nanoscale dimensions. In the present study, we propose an adiabatic quantum pump with a simple structure based on molybdenum disulfide, MoS₂, to generate large spin and valley resolved currents. We show that pure and fully polarized spin and valley currents can be easily generated by employing two potential gates and using an exchange magnetic field. Unlike graphene and silicene, in order to induce a valley resolved current in MoS₂, one does not need to induce strain and apply an electric field. The spin and valley resolved currents are completely coupled together, so that the spin up (down) current is exactly equal to the valley K(K') current. Hence, we can detect the valley resolved current by utilizing more straightforward and simple methods used for the detection of spin resolved currents. The other prominent feature of this proposed pump is its large current, which is two and three orders of magnitude larger than the maximum current of similar pump structures based on silicene and graphene, respectively. The results of this study are promising for the fabrication of quantum pumps with large spin and valley resolved currents, which opens up the possibility of further development of spintronics and valleytronics in 2D nanostructures.

Valley splitting in the transition-metal dichalcogenide monolayer via atom adsorption

In this letter we study the valley degeneracy splitting of the transition-metal dichalcogenide monolayer by first-principles calculations. The local magnetic moments are introduced into the system when the transition-metal atoms are adsorbed on the monolayer surface. The Zeeman effect arising from the local magnetic moment at transition-metal atom sites lifts the valley degeneracy. Anomalous charge, spin and valley Hall effects can be accessed due to valley splitting when we can only excite carriers of one valley. The valley splitting depends on the direction of magnetization and thus can be tuned continuously by an external magnetic field. This tunable valley splitting offers a practical avenue for exploring device paradigms based on the spin and valley degrees of freedom.

Graphene Nanobubbles as Valley Filters and Beam Splitters

The energy band structure of graphene has two inequivalent valleys at the K and K^{'} points of the Brillouin zone. The possibility to manipulate this valley degree of freedom defines the field of valleytronics, the valley analogue of spintronics. A key requirement for valleytronic devices is the ability to break the valley degeneracy by filtering and spatially splitting valleys to generate valley polarized currents. Here, we suggest a way to obtain valley polarization using strain-induced inhomogeneous pseudomagnetic fields (PMFs) that act oppositely on the two valleys. Notably, the suggested method does not involve external magnetic fields, or magnetic materials, unlike previous proposals. In our proposal the strain is due to experimentally feasible nanobubbles, whose associated PMFs lead to different real space trajectories for K and K^{'} electrons, thus allowing the two valleys to be addressed individually. In this way, graphene nanobubbles can be exploited in both valley filtering and valley splitting devices, and our simulations reveal that a number of different functionalities are possible depending on the deformation field.

Valley Filtering and Electronic Optics Using Polycrystalline Graphene

In this Letter, both the manipulation of valley-polarized currents and the optical-like behaviors of Dirac fermions are theoretically explored in polycrystalline graphene. When strain is applied, the misorientation between two graphene domains separated by a grain boundary can result in a mismatch of their electronic structures. Such a discrepancy manifests itself in a strong breaking of the inversion symmetry, leading to perfect valley polarization in a wide range of transmission directions. In addition, these graphene domains act as different media for electron waves, offering the possibility to modulate and obtain negative refraction indexes.

Controllable quantum valley pumping with high current in a silicene junction

We propose an efficient scheme for the generation and control of both pure and fully polarized valley currents in a silicene-based junction, using adiabatic quantum pumping. The pure and fully polarized valley currents are induced using ferromagnetic proximity and the application of a perpendicular electric field. We show that the valley polarized current can easily be switched from valley K to valley [Formula: see text] and vice versa, simply by reversing the direction of the electric field. Thus, the valley current is controllable electrically. Compared to the methods proposed for generation of valley current by quantum pumping in graphene, which are based on inducing strain on its sheet, our method is very simple and can be easily utilized in practical applications. Also, we show that the magnitude of pumped current in a silicene-based junction is roughly one order of magnitude greater than that of graphene. In addition to valley-related currents, our pump scheme can be used on its own to generate pure and fully polarized spin currents. A comparison between weak and strong adiabatic regimes is given, and the effects of some structural parameters that can significantly affect the pumping currents and polarizations are discussed.

Odd-parity currents induced by dynamic deformations in graphene-like systems

Reduced (3  +  1)-dimensional Dirac systems with inter-pseudo-spin and inter-valley scattering are employed to investigate current responses to (chiral) gauge fields in graphene-like systems. From (chiral) current-(chiral) current correlation functions, we derive the current responses. Except for electric currents induced by external gauge fields, we find the inter-valley scattering can break the topological nature of odd-parity currents. Given the proper conditions, this property can help us realize valley-polarized electric currents. Through the dynamic deformations generating the chiral gauge fields, we find the vortex-like currents while their profiles can be tuned by superposition of some deformations. In particular, we find a more manageable approach to realize the topological electric current by choosing a linear dynamic deformation.

Coherent Nonlinear Optical Response Spatial Self-Phase Modulation in MoSe2 Nano-Sheets

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are drawing increasing interest due to their relatively high carrier mobilities, valley pseudospins, and gapped electronic structures, which all indicate interesting nonlinear optical properties of these 2D materials. However, such nonlinear optical properties are so far less investigated and their correlation with the electronic structure of the material is rarely probed. In this work, we have systematically investigated the spatial self-phase modulation (SSPM) of MoSe₂ flakes in a suspension form, which is a coherent third-order nonlinear optical effect. The nonlinear susceptibility χ⁽³⁾ and its wavelength-dependence are measured, yielding a value of 1.1 × 10⁻⁹ e.s.u. (SI: 1.53 × 10⁻¹⁷ m²/V²) at 532 nm laser excitation for effective one-layer MoSe₂.

The positive piezoconductive effect in graphene

As the thinnest conductive and elastic material, graphene is expected to play a crucial role in post-Moore era. Besides applications on electronic devices, graphene has shown great potential for nano-electromechanical systems. While interlayer interactions play a key role in modifying the electronic structures of layered materials, no attention has been given to their impact on electromechanical properties. Here we report the positive piezoconductive effect observed in suspended bi- and multi-layer graphene. The effect is highly layer number dependent and shows the most pronounced response for tri-layer graphene. The effect, and its dependence on the layer number, can be understood as resulting from the strain-induced competition between interlayer coupling and intralayer transport, as confirmed by the numerical calculations based on the non-equilibrium Green's function method. Our results enrich the understanding of graphene and point to layer number as a powerful tool for tuning the electromechanical properties of graphene for future applications.

Quantum Spin-Quantum Anomalous Hall Insulators and Topological Transitions in Functionalized Sb(111) Monolayers

Electronic and topological behaviors of Sb(111) monolayers decorated with H and certain magnetic atoms are investigated by using ab initio methods. The drastic exchange field induced by the magnetic atoms, together with strong spin-orbit coupling (SOC) of Sb atoms, generates one new category of valley polarized topological insulators, called quantum spin-quantum anomalous Hall (QSQAH) insulators in the monolayer, with a band gap up to 53 meV. The strong SOC is closely related to Sb px and py orbitals, instead of pz orbitals in usual two-dimensional (2D) materials. Topological transitions from quantum anomalous Hall states to QSQAH states and then to time-reversal-symmetry-broken quantum spin Hall states are achieved by tuning the SOC strength. The behind mechanism is revealed. Our work is helpful for future valleytronic and spintronic applications in 2D materials.

Coupling Ferroelectricity with Spin-Valley Physics in Oxide-Based Heterostructures

The coupling of spin and valley physics is nowadays regarded as a promising route toward next-generation spintronic and valleytronic devices. In the aim of engineering functional properties for valleytronic applications, we focus on the ferroelectric heterostructure BiAlO3/BiIrO3, where the complex interplay among a trigonal crystal field, layer degrees of freedom, and spin-orbit coupling mediates a strong spin-valley coupling. Furthermore, we show that ferroelectricity provides a nonvolatile handle to manipulate and switch the emerging valley-contrasting spin polarization.

A valley beam splitter of massive Dirac electrons

We propose an electrically controllable valley beam splitter in a double-barrier resonant structure through valley-dependent Goos–Hänchen effects.

Nonlinear valley and spin currents from Fermi pocket anisotropy in 2D crystals

The controlled flow of spin and valley pseudospin is key to future electronics exploiting these internal degrees of freedom of carriers. Here, we discover a universal possibility for generating spin and valley currents by electric bias or temperature gradient only, which arises from the anisotropy of Fermi pockets in crystalline solids. We find spin and valley currents to the second order in the electric field as well as their thermoelectric counterparts, i.e., the nonlinear spin and valley Seebeck effects. These second-order nonlinear responses allow two unprecedented possibilities to generate pure spin and valley flows without net charge current: (i) by an ac bias or (ii) by an arbitrary inhomogeneous temperature distribution. As examples, we predict appreciable nonlinear spin and valley currents in two-dimensional (2D) crystals including graphene, monolayer and trilayer transition-metal dichalcogenides, and monolayer gallium selenide. Our finding points to a new route towards electrical and thermal generations of spin and valley currents for spintronic and valleytronic applications based on 2D quantum materials.

Spin-valley filtering in strained graphene structures with artificially induced carrier mass and spin-orbit coupling

The interplay of massive electrons with spin-orbit coupling in bulk graphene results in a spin-valley dependent gap. Thus, a barrier with such properties can act as a filter, transmitting only opposite spins from opposite valleys. In this Letter we show that a strain induced pseudomagnetic field in such a barrier will enforce opposite cyclotron trajectories for the filtered valleys, leading to their spatial separation. Since spin is coupled to the valley in the filtered states, this also leads to spin separation, demonstrating a spin-valley filtering effect. The filtering behavior is found to be controllable by electrical gating as well as by strain.

Magnetic quantum ratchet effect in Si-MOSFETs

We report on the observation of magnetic quantum ratchet effect in metal-oxide semiconductor field-effect-transistors on silicon surface (Si-MOSFETs). We show that the excitation of an unbiased transistor by ac electric field of terahertz radiation at normal incidence leads to a direct electric current between the source and drain contacts if the transistor is subjected to an in-plane magnetic field. The current rises linearly with the magnetic field strength and quadratically with the ac electric field amplitude. It depends on the polarization state of the ac field and can be induced by both linearly and circularly polarized radiation. We present the quasi-classical and quantum theories of the observed effect and show that the current originates from the Lorentz force acting upon carriers in asymmetric inversion channels of the transistors.