Valley-dependent Transport Research Articles

Valley-dependent transport through graphene quantum dots due to proximity-induced, staggered spin-orbit couplings

Valley-dependent transport through graphene quantum dots due to proximity-induced, staggered spin-orbit couplings

The study of spin- and valley-dependent electron transport properties in silicene modulated by metallic gates

The study of spin- and valley-dependent electron transport properties in silicene modulated by metallic gates

Valley filters using graphene blister defects from first principles

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone–Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from the K valley to the valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell (BHC) including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the BHC contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on the and channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.

Valley-dependent transport in a mescoscopic twisted bilayer graphene device

We study the valley-dependent electron transport in a four-terminal mesoscopic device of the two monolayer graphene nanoribbons vertically stacked together, where the intersection forms a bilayer graphene lattice with a controllable twist angle. Using a tight-binding lattice model, we show that the longitudinal and transverse conductances exhibit significant valley polarization in the low energy regime for small twist angles. As the twist angle increases, the valley polarization shifts to the high energy regime. This arises from the regrouping effect of the electron band in the twisted bilayer graphene region. But for relatively large twist angles, no significant valley polarization is observed. These results are consistent with the spectral densities of the twisted bilayer graphene.

Exploring valley polarized transport in graphene bilayer flakes

Valleytronics offers new routes to explore possible dual electronic responses related to the valley origin of the carriers. Graphene is known as an optimal material for devices based on valley-filter transport due to the double Dirac cones. Here we focus on understanding the role played by geometrical spatial details in the valley-dependent transport of bilayered graphene flakes, with long length, simulating nanoribbons. Flakes of different widths are considered at the stacking region. We also explore changing the valley transmission when a bias potential is applied. A tight-binding Hamiltonian is used to model the system, and the conductance is obtained by Landauer-Büttiker’s formalism and recursive Green’s function method. Our findings show that considering different bilayered graphene flake widths produces valley-polarized transmissions due to the presence of edge states. Furthermore, the corresponding energy range for which the valley filter occurs depends on the width asymmetries details. The theoretical analysis presented here indicates a potential route for valley-filter transmission scenarios of relatively easy engineering implementation.

Electrical tuning of valley polarization in monolayer transition metal dichalcogenides

The valley degree of freedom in two-dimensional crystalline materials is another promising candidate for designing dissipationless information devices in addition to the spin degree of freedom. The primary challenge for the application of valley degree of freedom is to achieve a significant and controllable valley polarization. Here, we discover an electrical tunable mechanism for generating valley polarization caused by the backscattering induced by Rashba spin-orbit coupling in monolayer nanoribbon of transition-metal dichalcogenides. Since the Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides can be induced by the external electric field, the valley polarization here can be dynamically controlled by means of the external gating. The results of the valley-dependent transport calculations show that a pronounced valley polarization can be generated when the external electric field-induced Rashba spin-orbit coupling is present in the central scattering region of the nanoribbon. Moreover, the efficiency and direction of the valley polarization can be controlled by tuning the Fermi energy and the strength of Rashba spin-orbit coupling. The calculated results also confirm that the valley polarization remains significant in the presence of atomic vacancy defects. This finding opens up a potential possibility for realizing electrical control of the valleytronics devices.

Realization of Photonic Topological Insulators at Terahertz Frequencies Characterized by Time-Domain Spectroscopy

Topological systems are inherently robust to disorder and continuous perturbations, resulting in dissipation-free edge transport of electrons in quantum solids, or reflectionless guiding of photons and phonons in classical wave systems characterized by topological invariants. Realization of robust, lossless topological insulators in the technologically important terahertz (THz) frequency range has drawn immense attention. In this paper, we propose a Kagome lattice structure that supports topological edge states with valley-dependent transport. Optical topological insulators are subsequently fabricated, in which mirror symmetry is broken and degenerate states are lifted at $K({K}^{\ensuremath{'}})$ valleys in the band structure. Theoretical and numerical analyses show that four types of edge states can be obtained. The performance of the fabricated topological waveguides is characterized with THz time-domain spectroscopy (TDS). Nearly identical THz wave transmission in the 0.437--0.453-THz range is verified with a straight-line topological waveguide and Z-shaped waveguides with multiple sharp turning corners, which quantitatively illustrate the strong backscattering suppression effects from the nontrivial topology edge states of the structures. Particularly, the THz TDS measurements provide important extra time-domain information about the topological waveguiding mode, which is generally unavailable from previous topological insulator investigations solely replying on power spectrum measurements.

Effect of the Rashba spin-orbit coupling on spin transport properties in WSe2 superlattice

We report a theoretical study of the spin and valley dependent transport properties in a WSe2 superlattice with external Rashba spin-orbit interaction (ERSOI). It is shown that the conductance with and without spin flip strongly depends on the ERSOI and has an oscillatory behavior with respect to ERSOI strength, thus WSe2 superlattice can act as a spin-converter. Also, the magnitude of spin-dependent conductance can be efficiently controlled by potential barrier height and number of superlattice barriers. Results show that the valley and spin polarization depend on the number of superlattice barriers and exhibit an oscillatory evolution versus ERSOI strength and potential barrier height. These results clarify that a WSe2 superlattice can be useful for the design of novel valleytronic and spintronic devices.

Valley filtering in graphene under a magnetic proximity

We investigate valley-dependent electronic transport properties of graphene in close proximity to a thick ferromagnetic insulator (FI). It is found that spin-resolved valley filtering can be achieved under the interplay between the stray field emanated from the FI and the exchange-proximity interaction. In the intraband-tunneling regime, there exists a window of Fermi energy where both the spin and valley polarization of the output current are almost complete $(\ensuremath{\approx}\ensuremath{-}100%)$. In the interband-tunneling regime, the valley polarization can approach $100%$ or $\ensuremath{-}70%$ near some conductance peaks, while the spin polarization is near $100%$ in a wide window of Fermi energy. The role of valley-Zeeman coupling is examined for both cases. The spin-valley filtering is found to be tunable by electric gating. Our findings are helpful for designing a spin-valley filter based on graphene with imprinted ferromagnetism.

Valley-Dependent Electronic Transport in a Graphene-Based Magnetic-Strained Superlattice

Valley-Dependent Electronic Transport in a Graphene-Based Magnetic-Strained Superlattice

Valley-dependent transport in strain engineering graphene heterojunctions

We study the effect of strain on band structure and valley-dependent transport properties of graphene heterojunctions. It is found that valley-dependent separation of electrons can be achieved by utilizing strain and on-site energies. In the presence of strain, the values of transmission can be effectively adjusted by changing the strengths of the strain, while the transport angle basically keeps unchanged. When an extra on-site energy is simultaneously applied to the central scattering region, not only are the electrons of valleys K and K′ separated into two distinct transmission lobes in opposite transverse directions, but the transport angles of two valleys can be significantly changed. Therefore, one can realize an effective modulation of valley-dependent transport by changing the strength and stretch angle of the strain and on-site energies, which can be exploited for graphene-based valleytronics devices.

Valley-resolved transport in zigzag graphene nanoribbon junctions

Applying the transfer matrix and Green’s function methods, we study the valley-resolved transport properties of zigzag graphene nanoribbon (ZGNR) junctions. The width of the left and right ZGNRs are N L and N R , and N L ≥ N R . The step/dip positions of the conductance G, the intravalley transmission coefficients (T KK and ), and the valley polarization efficiency correspond to the subband edges of the right/left ZGNR that are controlled by N R /N L . The intervalley transmission coefficients ( and ) exhibit peaks at most of the subband edge of the left and right ZGNRs. In the bulk gap of the right ZGNR, =0, and = ±1, the valley polarization is well preserved. As N R increases, the energy region for = ±1 decreases. When N L is fixed and N R decreases, G, T KK , and exhibit more and more dips, and the peaks of () become more and more high, especially when (N L − N R )/2 is odd. These characters are quite useful for manipulating the valley dependent transport properties of carriers in ZGNR junctions by modulating N L or N R , and our results are helpful to the design of valleytronics based on ZGNR junctions.

Effect of the strain on spin-valley transport properties in MoS2 superlattice

The effect of the strain on the spin and valley dependent transport properties, including the conductance and polarization, through a monolayer MoS2 superlattice under Rashba spin–orbit coupling is theoretically investigated. It is found that the conductance strongly depends on the spin and valley degrees of freedom, and spin-inversion can be achieved by MoS2 superlattice. Also, the spin and valley dependent conductance in a monolayer MoS2 superlattice can be efficiently adjusted via strain and the number of the superlattice barriers. Moreover, it is demonstrated that both the magnitude and sign of the spin and valley polarization depend on the strain strength, the number of barriers, and electrostatic barrier height. Both full spin and valley polarized current (with 100% or − 100% efficiency) can be realized in a MoS2 superlattice under strain.

Into the valley

Quantum Optics The topological features of band structures provide robust propagation paths for classical and quantum states of light. Another ingredient of band structure is the valley degree of freedom, in which the propagation path of light is dependent on its polarization state. Chen et al. used both topology and valley-dependent transport to design and fabricate a photonic crystal structure capable of forming the basis of optical quantum circuits. The authors demonstrate the input of left- and right-circularly polarized states of quantum light, their separation into different paths, and controlled interference as the single photons are brought back together again. The results provide a route for developing complex optical quantum circuits for on-chip quantum information processing. Phys. Rev. Lett. 126 , 230503 (2021).

Valley-dependent electronic transport in quantum Hall systems of α-T3 model

We study the quantum transport in α-T3 model lattice in the presence of a perpendicular magnetic field. It is found that valley pseudospin is also a very important degree of freedom for electrons in α-T3 model lattice, which can be modulated by the magnetic field. When a perpendicular magnetic field is applied to α-T3 model lattice, the electrons in the two valleys have different responses to the magnetic field. We found that the continuous subbands of α-T3 model lattice are splitted into discrete Landau levels by the perpendicular magnetic fields, and the Landau levels for the two valleys are different, which leads to high valley polarization. Our results may stimulate further experimental studies of the realization of valleytronic devices.

Strain-Controlled Valley Polarization in a Graphene Under the Modulation of a Realistic Magnetic Field

Valley polarization is one basic concept in valleytronic device applications. However, the coexistence of this scenario in graphene under the joint modulation of a realistic magnetic field and a strain has not been reported. Therefore, in this paper, we use the transfer-matrix method to illustrate the effect of a strain on the valley-dependent transport properties of electrons in a graphene with a realistic magnetic field. We find that the valley polarization strongly depends on the strength and the width of the strained barrier rather than on its position. We also find that the 100% valley polarization can be continuously achieved in a broad Fermi energy region by controlling the strain. This point makes it easier to design the new kinds of the valleytronic devices based on graphene modulated by the realistic magnetic field and the strain.

Spin and Valley Filter Based on Two-Dimensional WSe2 Heterostructures

In this work, we investigate the spin and valley transport properties of a $\mathrm{W}{\mathrm{Se}}_{2}$ monolayer placed on top of a ferromagnetic insulator. We are interested in controlling the transport properties by applying external potentials to the system. To obtain spin and valley polarizations, we consider a single- and a double-barrier structure with gate potentials. We analyze how the efficiency of these polarized transport properties depends on the gate-potential intensities and geometrical configurations. Additionally, we investigate how the spin and valley transport properties are modified when an ac potential is applied to the system. We obtain a controllable modulation of the spin and valley polarizations as a function of the intensity and frequency of the ac potential, mainly in the terahertz range. These results validate the proposal of double quantum well structures of $\mathrm{W}{\mathrm{Se}}_{2}$ as candidates to provide spin- and valley-dependent transport within an optimal geometrical parameter regime.

Anomalous Josephson effect and quantum anomaly in inversion asymmetric Weyl semimetals

We study a Josephson junction involving an inversion-asymmetric Weyl semimetal in presence of time-reversal symmetric (TRS) or time-reversal symmetry broken tilt in the Weyl spectra. We reveal that both types of tilts in the Weyl nodes lead to a Josephson $0$-$\pi$ transition and a zero bias valley/chiral supercurrent. Strikingly, the TRS tilt gives rise to a pure valley Josephson current (VJC) and TRS broken tilt induces a pure chirality Josephson current (CJC) in this system. The VJC and CJC are the manifestation of valley symmetry broken and $\mathbb{Z}_2$ symmetry broken by the respective tilt. We obtain the reversal of a pure VJC and pure CJC even in the zero bias condition controllable by the junction length. Our analysis of controllability of valley and chirality dependent transport in an inversion asymmetric Weyl semimetal junction could allow applications in valleytronics and chiralitytronics, respectively. The tilt induced Josephson effect provides an alternative route for supercurrent $0$-$\pi$ transition, different from the conventional ferromagnetism Josephson junctions where the spin polarization is essential. In the long junction and zero temperature limit, VJC and CJC are associated with a quantum anomaly which is manifested through a discontinuous jump in the current in absence of TRS and TRS breaking tilts, respectively.

Tunable valley filter efficiency by spin–orbit coupling in silicene nanoconstrictions* *Project supported by the Natural Science Foundation of Zhejiang Province, China (Grant No. LQ17A040001), the National Natural Science Foundation of China (Grant Nos. 61874078, 11647046, and 61904125), the National Key Research and Development Program of China (Grant No. 2018YFB2202100), and the Science and Technology Planning Project of Wenzhou City (Grant

Valley filter is a promising device for producing valley polarized current in graphene-like two-dimensional honeycomb lattice materials. The relatively large spin–orbit coupling in silicene contributes to remarkable quantum spin Hall effect, which leads to distinctive valley-dependent transport properties compared with intrinsic graphene. In this paper, quantized conductance and valley polarization in silicene nanoconstrictions are theoretically investigated in quantum spin-Hall insulator phase. Nearly perfect valley filter effect is found by aligning the gate voltage in the central constriction region. However, the valley polarization plateaus are shifted with the increase of spin–orbit coupling strength, accompanied by smooth variation of polarization reversal. Our findings provide new strategies to control the valley polarization in valleytronic devices.

Spin- and valley- dependent energy band and polarization in ferromagnetic silicene superlattice with circularly polarized light

We investigated the spin- and valley-dependent energy band and transport properties in ferromagnetic silicene superlattice with an off-resonant circularly polarized light. It is obviously found that the spin- and valley-dependent energy gap can be well tuned via the off-resonant circularly polarized light. Fully spin and valley-polarized can be realized in certain ranges of light intensity. What is particularly interesting is that by slightly adjusting the energy of the light field, the valley polarization can be suddenly reversed from 100% to -100%. Thus, our results are expected to be informative for switching devices related to valleytronics.