Valley Filter Research Articles

Reconfigurable Photonic Valley Filter in Hybrid Topological Heterostructures

AbstractTopological photonics has emerged as an important branch of photonics for its excellent ability to robustly manipulate light. As a widely used topological photonic platform, valley photonic crystals have attracted great attention recently due to the unique opportunities the valley degree of freedom provides to potentially encode and process binary photonic information. However, an efficient and controllable way to generate pure valley current is still lacking. Here, a perfect photonic valley filter with on‐demand routing and switching functionalities by exploiting the unique physics of magneto‐optic and valley photonic crystal is proposed. Particularly, an additional width degree of freedom is introduced by inserting an intermediate layer with matched Dirac points at specific valleys between two domains of topologically distinct photonic crystals. The resultant three‐layer topological heterostructures support large‐area valley polarized states with tunable mode width. Moreover, perfect photonic valley filters to generate and guide the pure valley current through reconfigurable propagation paths by only changing the directions of external magnetic fields are also demonstrated. The work not only lays a solid foundation on the principle and design of photonic valley filters, the great reconfigurability of the design also provides broad application prospects in photonic integrated networks and on‐chip integrated communication systems.

Valley filter of black phosphorus using irradiation by linearly polarized light

Valley filter of black phosphorus using irradiation by linearly polarized light

Floquet-engineered valley topotronics in Kekulé-Y bond textured graphene superlattice

The exquisite distortion in a Kekulé -Y (Kek-Y) superlattice merges the two inequivalent Dirac cones (from the K- and the K′- points) into the highest symmetric Γ-point in the hexagonal Brillouin zone. Here, we report that UV circularly polarized light not only opens up a topological gap at the Γ-point, but also lifts the valley degeneracy at that point. Endowed with Floquet dynamics and by devising a scheme of high-frequency approximation, we propose that the left/right-handedness in polarized light offers the possibility to realize valley-selective circular dichroism in a Kek-Y-shaped graphene superlattice. In addition, the non-vanishing Berry curvature and enumeration of the valley-resolved Chern number CK/CK′=+1/−1 enable us to assign two pseudospin flavors (up/down) with the two valleys. Thereby, the above observations confirm the topological transition, suggesting the ease of realizing the valley quantum anomalous Hall state within the photon-dressed Kek-Y. These findings further manifest a non-zero optical valley polarization that is maximal at the Γ-point. Our paper thus proposes an optically switchable topological valley filter, which is desired in the evolving landscape of valleytronics.

Optoelectronically controlled spin-valley filter and nonlocal switch based on an asymmetrical silicene magnetic superconducting heterostructure

We investigate the effects of the circularly polarized light (CPL) and the electric field (EF) on the nonlocal transport in a silicene-based antiferromagnet/superconductor/ferromagnet (AF/S/F) asymmetrical junction. For case I (II), the CPL and the EF are applied simultaneously in the antiferromagnetic (ferromagnetic) region, whereas in the ferromagnetic (antiferromagnetic) region, only a constant EF is considered. The spin-valley-resolved conductance can be turned on or off by adjusting the CPL or the EF. The AF/S/F junction can be manipulated as a spin-locked valley filter for case I, while for case II, it can be used not only as a valley-locked spin filter but also as a nonlocal switch between two pure nonlocal processes. Such interesting nonlocal switch effect can be effectively controlled by reversing the direction of the incident energy axis, the handedness of the CPL, or the direction of the EF. These findings may open an avenue to the design and manufacture of the spintronic and valleytronic devices based on the asymmetrical silicene magnetic superconducting heterostructure.

Optical snake states in a photonic graphene.

We propose an optical analog of electron snake states based on an artificial gauge magnetic field in a photonic graphene implemented by varying distances between cavity pillars. We develop an intuitive and exhaustive continuous model based on tight-binding approximation and compare it with numerical simulations of a realistic photonic structure. The allowed lateral propagation direction is shown to be strongly coupled to the valley degree of freedom, and the proposed photonic structure may be used as a valley filter.

Valley filtering using magnetic proximity effect in monolayer WS[formula omitted]

Valley filtering using magnetic proximity effect in monolayer WS[formula omitted]

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.

Strain-engineered magnon states in two-dimensional ferromagnetic monolayers

We systematically investigate the strain-engineered magnon states in two-dimensional (2D) ferromagnetic monolayers. By suitable engineering of an inhomogeneous strain, we demonstrate the emergence of magnon Landau levels and magnon snake states in 2D ferromagnetic monolayers. We show a magnon valley Hall effect and valley filter without relying on any external fields. Our proposal offers us another way to manipulate magnon valley transport and construct different types of flexible spintronic devices, and is experimentally feasible for 2D ferromagnetic materials by using state-of-art techniques. Published by the American Physical Society 2024

A generalised FK model for the 558 line defect in graphene

ABSTRACT Defects in two-dimensional nanomaterials do not always affect their properties negatively, but often have useful functionality. The 558 line defect in graphene distinctively acts as a quasi-one-dimensional metallic wire and a valley filter, which is already experimentally realised. To have a thorough understanding of the 558 line defect in graphene, we have extended the conventional FK model and have applied the model to theoretically investigate the structure of the 558 line defect, which agrees well with the first-principle simulation result. In the generalised FK model, it is vital to take into account bond energy correction to the interatomic force of atoms on the defect line. The generalised FK model can be used to predict structures of one-dimensional topological defects in other two-dimensional materials.

Valley filtering in black phosphorus nanofilms under a magnetic-electric barrier

Valley filtering in black phosphorus nanofilms under a magnetic-electric barrier

Half-valley semimetal and largest inverse topological caloric effect at topological quantum criticality in the chiral-symmetric AIII class

Half-valley semimetal (HVSM) and single-valley states are the hallmark of valleytronics in two-dimensional honeycomb lattice materials, but their quasi-one-dimensional analog that takes advantage of quantum manipulation has not yet been realized. We propose a double-helical ladder model described by a coupled double Su–Schrieffer–Heeger chain, wherein the interchain coupling controlled by magnetic flux induces time-reversal and particle-hole symmetry breaking and preserves only the chiral symmetry, which is classified into the AIII symmetry class. It realizes valley polarization, single-valley topological insulator, and HVSM as the topological quantum criticality (TQC), signaling well valley filter or valve effects. Furthermore, the TQC produces the largest inverse topological caloric effect accompanied by a T-linear relation of isothermal entropy change at ultra-low temperatures. Our findings not only open alternative perspectives for multifunctional quantum devices in valleytronics but also shed light on the thermodynamic characterization of TQC and promote the rapid development of topological quantum refrigeration technology.

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.

Valleytronic topological filters in silicene-like inner-edge systems

Inner edge state with spin and valley degrees of freedom is a promising candidate for designing a dissipationless device due to the topological protection. The central challenge for the application of the inner edge state is to generate and modulate the polarized currents. In this work, we discover a new mechanism to generate fully valley- and spin–valley-polarized current caused by the Bloch wavevector mismatch (BWM). Based on this mechanism, we design some serial-typed inner-edge filters. By using once of the BWM, the coincident states could be divided into transmitted and reflected modes, which can serve as a valley or spin–valley filter. In particular, while with twice of the BWM, the incident current is absolutely reflected to support an off state with a specified valley and spin, which is different from the gap effect. These findings give rise to a new platform for designing valleytronics and spin-valleytronics.

High-efficiency spin and valley filter in a monolayer WSe2 superlattice modulated by an anti-handed off-resonance circularly polarized light

We have theoretically studied spin and valley transport properties in an optically-controlled electromagnetic superlattice based on monolayer WSe2. Owing to the regulation of the anti-handed off-resonance circularly polarized light, we find that spin- and valley-dependent line-type resonant peaks appear in the transport forbidden region for small incident angles, so that high-efficiency spin-valley polarization can be obtained. The result indicates that the high-efficiency spin and valley filter can be more conveniently implemented based on the 2D transition metal dichalcogenides superlattice. In addition, the number of spin- and valley-dependent line-type resonant peaks is tunable using an incident angle, gate voltage and the width of the modulated regions.

Atomic Valley Filter Effect Induced by an Individual Flower Defect in Graphene

Owing to the bipartite nature of honeycomb lattice, the electrons in graphene host valley degree of freedom, which gives rise to a rich set of unique physical phenomena including chiral tunneling, Klein paradox, and quantum Hall ferromagnetism. Atomic defects in graphene can efficiently break the local sublattice symmetry, and hence, have significant effects on the valley-based electronic behaviors. Here we demonstrate that an individual flower defect in graphene has the ability of valley filter at the atomic scale. With the combination of scanning tunneling microscopy and Landau level measurements, we observe two valley-polarized density-of-states peaks near the outside of the flower defects, implying the symmetry breaking of the K and K′ valleys in graphene. Moreover, the electrons in the K valley can highly penetrate inside the flower defects. In contrast, the electrons in the K′ valley cannot directly penetrate, instead, they should be assisted by the valley switch from the K′ to K. Our results demonstrate that an individual flower defect in graphene can be regarded as a nanoscale valley filter, providing insight into the practical valleytronics.

Exploring valley polarized transport in graphene bilayer flakes

Exploring valley polarized transport in graphene bilayer flakes

Effectuating tunable valley selection via multiterminal monolayer graphene devices

Valleytronics using two-dimensional materials opens unprecedented opportunities for information processing using a valley polarizer as a basic building block. Various methodologies, such as strain engineering, the inclusion of line defects, and the application of static magnetic fields, have been widely explored for creating valley polarization. However, these methods suffer from low transmission or lack of polarization directionality. To overcome the above limitations, we propose an all-electrical valley polarizer using zigzag edge graphene nanoribbons in a multiterminal device geometry. The proposed device can be gate-tuned to operate along two independent regimes: (i) a terminal-specific valley filter that utilizes band-structure engineering, and (ii) a parity-specific valley filter that exploits the parity selection rule in zigzag edge graphene. We show that the device exhibits intriguing physics in the multimode regime of operation affecting the valley polarization; we investigate various factors influencing polarization in wide-device geometries, such as optical analogs of graphene Dirac fermions, angle-selective transmission via $p\ensuremath{-}n$ junctions, and the localization of edge states. Furthermore, we evaluate the performance of the proposed device structures in the presence of Anderson short-range disorder at the edges and the bulk, and we find it to be resilient to edge disorder even for a higher disorder strength. The device geometry is optimized to achieve maximum valley polarization, thereby paving the way for a physics-based tunable valleytronic device.

Computational discovery of two-dimensional rare-earth iodides: promising ferrovalley materials for valleytronics

Two-dimensional Ferrovalley materials with intrinsic valley polarization are rare but highly promising for valley-based nonvolatile random access memory and valley filter devices. These ferromagnetic materials exhibit valleys at or near the Fermi level with intrinsic magnetism. The strong coupling between magnetism and spin–orbit coupling induces intrinsic valley polarization. Using Kinetically Limited Minimization, an unconstrained crystal structure prediction algorithm, and prototype sampling based on first-principles calculations, we have discovered new Ferrovalley materials, rare-earth iodides RI2, where R is a rare-earth element belonging to Sc, Y, or La-Lu, and I is Iodine. The rare-earth iodides are layered and demonstrate either 2H, 1T, or 1T d phase as the ground state in bulk, analogous to transition metal dichalcogenides (TMDCs). The calculated exfoliation energy of monolayers (MLs) is comparable to that of graphene and TMDCs, suggesting possible experimental synthesis. The MLs in the 2H phase exhibit ferromagnetism due to unpaired electrons in d and f orbitals. Throughout the rare-earth series, d bands have valley polarization at K and points in the Brillouin zone in the vicinity of the Fermi level. Large intrinsic valley polarization in the range of 15–143 meV without external stimuli is observed in these Ferrovalley materials, which can be enhanced further by applying an in-plane bi-axial strain. These valleys can selectively be probed and manipulated for information storage and processing, potentially offering superior performance beyond conventional electronics and spintronics. We further show that the 2H ferromagnetic phase of RI2 MLs possesses non-zero Berry curvature and exhibits anomalous valley Hall effect with considerable anomalous Hall conductivity. Our work will incite exploratory synthesis of the predicted Ferrovalley materials and their application in valleytronics and beyond.

Robust valley filter induced by quantum constructive interference in graphene with line defect and strain

In this paper, we theoretically investigate the manipulation of valley-polarized currents and the optical-like behaviours of Dirac fermions in graphene with single line defect. After the introduction of a local uniaxial strain, the valley transmission probability increases and transmission plateau emerges in a large angle range. Such phenomenon originates from resonant tunnelling, and the strain act as an antireflective coating for the valley states, analogous to the antireflective coating in an optical device. This indicates that perfect valley polarization can occur in a larger incident angle range compared with solely line defect. Interestingly, in the presence of Anderson disorder, even though the transmission decreases, the valley polarization is still robust. Our theoretical findings may be experimentally observable and valuable for valleytronic applications based on graphene.

All-electrical valley filtering in graphene systems. I. A path to integrated electro-valleytronics

Probing and controlling the valley degree of freedom in graphene systems by transport measurements has been a major challenge to fully exploit the unique properties of this two-dimensional material. In this theoretical work, we show that this goal can be achieved by a quantum-wire geometry made of gapped graphene that acts as a valley filter with the following favorable features: (i) all electrical gate control, (ii) electrically switchable valley polarity, (iii) robustness against configuration fluctuation, and (iv) potential for room temperature operation. This valley filtering is accomplished by a combination of gap opening in either bilayer graphene with a vertical electrical field or single layer graphene on h-BN, valley splitting with a horizontal electric field, and intervalley mixing by defect scattering. In addition to functioning as a building block for valleytronics, the proposed configuration makes it possible to convert signals between electrical and valleytronic forms, thus allowing for the integration of electronic and valleytronic components for the realization of electro-valleytronics.