Pseudospin-orbit Coupling Research Articles

Broadband acoustic pseudospin topological states based on the reverse spin-orbit coupling in generalized insulators

Acoustic topological insulators have the excellent characteristic of the pseudospin-dependent one-way transmission of sound edge states immune to backscattering. We realize the broadband acoustic pseudospin topological edge states with subwavelength generalized topological insulators, which is achieved by reverse pseudospin-orbit coupling. The subwavelength band and broadband nontrivial bandgap can be achieved by adjusting the topological structure of the scatterers and introducing resonators. The results demonstrate that the resonator can significantly reduce the frequencies of p-states and d-states by introducing resonance scattering; the scattering size and rotation angles change the frequencies of p-states and d-states in opposite directions by adjusting the distribution of the sound field. Then, we experimentally realize the pseudospin-dependent one-way transmission of sound edge states along the interface separating phononic crystals with distinct topological phases. Our research provides a systematic scheme for the design of acoustic topological insulators with versatile applications.

Soliton in parity-time optical lattice with pseudo spin-orbit coupling

We investigated the existence and stability of solitons in parity-time optical lattice with pseudo spin-orbit coupling (SOC). The numerical results show that the symmetry of solitons still accords with the characteristics of traditional parity-time (PT) soliton symmetry. Due to the existence of PT optical lattice, the polarization component of solitons is not purely real or purely imaginary. And the strength of SOC has a significant impact on the band-gap structure of the system.

Pseudospin-orbit coupling and non-Hermitian effects in the quantum geometric tensor of a plasmonic lattice

Pseudospin-orbit coupling and non-Hermitian effects in the quantum geometric tensor of a plasmonic lattice

Pseudo-spin-orbit-coupling-based manipulation of vector beams using electromagnetically induced transparency.

Based on the electromagnetically induced transparency (EIT) model and the higher-order Poincaré sphere (HOPS) framework, we establish a general paradigm to investigate the paraxial evolution of a vector beam in a tripod EIT system. By quantum-optical analogy, we introduce a formalism with a generalized Pauli-like equation under rotational invariance, in which the pseudo-spin-orbit coupling (PSOC) and the spin-orbit nonseparability of light can coexist. More importantly, we find that both the PSOC-based real and imaginary potentials play a key role in controlling and modulating the nonseparable state of the vector beam to traverse the entire HOPS, where the orientation and ellipticity of the transmitted polarization can be modified by varying the PSOC coefficients. Therefore, an all-optical scheme can be proposed to improve the flexibility for tailoring the space-variant polarization of light in coherent media, where the tunable spatial-polarization multiplexing may be useful in conventional and quantum information processing.

Nonlinear photoconductivities and quantum geometry of chiral multifold fermions

Chiral multifold fermions are quasiparticles that appear only in chiral crystals such as transition metal silicides in the cubic B20 structure (i.e., the CoSi family), and they may show exotic physical properties. Here we study the injection and shift photoconductivities and also the related geometrical quantities for several types of chiral multifold fermions, including spin-1/2 as well as pseudospin-1 and -3/2 fermions, dubbed as Kramers Weyl, triple-point, and Rarita-Schwinger-Weyl (RSW) fermions, respectively. We utilize the minimal symmorphic model to describe the triple-point fermions (TPF). We also consider the more realistic model Hamiltonian for the CoSi family including both linear and quadratic terms. We find that injection currents due to circularly polarized light are quantized as a result of the Chern numbers carried by the multifold fermions within the linear models. Surprisingly, we discover that in the TPF model, the linear shift conductivities, responsible for the shift current generation by linearly polarized light, are proportional to the pseudo spin-orbit coupling and independent of photon frequency. In contrast, for the RSW and Kramer Weyl fermions, the linear shift conductivity is linearly proportional to photon frequency. The numerical results agree with the power-counting analysis for quadratic Hamiltonians. The frequency independence of the linear shift conductivity could be attributed to the strong resonant symplectic Christoffel symbols of the flat bands. Moreover, the calculated symplectic Christoffel symbols show significant peaks at the nodes, suggesting that the shift currents are due to the strong geometrical response near the topological nodes.

Topological acoustic tweezer and pseudo-spin states of acoustic topological insulators

Topological protection wave engineering in artificially structured media is at the forefront of metamaterials research. Acoustic analogs of electronic topological insulators have brought a lot of new opportunities for manipulating sound propagation and have attracted the attention of many scholars. A variety of artificial acoustic systems hosting topological edge states have been proposed. However, the pseudo-spin edge states at a boundary have not been clearly and intuitively characterized. In this article, we provide experimental verification of pseudo-spin states by the method of acoustic particle manipulation by using the rotation of the particle to describe the pseudo-spin sound field characteristics at the boundary of the acoustic topological insulator in detail. It indicates the existence of pseudo-spin–orbit coupling effects and pseudo-spin-related acoustic unidirectional transmission even if the defect exists. Furthermore, the pseudo-spin sound field can be used to control the rotation of multiple particles and droplets. This kind of topological acoustic tweezer with defect immunity has great application potential in the fields of biomedicine and materials science.

Pseudospin–Orbit Coupling for Chiral Light Routings in Gauge-Flux-Biased Coupled Microring Resonators

Pseudospin–Orbit Coupling for Chiral Light Routings in Gauge-Flux-Biased Coupled Microring Resonators

Spin-Chern insulator induced by pseudospin-orbit coupling in phononic crystals

The boundary modes of classical waves protected by the topology are of great significance for robust energy transmission and reliable information processing. Many endeavors have been made to realize topological insulators in electromagnetic, mechanical and acoustic systems. Previous studies on acoustic boundary modes mainly focused on the interface states at the domain walls between different topological phases of phononic crystals. In electromagnetic and mechanical systems, although helical boundary states have been observed, the pseudospin in them is conserved, thus these systems can be considered as two independent copies of Chern insulators. When the pseudospin conservation is broken, the topology cannot be described by Chern numbers, and the spin will vary in transport. This general case remains to be explored both in electronic and classical systems. In this work, we constructed a two-dimensional topological phononic crystal induced by pseudospin-orbit coupling, which hosts the gapless helical boundary modes at the edge of a phononic crystal. Since these boundary states originate from non-zero spin-Chern numbers, the system is called the spin-Chern insulator. We have observed these boundary states in experiments, and observed the spin-flipping transport in the H-shaped sample, proving the spin non-conservation of the boundary states. Different from the earlier work, this work demonstrates two important aspects: first, the acoustic topological insulator is induced by the pseudospin-orbit coupling, and possesses the gapless helical edge states on its boundary, thus is the first analogue of topological insulator for acoustic wave; second, the inherent spin conservation is broken, which expands and deepens the knowledge of current topological physics in classical and electronic systems. The helical boundary states may have potential applications in new topological acoustic devices.

Negative thermal Hall conductance in a two-dimer Shastry-Sutherland model with a π -flux Dirac triplon

We introduce an effective 2-dimer tight-binding model for the family of Shastry-Sutherland models with geometrically tunable triplon excitations. The Rashba pseudospin-orbit coupling induced by the tilted external magnetic field leads to elementary excitations having nontrivial topological properties with {\pi}-Berry flux. The interplay between the in-plane and out-of-plane magnetic field thus allows us to effectively engineer the band structure in this bosonic system. In particular, the in-plane magnetic field gives rise to Berry curvature hotspot near the bottom of the triplon band, and at the same time significantly increases the critical magnetic field for the topological triplon band. We calculate explicitly the experimental signature of the thermal Hall effect of triplons in SrCu2(BO3)2, and show a pronounced and tunabled transport signals within the accessible parameter range, particularly with a change of sign of the thermal Hall conductance. The tilted magnetic field is also useful in reducing the bandwidth of the lowest triplon band. We show it can thus be a flexible theoretical and experimental platform for the correlated bosonic topological system.

Interface dark excitons at sharp lateral two-dimensional heterostructures

We study the dark excitons at the interface of a sharp lateral heterostructure of two-dimensional transition metal dichalcogenides (TMDs). By introducing a low-energy effective Hamiltonian model, we find the energy dispersion relation of exciton and show how it depends on the onsite energy of composed materials and their spin–orbit coupling strengths. It is shown that the effect of the geometrical structure of the interface, as a deformation gauge field (pseudo-spin–orbit coupling), should be considered in calculating the binding energy of exciton. By discretization of the real-space version of the dispersion relation on a triangular lattice, we show that the binding energy of exciton depends on its distance from the interface line. For exciton near the interface, the binding energy is equal to 0.36 eV, while for the exciton far enough from the interface, it is equal to 0.26 eV. Also, it has been shown that for a zigzag interface the binding energy increases by 0.34 meV compared to an armchair interface due to the pseudo-spin-orbit interaction (gauge filed). The results can be used for designing 2D-dimensional-lateral-heterostructure- based optoelectronic devices to improve their characteristics.

Vector solitons in nonlocal optical media with pseudo spin-orbit-coupling.

We numerically investigate the existence and stability of nonlocal vector solitons with pseudo spin-orbit-coupling (SOC). The pseudo SOC is realized by a framework based on the spatial-domain copropagation of two beams with mutually orthogonal polarizations and opposite transverse components of the carrier wave vectors in nonlocal optical media. The numerical results show that there are two kinds of solutions for vector solitons, one is central symmetric, and the other is noncentral symmetric. The solitons may exist below a certain threshold value of the effective SOC strength in the system.

Elastic phononic topological plate with edge and corner sates based on pseudospin-valley-coupling

Topologically protected gapless edge states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with topological corner states have been theoretically predicted in electric systems, and experimentally realized in two-dimensional (2D) mechanical and electromagnetic systems, electrical circuits, optical and sonic crystals, and elastic phononic plates. Here, we demonstrate a pseudospin-valley-coupled phononic TI, which simultaneously exhibits gapped edge states and topological corner states. Pseudospin-orbit coupling edge states and valley-polarized edge state are respectively induced by the lattice deformation and the symmetry breaking. When both of them coexist, these topological edge states will be greatly gapped and the topological corner state emerges. Under direct field measurements, the robust edge propagation behaving as an elastic waveguide and the topological corner mode working as a robust localized resonance are experimentally confirmed. The pseudospin-valley coupling in our phononic TIs can be well-controlled which provides a reconfigurable platform for the multiple edge and corner states, and exhibits well applications in the topological elastic energy recovery and the highly sensitive sensing.

Thermal Hall effect and topological edge states in a square-lattice antiferromagnet

We show that the two dimensional spatial inversion-symmetry (SIS) broken square lattice antiferromagnet with easy-plane spin anisotropy exhibits a thermal Hall effect and the edge modes characterized by the Z 2 topological invariant. These topological properties require a nonzero Berry curvature, and its origin is ascribed to the Dzyaloshinskii-Moriya (DM) interactions or the noncoplanar magnetic ordering generating a U(1) gauge field that couples to the kinetic motion of magnons. Although this picture is established in ferromagnets on the kagome and pyrochlore lattices, it does not apply to our square lattice model since such gauge field cancels out in an edge shared geometry. Instead, our case has an analogy with the anomalous Hall effect of Rashba electronic system where the spin orbit coupling generates an SU(2) gauge field. The two species of magnons defined on antiferromagnetic sublattices can be regarded as the pseudo-spin degrees of freedom of magnons. The DM interactions that emerge due to the SIS breaking serve as a pseudo-spin orbit coupling of magnons and generate a Berry curvature, when the direction of the magnetic moments is properly controlled by the magnetic field. The thermal Hall conductivity of our antiferromagnet shows rapid growth in temperature T beyond the power-law, reflecting the almost gapless low energy branch of the antiferromagnet, which distinctively differs from the T^7/2 -dependence in the pyrochlore ferromagnets. The present system serves as a standard model for the noncentrosymmetric crystals Ba2MnGe2O7 and Ba2CoGe2O7.

Acoustic spin Hall-like effect in hyperbolic metamaterials controlled by the helical wave

Because of the spin-less nature of sound, acoustic helical wave with different helical directions can be taken as a “spin-like” degree of freedom. In this Letter, we examine the pseudospin-orbit coupling effect in acoustics when an acoustic helical wave emitter interacts with the acoustic hyperbolic metamaterial (AHMM). The acoustic helical wave emitter is situated at the boundary of the AHMM, which gives rise to the unidirectional excitation with the trajectory controlled by the helical directions, and hence the acoustic spin Hall-like effect (ASHE) is observed. The ASHE is further demonstrated for the string-type and the membrane-type AHMM based on the hyperbolic dispersion. The reported ASHE paves a new way to exploiting signal routing and unidirectional excitation controlled by the helical directions of the acoustic helical wave.

Self-ordering induces multiple topological transitions for in-plane bulk waves in solid phononic crystals

Topological defects with symmetry-breaking phase transitions have captured much attention. Vortex generated by topological defects exhibits exotic properties and its flow direction can be switched by altering the spin configurations. Contrary to electromagnetic and acoustic domains, the topological transport of elastic waves in periodic structures with topological defects is not well explored due to the mode conversion between the longitudinal and transverse modes. Here, we propose an elastic topological insulator with spontaneously broken symmetry based on the topological theory of defects and homotopy theory. Multiple topological transitions for elastic waves are achieved by topologically modifying the ellipse orientation in a triangular lattice of elliptical cylinders. The solid system, independent of the number of molecules in order parameter space, breaks through the limit of the point-group symmetry to emulate elastic pseudospin-orbit coupling. The transport robustness of the edge states is experimentally demonstrated. Our approach provides new possibilities for controlling and transporting elastic waves.

Reconfigurable topological phononic crystal slabs

We propose two-dimensional reconfigurable phononic crystal slabs that support topologically protected edge states for Lamb waves. It consists of a triangular lattice of triangular-like air holes perforated in a slab and two sandwich cylinders are placed in the matrix of each unit cell. The pseudospin–orbit coupling is achieved by shifting the sandwich cylinders up or down. The robust transport of Lamb waves along reconfigurable interfaces is further demonstrated. The dynamically reconfigurable design provides an excellent platform for tuning the frequency range of topological edge states and steering efficient transport of Lamb waves along arbitrary pathways, which can be employed for elastic-wave communications, signal processing, and sensing.

Chirald-Wave Superfluid in Periodically Driven Lattices

A chiral d-wave superfluid is a preliminary example of interacting topological matter. However, unlike s-wave superfluids prevalent in nature, its existence requires a strong d-wave interaction, a criterion that is difficult to access in ordinary systems. There is no experimental observation of such unconventional superfluid at the moment. Here, we present a new principle for creating a two-dimensional (2D) chiral d-wave superfluid using periodically driven lattices. Because of an imprinted 2D pseudospin-orbit coupling, where the sublattice index serves as the pseudospin, the s-wave interaction between two hyperfine spin states naturally creates a chiral d-wave superfluid. This scheme can be directly implemented in current experiments.

Spin Force on Pseudo Spin Current in Graphene

The present study investigated spin force by using pseudo spin-orbit coupling in an artificial atomic structure for graphene. This spin force, which is applied on pseudo spin current, can be controlled by graphene gap and electric field due to charged nanodot. It seems that, by this force can be constructed a logical system from graphene and semiconductors. In addition, this force can be used for understanding the Zitterbewegung of electron wave packet with pseudo spin-orbit coupling in graphene and create of the charge Hall effect in semiconductors and graphene.

Pseudo-Spin–Orbit Coupling in Graphene within Hydrogenic Impurity Context

In graphene, we report the first theoretical demonstration of how the intrinsic spin orbit interaction can be deduced from the theory and how it can be controlled by tuning a uniform magnetic field, and/or by changing the strength of a long range Coulomb like impurity (adatom), as well as gap parameter. In the impurity context, we find that intrinsic spin-orbit interaction energy may be enhanced by increasing the strength of magnetic field and/or by decreasing the band gap mass term. Additionally, it may be strongly enhanced by increasing the impurity strength. Furthermore, from the proposal of Kane and Mele [Phys. Rev. Lett. 95, 226801 (2005)], it was discussed that the pristine graphene has a quantized spin Hall effect regime where the Rashba type spin orbit interaction term is smaller than that of intrinsic one. Our analysis suggest the nonexistence of such a regime in the ground state of flat graphene.

Commensuration and interlayer coherence in twisted bilayer graphene

The low energy electronic spectra of rotationally faulted graphene bilayers are studied using a long wavelength theory applicable to general commensurate fault angles. Lattice commensuration requires low energy electronic coherence across a fault and preempts massless Dirac behavior near the neutrality point. Sublattice exchange symmetry distinguishes two families of commensurate faults that have distinct low energy spectra which can be interpreted as energy-renormalized forms of the spectra for the limiting Bernal and AA stacked structures. Sublattice-symmetric faults are generically fully gapped systems due to a pseudospin-orbit coupling appearing in their effective low energy Hamiltonians.