Valley States Research Articles

Multiple Broadband Infrared Topological Photonic Crystal Valley States Based on Liquid Crystals

Spectral tunable technology has to meet the requirements of strong robustness and wide spectral range. We propose a method for the transmission and manipulation of infrared topological photonic crystal valley states based on tunable refractive index method that exhibits broad-spectrum and multi-band characteristics, along with a tunable emission angle. With this structure, different rotational directions of vortex light sources can independently excite the K valley and K′ valley within the frequency band ranging from 75.64 THz to 99.61 THz. At frequencies from 142.60 THz to 171.12 THz, it is possible to simultaneously excite both the K valley and K′ valley. The dual refractive index tunable design allows for the adjustment of the emission angle at a fixed frequency, enabling control over the independent excitation of either a single K valley or K′ valley, as well as their simultaneous excitation. This capability has significant implications for photonic computation and tunable filtering, offering enhanced operational flexibility and expanded functionality for future optical communications and integrated optical circuits.

Tunable valley states in two-dimensional ScBr2

Effective manipulation of valley degrees of freedom can offer significant opportunities for both fundamental research and practical applications. In this work, based on the first-principles calculations, we, respectively, studied the modulation of the valley states of the two-dimensional (2D) ferrovalley material ScBr2 in its bilayers and multiferroic heterostructures. The sliding ferroelectricity is found in ScBr2 bilayers, and the ferroelectric polarization is coupled with valley polarization, which can enable the switching of layer-polarization anomalous Hall effect. The switching of magnetic ground states can also be achieved through layer sliding. On the other hand, the reversal of the ferroelectric polarization of Ga2S3 in the ScBr2/Ga2S3 heterojunction can induce a semiconductor to half-metal phase transition, thereby enabling control of the anomalous valley Hall effect for “on” and “off” states. Our work provides two effective ways to manipulate the valley states in 2D materials.

Atomistic Compositional Details and Their Importance for Spin Qubits in Isotope-Purified Silicon Quantum Wells.

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope concentration depth profiles in a SiGe/28Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time-of-flight secondary-ion mass spectrometry down to their respective limits of isotope concentrations and depth resolution. Spin-echo dephasing times and valley energy splittings EVS around have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing toward the suppression of qubit decoherence through hyperfine interaction with crystal host nuclear spins or via scattering between valley states. The concentration of nuclear spin-carrying 29Si is 50 ± 20ppm in the 28Si QW. The resolution limits of APT allow to uncover that both the SiGe/28Si and the 28Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment, representative of the thermal budget experienced by the heterostructure during qubit device processing, broadens the top SiGe/28Si QW interface by about two monolayers, while the width of the bottom 28Si/SiGe interface remains unchanged. Using a tight-binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of % in the QW, resulting from segregation, is instrumental for the observed large . Minimal Ge additions <1%, which get more likely in thin QWs, will hence support high EVS without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubitproperties.

Square-root topological insulator for a dual-band photonic waveguide

We show that square-root topological insulators can be utilized as a platform for dual-band topological waveguides in 2-dimensional photonic crystals. Valley states for each of two frequency band gaps in differently perturbed decorated honeycomb lattices have opposite directions of the phase rotations, revealing opportunities for dual-band unidirectional topological valley transport. The topological valley edge states exist at the boundary between differently perturbed photonic crystals in the two frequency bands. In addition, unidirectional, and robust propagation of electromagnetic waves along the boundary in the two frequency bands are numerically performed. Our work provides extensions of square-root topological insulators to robust, unidirectional, and dual-band photonic waveguides.

Bending immunity in valley edge states and non-Hermitian supercoupling effects

Bending immunity in valley edge states and non-Hermitian supercoupling effects

Ultrafast Coherent Exciton Couplings and Many-Body Interactions in Monolayer WS2.

Transition metal dichalcogenides (TMDs) are quantum confined systems with interesting optoelectronic properties, governed by Coulomb interactions in the monolayer (1L) limit, where strongly bound excitons provide a sensitive probe for many-body interactions. Here, we use two-dimensional electronic spectroscopy (2DES) to investigate many-body interactions and their dynamics in 1L-WS2 at room temperature and with sub-10 fs time resolution. Our data reveal coherent interactions between the strongly detuned A and B exciton states in 1L-WS2. Pronounced ultrafast oscillations of the transient optical response of the B exciton are the signature of a coherent 50 meV coupling and coherent population oscillations between the two exciton states. Supported by microscopic semiconductor Bloch equation simulations, these coherent dynamics are rationalized in terms of Dexter-like interactions. Our work sheds light on the role of coherent exciton couplings and many-body interactions in the ultrafast temporal evolution of spin and valley states in TMDs.

Mapping of valley splitting by conveyor-mode spin-coherent electron shuttling

In Si/SiGe heterostructures, the low-lying excited valley state seriously limits the operability and scalability of electron spin qubits. For characterizing and understanding the local variations in valley splitting, fast probing methods with high spatial and energy resolution are lacking. Leveraging the spatial control granted by conveyor-mode spin-coherent electron shuttling, we introduce a method for two-dimensional mapping of the local valley splitting by detecting magnetic field-dependent anticrossings of ground and excited valley states using entangled electron spin-pairs as a probe. The method has sub-μeV energy accuracy and a nanometer lateral resolution. The histogram of valley splittings spanning a large area of 210 nm by 18 nm matches well with statistics obtained by the established but time-consuming magnetospectroscopy method. For the specific heterostructure, we find a nearly Gaussian distribution of valley splittings and a correlation length similar to the quantum dot size. Our mapping method may become a valuable tool for engineering Si/SiGe heterostructures for scalable quantum computing.

Electric Dipole Coupling of a Bilayer Graphene Quantum Dot to a High-Impedance Microwave Resonator.

We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform that can host long-lived spin and valley states. Our device combines a high-impedance (Zr ≈ 1 kΩ) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection of the interdot transition with a signal-to-noise ratio of 3.5 within 1 μs integration time. The charge-photon interaction is quantified in the dispersive and resonant regimes by comparing the resonator response to input-output theory, yielding a coupling strength of g/2π = 49.7 MHz. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge-photon coupling with bilayer graphene quantum dots.

Valley edge states with opposite chirality in temperature dependent acoustic media

The valley degree of freedom in phononic crystals and metamaterials holds immense promise for manipulating acoustic and elastic waves. However, the impact of acoustic medium properties on valley edge state frequencies and their robustness to one-way propagation in valley topological phononic crystals remains unexplored. While significant attention has been devoted to scatterer design embedded in honeycomb lattices within acoustic and elastic media to achieve valley edge states and topologically protected nontrivial bandgaps, the influence of variations in acoustic medium properties, such as wave velocity and density affected by environmental temperature, has been overlooked. In this study, we investigate the effect of valley edge states and topological phases exhibited by topological phononic lattices in a temperature-dependent acoustic medium. We observe that a decrease in wave velocity and density, influenced by changing environmental temperature, shifts the topological valley edge states to lower frequencies. Therefore, alongside phononic lattice design, it is crucial to consider the impact of acoustic medium properties on the practical application of acoustic topological insulators. This issue becomes particularly significant when a topological phononic crystal is placed in a wave medium that transitions from incompressible to compressible, where wave velocity and density are no longer constant. Our findings offer a novel perspective on investigating topological insulators in variable acoustic media affected by changing thermodynamic and fluid properties.

Tailored Triggering of High-Quality Multi-Dimensional Coupled Topological States in Valley Photonic Crystals

The combination of higher-order topological insulators and valley photonic crystals has recently aroused extensive attentions due to the great potential in flexible and efficient optical field manipulations. Here, we computationally propose a photonic device for the 1550 nm communication band, in which the topologically protected electromagnetic modes with high quality can be selectively triggered and modulated on demand. Through introducing two valley photonic crystal units without any structural alteration, we successfully achieve multi-dimensional coupled topological states thanks to the diverse electromagnetic characteristics of two valley edge states. According to the simulations, the constructed topological photonic devices can realize Fano lines on the spectrum and show high-quality localized modes by tuning the coupling strength between the zero-dimensional valley corner states and the one-dimensional valley edge states. Furthermore, we extend the valley-locked properties of edge states to higher-order valley topological insulators, where the selected corner states can be directionally excited by chiral source. More interestingly, we find that the modulation of multi-dimensional coupled photonic topological states with pseudospin dependence become more efficient compared with those uncoupled modes. This work presents a valuable approach for multi-dimensional optical field manipulation, which may support potential applications in on-chip integrated nanophotonic devices.

Valley edge states as bound states in the continuum

Bound states in the continuum (BICs) are spatially localized states with energy embedded in the continuum spectrum of extended states. The combination of BICs physics and nontrivial band topology theory givs rise to topological BICs, which are robust against disorders and meanwhile, the merit of conventional BICs is attracting wide attention recently. Here, we report valley edge states as topological BICs, which appear at the domain wall between two distinct valley topological phases. The robustness of such BICs is demonstrated. The simulations and experiments show great agreement. Our findings of valley related topological BICs shed light on both BICs and valley physics, and may foster innovative applications of topological acoustic devices.

Electric field dependence of spin qubit in a Si-MOS quantum dot

Abstract Valley, the intrinsic feature of silicon, is an inescapable subject in silicon-based quantum computing. At the spin–valley hotspot, both Rabi frequency and state relaxation rate are significantly enhanced. With protection against charge noise, the valley degree of freedom is also conceived to encode a qubit to realize noise-resistant quantum computing. Here, based on the spin qubit composed of one or three electrons, we characterize the intrinsic properties of valley in an isotopically enriched silicon quantum dot (QD) device. For one-electron qubit, we measure two electric-dipole spin resonance (EDSR) signals which are attributed to partial occupation of two valley states. The resonance frequencies of two EDSR signals have opposite electric field dependences. Moreover, we characterize the electric field dependence of the upper valley state based on three-electron qubit experiments. The difference of electric field dependences of the two valleys is 52.02 MHz/V, which is beneficial for tuning qubit frequency to meet different experimental requirements. As an extension of electrical control spin qubits, the opposite electric field dependence is crucial for qubit addressability, individual single-qubit control and two-qubit gate approaches in scalable quantum computing.

Symmetry and magnetic direction dependent spin/valley splitting and anomalous Hall conductivity of antiferromagnetic monolayer MnPTe3

Due to the time reversal symmetry breaking or spin-valley coupling, there are intrinsic valley splittings in two-dimensional magnetic materials, which are closely related to the symmetry and magnetic order. Based on MnPTe3 monolayers with different symmetries, we present an understanding of direction dependent spin/valley splitting and anomalous Hall conductivity from the perspective of orbital interactions. Unlike T-MnPTe3 with central inversion symmetry, in-plane magnetization leads to further non-degeneracy of valley electrons of H–MnPTe3, and the splitted valley states are spin polarized, which is perpendicular to the in-plane magnetization direction. The model and first-principle calculations consistently indicate that the splitting of the valley states is almost proportional to sinθxz, which originates from the difference in spin-up and spin-down interactions introduced by the breaking of spatial inversion symmetry. Regardless of spin degeneracy, non-zero anomalous Hall conductivity occurs with out-of-plane magnetization, and this direction dependent anomalous Hall effect is caused by mirror symmetry breaking from the neighboring Te atoms and spin-orbit coupling. Magnetization direction is an important means of regulating valley/spin splittings and anomalous Hall effect, which has certain significance for realizing spin or valley polarized Hall effect and the research of spintronic or valleytronic devices.

Observation of Free-Boundary-Induced Chiral Anomaly Bulk States in Elastic Twisted Kagome Metamaterials.

Chiral anomaly bulk states (CABSs) can be realized by choosing appropriate boundary conditions in a finite-size waveguide composed of two-dimensional Dirac semimetals, which have unidirectional and robust transport similar to that of valley edge states. CABSs use almost all available guiding space, which greatly improves the utilization of metamaterials. Here, free-boundary-induced CABSs in elastic twisted kagome metamaterials with C_{3v} symmetry are experimentally confirmed. The robust valley-locked transport and complete valley state conversion are experimentally observed. Importantly, the sign of the group velocity near the K and K^{'} points can be reversed by suspending masses at the boundary to manipulate the onsite potential. Moreover, CABSs are demonstrated in nanoelectromechanical phononic crystals by constructing an impedance-mismatched hard boundary. These results open new possibilities for designing more compact, space-efficient, and robust elastic wave macro- and microfunctional devices.

Long-lived valley states in bilayer graphene quantum dots

Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanisms are unlikely to be at work for valleys, promising more robust qubits. The Berry curvature associated with valley states allows for electrical control of their energies, suggesting routes for coherent qubit manipulation. However, the relaxation time of valley states—which ultimately limits these qubits’ coherence properties and therefore their suitability as practical qubits—is not yet known. Here we measure the characteristic relaxation times of these spin and valley states in gate-defined bilayer graphene quantum dot devices. Different valley states can be distinguished from each other with a fidelity of over 99%. The relaxation time between valley triplets and singlets exceeds 500 ms and is more than one order of magnitude longer than for spin states. This work facilitates future measurements on valley-qubit coherence, demonstrating bilayer graphene as a practical platform hosting electrically controlled, long-lived valley qubits.

Light-Shaping of Valley States.

The established paradigm to create valley states, excitations at local band extrema ("valleys"), is through selective occupation of specific valleys via circularly polarized laser pulses. Here we show a second way exists to create valley states, not by valley population imbalance but by "light-shaping" in momentum space, i.e. controlling the shape of the distribution of excited charge at each valley. While noncontrasting in valley charge, such valley states are instead characterized by a valley current, identically zero at one valley and finite and large at the other. We demonstrate that these (i) are robust to quantum decoherence, (ii) allow lossless toggling of the valley state with successive femtosecond laser pulses, and (iii) permit valley contrasting excitation both with and without a gap. Our findings open a route to robust ultrafast and switchable valleytronics in a wide scope of 2d materials, bringing closer the promise of valley-based electronics.

Tunable bifunctional acoustic logic gates based on topological valley transport

Valley degree of freedom has attracted great interest in the realization of topological edge states in acoustic systems owing to its rich valley-contrasting physics and great potential applications. However, the practice of valley acoustic topological insulators (ATIs) in designing tunable multifunctional devices without changing their structures still remains a great challenge. Here, we show that the antisymmetric and symmetric distribution nature of valley edge states in the valley ATIs with two different domain walls can be utilized to design tunable robust acoustic logic gates (ALGs). We experimentally demonstrate two types of tunable bifunctional ALGs (denoted as ALG-I and ALG-II), in which ALG-I is composed of a single domain wall, and ALG-II is constructed by a bent topological waveguide containing two domain walls. For ALG-I, the functions of logical inclusive OR and logical exclusive OR (denoted as OR and XOR, respectively) can be switched by actively tuning the phases of two input sound sources without changing the structure. For ALG-II, the logic functions OR and XOR can be implemented through the left and right incidences, respectively, of a pair of sound sources. Similarly, the switching of the logic functions OR and XOR on both sides of ALG-II can be realized by simply adjusting the phases of two sound sources. The designed ALGs have the advantages of simple structure, high robustness, as well as active tunability, leading to a wide range of potential applications in integrated acoustics, acoustic communications, and information processing.

Phase-change in topological chiral phononic crystal for directional coupling switch

Recently, acoustic valley Hall topological insulators have become a cutting-edge area of acoustic physics, where the topological phase transition in phononic crystals shows the presence of band inversion through the Dirac point in the momentum space. We developed a 2D hexagonal lattice chiral phononic crystal using reconfigurable construction by extending one side of the original rectangular rods. When the variation of the side length was from left to right, the topological phase transition is triggered by reopening the Dirac degeneracies beyond high-symmetry points in the first Brillouin zone. We numerically showed valley edge state’s propagation through the interface bent toward distinct chiral topological phononic crystals. Moreover, we assembled 2 × 2 cross-waveguides with a defect cavity based on double heterostructure interfaces. The simulated results verify that the phase change is achieved by the directional coupling switching. This research possibly paves the way for exploiting valley edge states to design the complex acoustic waveguide.

Orbital multiferroicity in pentalayer rhombohedral graphene.

Ferroic orders describe spontaneous polarization of spin, charge and lattice degrees of freedom in materials. Materials exhibiting multiple ferroic orders, known as multiferroics, have important parts in multifunctional electrical and magnetic device applications1-4. Two-dimensional materials with honeycomb lattices offer opportunities to engineer unconventional multiferroicity, in which the ferroic orders are driven purely by the orbital degrees of freedom and not by electron spin. These include ferro-valleytricity corresponding to the electron valley5 and ferro-orbital-magnetism6 supported by quantum geometric effects. These orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements and electric control of the valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low-temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tanΘH > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E, we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite fieldE·B(where Bis the magnetic field), but not to the individual fields. Tuning E would switch each ferroic order independently and achieve non-volatile switching of them together. Our observations demonstrate a previously unknown type of multiferroics and point to electrically tunable ultralow-power valleytronic and magnetic devices.

Defect‐Adjusted Valley Edge States and Rainbow Trapping of Elastic Waves in 2D Topological Phononic Crystals

Topological phononic crystals (PnCs) with topologically protected boundary states have important applications in the fields of acoustic wave transmission and control. However, previous studies based on solid‐state PnC systems are mostly limited by fixed structures, resulting in the difficulty to deform the edge states, which partly limits its practical applications. Herein, a 2D solid topological PnC coupled with the defect is designed to achieve the adjustable valley edge state and rainbow trapping. First, by breaking the spatial inversion symmetry, the valley Hall phase transition of elastic wave is realized and valley edge states are obtained. Next, by introducing defects of different widths between the two different valleys’ topological PnCs, both the defect‐adjusted valley edge state and defect state are achieved. Then, by designing different topological PnCs waveguides, the robust transport characteristics of the two above states are compared. Subsequently, a new power divider based on the defect‐adjusted valley edge state is designed, which is found to possess various manners of operation such as equal and unequal power divisions. Finally, based on defect adjustment of the edge states, a rainbow trapping is implemented. This research provides an important guidance for ultrasonic devices, such as waveguides, energy harvesters, and power dividers.