Pseudomagnetic Field Research Articles

Hinge Modes of Surface Arcs in a Synthetic Weyl Phononic Crystal.

Chiral bulk Landau levels and surface arcs, as the two distinctive features unique to Weyl semimetals, have each attracted enormous interest. Recent works have revealed that surface-arc modes can support one-sided chiral hinge modes, a hallmark of the three-dimensional quantum Hall effect, as a combined result of chiral Landau levels of bulk states and magnetic response of surface arcs. Here, we exploit a two-dimensional phononic crystal to construct an ideal Weyl semimetal under a pseudomagnetic field, in which a structural parameter is combined to construct a synthetic three-dimensional space. By directly measuring the acoustic pressure fields, we have not only visualized the one-sided chiral hinge modes, but also observed the quantized Landau level modes. The results pave the way to explore the high-dimensional quantum Hall physics in low-dimensional phononic platforms.

Valley-selective manipulation of moiré excitons through optical Stark effect

Abstract Semiconductor moiré superlattices provide great platforms for exploring exotic collective excitations. Optical Stark effect, a shift of the electronic transition in the presence of light field, provides an ultrafast and coherent method of manipulating matter states, which, however, has not been demonstrated in moiré materials. Here, we report the valley-selective optical Stark effect of moiré excitons in the WSe2/WS2 superlattice by using transient reflection spectroscopy. Prominent valley-selective energy shifts up to 7.8 meV have been observed for moiré excitons, corresponding to pseudo-magnetic fields as large as 34 T. Our results provide a route to coherently manipulate exotic states in moiré superlattices.

Electric, thermal, and thermoelectric magnetoconductivity for Weyl/multi-Weyl semimetals in planar Hall set-ups induced by the combined effects of topology and strain

We continue our investigation of the response tensors in planar Hall (or planar thermal Hall) configurations where a three-dimensional Weyl/multi-Weyl semimetal is subjected to the combined influence of an electric field E\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{E}}$$\\end{document} (and/or temperature gradient ∇rT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ abla _{{\ extbf{r}} } T$$\\end{document}) and an effective magnetic field Bχ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{B}}_\\chi$$\\end{document}, generalizing the considerations of Phys. Rev. B 108 (2023) 155132 and Physica E 159 (2024) 115914. The electromagnetic fields are oriented at a generic angle with respect to each other, thus leading to the possibility of having collinear components, which do not arise in a Hall set-up. The net effective magnetic field Bχ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{B}}_\\chi$$\\end{document} consists of two parts—(a) an actual/physical magnetic field B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{B}}$$\\end{document} applied externally; and (b) an emergent magnetic field B5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{B}}_5$$\\end{document} which quantifies the elastic deformations of the sample. B5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ extbf{B}}_5$$\\end{document} is an axial pseudomagnetic field because it couples to conjugate nodal points with opposite chiralities with opposite signs. Using a semiclassical Boltzmann formalism, we derive the generic expressions for the response tensors, including the effects of the Berry curvature (BC) and the orbital magnetic moment (OMM), which arise due to a nontrivial topology of the bandstructures. We elucidate the interplay of the BC-only and the OMM-dependent parts in the longitudinal and transverse (or Hall) components of the electric, thermal, and thermoelectric response tensors. Especially, for the co-planar transverse components of the response tensors, the OMM part acts exclusively in opposition (sync) with the BC-only part for the Weyl (multi-Weyl) semimetals.

Phonon Landau Quantization and Enhanced Lifetime in Deformed Graphene.

The pseudomagnetic field effect may offer unique opportunities for the emergence of intriguing phenomena. To date, investigations into pseudomagnetic field effects on phonons have been limited to sound waves in metamaterials. The revelation of this exotic effect on the atomic vibration of natural materials remains elusive. Our simulations of twisted graphene nanoribbons reveal well-defined Landau spectra and sublattice polarization of phonon states, mimicking the behavior of Dirac Fermions in magnetic fields. Both valley-specified helical edge currents and snake orbits are obtained. Analysis of dynamics indicates that phonon Landau states have extended lifetimes, which are crucial for the realization of Landau-level lasing. Our findings demonstrate the occurrence of the phonon pseudomagnetic field effect in natural materials, which has important implications for the mechanical tuning of phonon quantum states at the atomic scale.

Valley-polarized edge plasmons in graphene p–n junctions with pseudomagnetic fields

Abstract Owing to the inherent characteristics of collective excitations in graphene, electrical control of edge plasmons is highly desirable for nanoplasmonic applications. This study investigates valley-polarized edge pseudomagneplasmons in a graphene p–n junction subjected to a strain-induced pseudomagnetic field. A four-component hydrodynamic model is employed and solved via the Wiener–Hopf method, revealing the coexistence of three plasmon modes, including counterpropagating acoustic edge modes, gapless topological edge states, and zero modes. The valley polarization, as determined from the numerically exact solution, is stronger than that predicted by the approximate models. Notably, the confinement of edge plasmons at the graphene p–n junction significantly exceeds that at the graphene/vacuum interface, possibly because of the electron–hole attraction. Furthermore, gate-controlled subwavelength confinement is successfully achieved by applying an appropriate gate voltage, thereby highlighting a unique and promising attribute of edge pseudomagnetoplasmons in graphene p–n junctions.

Effect of a perpendicular magnetic field on bilayer graphene under dual gating

By studying the impact of a perpendicular magnetic field B on AB-bilayer graphene (AB-BLG) under dual gating, we yield several key findings for the ballistic transport of gate U∞. Firstly, we discover that the presence of B leads to a decrease in transmission. At a high value of B, we notice the occurrence of anti-Klein tunneling over a significant area. Secondly, in contrast to the results reported in the literature, where high peaks were found with an increasing in-plane pseudomagnetic field applied to AB-BLG, we find a decrease in conductivity as B increases. However, it is worth noting that in both cases, the number of oscillations decreases compared to the result in the study where no magnetic field was present (B=0). Thirdly, at the neutrality point, we demonstrate that the conductivity decreases and eventually reaches zero for a high value of B, which contrasts with the result that the conductivity remains unchanged regardless of the value taken by the in-plane field. Finally, we consider the diffusive transport with gate U∞=0.2γ1 and observe two scenarios. The amplitude of conductivity oscillations increases with B for energy E less than U∞ but decreases in the opposite case E>U∞.

Photonic Dirac waveguide in inhomogeneous spoof surface plasmonic metasurfaces

Abstract The metamaterial with artificial synthetic gauge field has been proved as an excellent platform to manipulate the transport of the electromagnetic wave. Here we propose an inhomogeneous spoof surface plasmonic metasurface to construct an in-plane pseudo-magnetic field, which is generated by engineering the gradient variation of the opened Dirac cone corresponding to spatially varying mass term. The chiral zeroth-order Landau level is induced by the strong pseudo-magnetic field. Based on the bulk state propagation of the chiral Landau level, the photonic Dirac waveguide is designed and demonstrated in the experimental measurement, in which the unidirectionally guided electromagnetic mode supports the high-capacity of energy transport. Without breaking the time-reversal symmetry, our proposal structure paves a new way for realizing the artificial in-plane magnetic field and photonic Dirac waveguide in metamaterial, and have potential for designing integrated photonic devices in practical applications.

Preparation and Modeling of Graphene Bubbles to Obtain Strain-Induced Pseudomagnetic Fields.

It has been both theoretically predicted and experimentally demonstrated that strain can effectively modulate the electronic states of graphene sheets through the creation of a pseudomagnetic field (PMF). Pressurizing graphene sheets into bubble-like structures has been considered a viable approach for the strain engineering of PMFs. However, the bubbling technique currently faces limitations such as long manufacturing time, low durability, and challenges in precise control over the size and shape of the pressurized bubble. Here, we propose a rapid bubbling method based on an oxygen plasma chemical reaction to achieve rapid induction of out-of-plane deflections and in-plane strains in graphene sheets. We introduce a numerical scheme capable of accurately resolving the strain field and resulting PMFs within the pressurized graphene bubbles, even in cases where the bubble shape deviates from perfect spherical symmetry. The results provide not only insights into the strain engineering of PMFs in graphene but also a platform that may facilitate the exploration of the strain-mediated electronic behaviors of a variety of other 2D materials.

Flat bands and extreme pseudomagnetic fields in monolayer graphene by topography strain engineering

Flat bands and extreme pseudomagnetic fields in monolayer graphene by topography strain engineering

Exciton spin Hall effect in arc-shaped strained WSe2

Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in strained transition metal dichalcogenides (TMDs) to achieve the in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations. Published by the American Physical Society 2024

Engineering the Band Topology in a Rhombohedral Trilayer Graphene Moiré Superlattice.

Moiré superlattices have become a fertile playground for topological Chern insulators, where the displacement field can tune the quantum geometry and Chern number of the topological band. However, in experiments, displacement field engineering of spontaneous symmetry-breaking Chern bands has not been demonstrated. Here in a rhombohedral trilayer graphene moiré superlattice, we use a thermodynamic probe and transport measurement to monitor the Chern number evolution as a function of the displacement field. At a quarter filling of the moiré band, a novel Chern number of three is unveiled to compete with the well-established number of two upon turning on the electric field and survives when the displacement field is sufficiently strong. The transition can be reconciled by a nematic instability on the Fermi surface due to the pseudomagnetic vector field potentials associated with moiré strain patterns. Our work opens more opportunities to active control of Chern numbers in van der Waals moiré systems.

Pseudomagnetic fields in bilayer phononic crystals

Pseudomagnetic field (PMF), as an artificial gauge field, has attracted widespread attention in the exploration of magnetic-like effects in artificial structural materials. It offers a novel mechanism for manipulating wave fields in classical wave systems where there is no or weak response to actual magnetic fields. In this work, we construct acoustic PMFs in bilayer phononic crystals by imposing uniaxial linear gradient strain on the scatterers of both layers. Under the PMFs, the linear nodal rings, occurring at around the K and points of the bilayer phononic crystals, split into acoustics Landau levels (LLs). Specifically, the n = 0 plateau of the LLs splits into two discrete ones due to the interlayer coupling. Furthermore, we construct two heterostructures by splicing two phononic crystals with opposite PMFs and observe unique in-plane snake-like propagations of the edge state as well as oscillations between the upper and lower layers. Bilayer structure provides additional degree of freedom to generate PMFs in various types of semimetals and enriches the manipulation of acoustic wave propagations. In addition, it can be extended to other classical wave systems, such as electromagnetic wave and mechanical systems.

Pseudomagnetic fields in fully relaxed twisted bilayer and trilayer graphene

We present simple models to describe the in-plane and the out-of-plane lattice relaxation in twisted bilayer and symmetrically twisted trilayer graphene. Analytical results and series expansions show that for twist angles θ>1.4∘ , the in-plane atomic displacements lead to pseudomagnetic fields weakly dependent on θ. In symmetrically twisted trilayer graphene, the central layer in-plane relaxation is greatly enhanced. The joint effect of the relaxation-induced pseudoscalar potentials and the associated energy difference between interlayer dimer and non-dimer pairs resulted in a significant electron–hole asymmetry both in twisted bilayer and trilayer graphene.

Motion of Two-Dimensional Excitons in Momentum Space Leads to Pseudospin Distribution Narrowing on the Bloch Sphere.

Motional narrowing implies narrowing induced by motion; for example, in nuclear magnetic resonance, the thermally induced random motion of the nuclei in an inhomogeneous environment leads to a counterintuitive narrowing of the resonance line. Similarly, the excitons in monolayer semiconductors experience magnetic inhomogeneity: the electron-hole spin-exchange interaction manifests as an in-plane pseudomagnetic field with a periodically varying orientation inside the exciton band. The excitons undergo random momentum scattering and pseudospin precession repeatedly in this inhomogeneous magnetic environment, typically resulting in fast exciton depolarization. On the contrary, we show that such magnetic inhomogeneity averages out at high scattering rates due to motional narrowing. Physically, a faster exciton scattering leads to a narrower pseudospin distribution on the Bloch sphere, implying a nontrivial improvement in exciton polarization. The in-plane nature of the pseudomagnetic field enforces a contrasting scattering dependence between the circularly and linearly polarized excitons, providing a spectroscopic way to gauge the sample quality.

Observation of Landau levels and chiral edge states in photonic crystals through pseudomagnetic fields induced by synthetic strain

Observation of Landau levels and chiral edge states in photonic crystals through pseudomagnetic fields induced by synthetic strain

Pseudomagnetic suppression of non-Hermitian skin effect

It has recently been shown that the non-Hermitian skin effect can be suppressed by magnetic fields. In this work, using a two-dimensional tight-binding lattice, we demonstrate that a pseudomagnetic field can also lead to the suppression of the non-Hermitian skin effect. With an increasing pseudomagnetic field, the skin modes are found to be pushed into the bulk, accompanied by the reduction of skin topological area and the restoration of Landau level energies. Our results provide a time-reversal invariant route to localization control and could be useful in various classical wave devices that are able to host the non-Hermitian skin effect but inert to magnetic fields.

A perspective of recent advances in PECVD-grown graphene thin films for scientific research and technological applications

The purpose of this review is to provide a perspective on some of the recent advances in applying graphene thin films, including monolayer and multilayer graphene sheets synthesized by a scalable approach based on plasma-enhanced chemical vapor deposition (PECVD), to scientific research and technologies. The technological applications present here include the use of graphene in flexible electronics, for anticorrosion of metals, and for Si-based technologies including superlubricity and optoelectronics. In the case of fundamental scientific research, we demonstrate that periodic patterns of nanoscale strain distributions in monolayer graphene can lead to local giant pseudomagnetic fields as well as global modifications to the electronic properties of monolayer graphene, including strain-induced valley Hall signals and quantized Landau levels in the absence of external magnetic fields, edge states, as well as increasing electronic correlation with increasing strain. These findings suggest new approaches towards developing emerging quantum states with tunable electronic correlation based on graphene straintronics.

Observation of 3D acoustic quantum Hall states

Quantum Hall effect, the quantized transport phenomenon of electrons under strong magnetic fields, remains one of the hottest research topics in condensed matter physics since its discovery in 2D electronic systems. Recently, as a great advance in the research of quantum Hall effects, the quantum Hall effect in 3D systems, despite its big challenge, has been achieved in the bulk ZrTe5 and Cd3As2 materials. Interestingly, Cd3As2 is a Weyl semimetal, and quantum Hall effect is hosted by the Fermi arc states on opposite surfaces via the Weyl nodes of the bulk, and induced by the unique edge states on the boundaries of the opposite surfaces. However, such intriguing edge state distribution has not yet been experimentally observed. Here, we aim to reveal experimentally the unusual edge states of Fermi arcs in acoustic Weyl system with the aid of pseudo-magnetic field. Benefiting from the macroscopic nature of acoustic crystals, the pseudo-magnetic field is introduced by elaborately designed the gradient on-site energy, and the edge states of Fermi arcs on the boundaries of the opposite surfaces are unambiguously demonstrated in experiments. Our system serves as an ideal and highly tunable platform to explore the Hall physics in 3D system, and has the potential in the application of new acoustic devices.

Chiral effective dynamo and torsion time loops holonomy on dislocated Dirac materials

Chiral torsional anomalies have recently been applied in condensed matter and models of gravitation and quantum field theory, in particular in Dirac and Weyl materials. Moreover, Einstein–Cartan space–time elastic gauge spaces with vanishing spin connection have appeared in the teleparallelism framework. Recently, Ciappina et al. (PRD (2020)) have investigated torsion in quantum field theory through time loops on Dirac materials endowed with torsion in (2 + 1)-dimensional space–time where the third dimension is replaced by time dimension. They considered Riemann-flat spaces graphenes for example. In this paper, a space–time teleparallel geometry is used where we encoded physical information of Burgers vectors on Dirac curved materials (GdA, CQG 38 (2021)). It is shown that when the magnetic or pseudo-magnetic field is encoded in this metric one obtains an interesting (2 + 1)-dimensional space–time is shown to lead to pseudo-Maxwell equations. We also show that a Riemann-flat grapheno, for example, imposes a vanishing Nieh–Yan (NY) torsional anomaly, whereas a curved Dirac material presents a non-vanishing NY anomaly. Torsion-induced holonomy on Dirac materials is investigated. Signatures of chiral dynamo effects in Dirac materials from the chiral chemical potential encoded into the covariant derivative are found. It is shown that in the absence of chiral effect the magnetic field torsion contribution decays, whereas in the chiral dynamo case, a dynamo effect is found. It is shown that from the interaction of chiral chemical potential with torsion an effective chiral chemical potential is found, which depends on the zero-component of Cartan torsion.

Surface potential-adjusted surface states in 3D topological photonic crystals

Surface potential in a topological matter could unprecedentedly localize the waves. However, this surface potential is yet to be exploited in topological photonic systems. Here, we demonstrate that photonic surface states can be induced and controlled by the surface potential in a dielectric double gyroid (DG) photonic crystal. The basis translation in a unit cell enables tuning of the surface potential, which in turn regulates the degree of wave localization. The gradual modulation of DG photonic crystals enables the generation of a pseudomagnetic field. Overall, this study shows the interplay between surface potential and pseudomagnetic field regarding the surface states. The physical consequences outlined herein not only widen the scope of surface states in 3D photonic crystals but also highlight the importance of surface treatments in a photonic system.