arXiv subjects

Chuanwei Zhang

Publications and source records attributed to Chuanwei Zhang.

At least 55 records · Page 3Linked to original sources

arXiv2020

In-plane Zeeman field induced Majorana corner and hinge modes in an $s$-wave superconductor heterostructure

Second-order topological superconductors host Majorana corner and hingemodes in contrast to conventional edge and surface modes in two and three dimensions. However, the realization of such second-order corner modes usually demands unconventional superconducting pairing or complicated junctions or layered structures. Here we show that Majorana corner modes could be realized using a 2D quantum spin Hall insulator in proximity contact with an $s$-wave superconductor and subject to an in-plane Zeeman field. Beyond a critical value, the in-plane Zeeman field induces opposite effective Dirac masses between adjacent boundaries, leading to one Majorana mode at each corner. A similar paradigm also applies to 3D topological insulators with the emergence of Majorana hinge states. Avoiding complex superconductor pairing and material structure, our scheme provides an experimentally realistic platform for implementing Majorana corner and hinge states.

cond-mat.mes-hall↗

arXiv2020

Two-dimensional non-Hermitian topological phases induced by asymmetric hopping in a one-dimensional superlattice

Non-Hermitian systems can host topological states with novel topological invariants and bulk-edge correspondences that are distinct from conventional Hermitian systems. Here we show that two unique classes of non-Hermitian 2D topological phases, a 2$\mathbb{Z}$ non-Hermitian Chern insulator and a $\mathbb{Z}_{2}$ topological semimetal, can be realized by tuning staggered asymmetric hopping strengths in a 1D superlattice. These non-Hermitian topological phases support real edge modes due to robust $\mathcal{PT}$-symmetric-like spectra and can coexist in certain parameter regime. The proposed phases can be experimentally realized in photonic or atomic systems and may open an avenue for exploring novel classes of non-Hermitian topological phases with 1D superlattices.

cond-mat.mes-hall↗

arXiv2020

Topological bands and triply-degenerate points in non-Hermitian hyperbolic metamaterials

Hyperbolic metamaterials (HMMs), an unusual class of electromagnetic metamaterials, have found important applications in various fields due to their distinctive properties. A surprising feature of HMMs is that even continuous HMMs can possess topological edge modes. However, previous studies based on equal-frequency surface (analogy of Fermi surface) may not correctly capture the topology of entire bands. Here we develop a topological band description for continuous HMMs that can be described by a non-Hermitian Hamiltonian formulated from Maxwell's equations. We find two types of three dimensional non-Hermitian triply-degenerate points with complex linear dispersions and topological charges $\pm 2$ and 0 induced by chiral and gyromagnetic effects. Because of the photonic nature, the vacuum band plays an important role for topological edge states and bulk-edge correspondence in HMMs. The topological band results are numerically confirmed by direct simulation of Maxwell's equations. Our work presents a general non-Hermitian topological band treatment of continuous HMMs, paving the way for exploring interesting topological phases in photonic continua and device implementations of topological HMMs.

cond-mat.mes-hall↗

arXiv2020

Magnetic stripe soliton and localized stripe wave in spin-1 Bose-Einstein condensates

The recent experimental realization of spin-orbit coupling for ultracold atomic gases opens a new avenue for engineering solitons with internal spatial structures through tuning atomic band dispersions. However, the types of the resulting stripe solitons in a spin-1/2 Bose-Einstein condensate (BEC) have been limited to dark-dark or bright-bright with the same density profiles for different spins. Here we propose that general types of stripe solitons, including magnetic stripe (e.g., dark-bright) and localized stripe waves (neither bright nor dark), could be realized in a spin-1 BEC with widely tunable band dispersions through modulating the coupling between three spin states and the linear momentum of atoms. \ Surprisingly, a moving magnetic stripe soliton can possess both negative and positive effective masses at different velocities, leading to a zero mass soliton at certain velocity. Our work showcases the great potential of realizing novel types of solitons through band dispersion engineering, which may provide a new approach for exploring soliton physics in many physical branches.

cond-mat.quant-gas↗

arXiv2020

Quantum spiral spin-tensor magnetism

The characterization of quantum magnetism in a large spin ($\geq 1$) system naturally involves both spin-vectors and -tensors. While certain types of spin-vector (e.g., ferromagnetic, spiral) and spin-tensor (e.g., nematic in frustrated lattices) orders have been investigated separately, the coexistence and correlation between them have not been well explored. Here we propose a novel quantum spiral spin-tensor order on a spin-1 Heisenberg chain subject to a spiral spin-tensor Zeeman field, which can be experimentally realized using a Raman-dressed cold atom optical lattice. We develop a method to fully characterize quantum phases of such spiral tensor magnetism with the coexistence of spin-vector and spin-tensor orders as well as their correlations using eight geometric parameters. Our method provides a powerful tool for characterizing spin-1 quantum magnetism and opens an avenue for exploring novel magnetic orders and spin-tensor electronics/atomtronics in large-spin systems.

cond-mat.quant-gas↗

arXiv2020

Topological phases in pseudospin-1 Fermi gases with two-dimensional spin-orbit coupling

The recent experimental realization of spin-orbit (SO) coupling for ultracold bosons and fermions opens an exciting avenue for engineering quantum matter that may be challenging to realize in solid state materials such as SO coupled spin-1 fermions. While one-dimensional SO coupling for spin-1 bosons has been experimentally realized, the generation of two-dimensional (2D) SO coupling and its topological properties are largely unexplored. Here we propose an experimental scheme for realizing a 2D Dresselhaus-type SO coupling in a square lattice for spin-1 Fermi gases. Because of the extended spin degree of freedom, many interesting topological phases could exist without relying on lattice point group symmetries that are crucial in solid state materials. These exotic phases include triply-degenerate points, quadratic band touching, a large Chern number ($C=5$) superfluid with 5 Majorana modes, triple-Weyl fermions, etc. Our scheme can be generalized to larger spins and provides a new route for engineering topological quantum matter by utilizing large spin degrees of freedom, instead of specific lattice symmetries.

cond-mat.quant-gas↗

arXiv2020

Active Perovskite Hyperbolic Metasurface

A special class of anisotropic media, hyperbolic metamaterials and metasurfaces (HMMs), has attracted much attention in recent years due to its unique abilities to manipulate and engineer electromagnetic waves on the subwavelength scale. Because all HMM designs require metal dielectric composites, the unavoidable metal loss at optical frequencies inspired the development of active HMMs, where gain materials is incorporated to compensate the metal loss. Here, we experimentally demonstrate an active type II HMM that operates at vacuum wavelength near 750 nm on a silicon platform. Different from previous active HMMs operating below 1 μm, the dielectric constituent in our HMM is solely composed of gain medium, by utilizing solution processed and widely tunable metal halide perovskite gain. Thanks to the facile fabrication, tunability and silicon compatibility of our active HMM, this work paves the way towards HMM's integration into on chip components, and eventually, into photonic integrated circuits.

physics.optics↗

arXiv2019

Tunable spin-orbit coupling and magnetic superstripe phase in a BEC

Superstripe phases in Bose-Einstein condensates (BECs), possessing both crystalline structure and superfluidity, opens a new avenue for exploring exotic quantum matters---supersolids. However, conclusive detection and further exploration of a superstripe is still challenging in experiments because of its short period, low visibility, fragility against magnetic field fluctuation or short lifetime. Here we propose a scheme in a spin-orbit coupled BEC which overcomes these obstacles and generates a robust magnetic superstripe phase, with only spin (no total) density modulation due to the magnetic translational symmetry, ready for direct real-space observation. In the scheme, two hyperfine spin states are individually Raman coupled with a largely-detuned third state, which induce a momentum-space separation between two lower band dispersions, yielding an effective spin-1/2 system with tunable spin-orbit coupling and Zeeman fields. Without effective Zeeman fields, spin-dependent interaction dominates, yielding a magnetic superstripe phase with a long tunable period and high visibility. Our scheme provides a platform for observing and exploring exotic properties of superstripe phases as well as novel physics with tunable spin-orbit coupling.

cond-mat.quant-gas↗

arXiv2019

Higher-order topological corner states induced by gain and loss

The concept of topological phases has been generalized to higher-order topological insulators and superconductors with novel boundary states on corners or hinges. Meanwhile, recent experimental advances in controlling dissipation (such as gain and loss) open new possibilities in studying non-Hermitian topological phases. Here, we show that higher-order topological corner states can emerge by simply introducing staggered on-site gain/loss to a Hermitian system in trivial phases. For such a non-Hermitian system, we establish a general bulk-corner correspondence by developing a biorthogonal nested-Wilson-loop and edge-polarization theory, which can be applied to a wide class of non-Hermitian systems with higher-order topological orders. The theory gives rise to topological invariants characterizing the non-Hermitian topological multipole moments (i.e., corner states) that are protected by reflection or chiral symmetry. Such gain/loss induced higher-order topological corner states can be experimentally realized using photons in coupled cavities or cold atoms in optical lattices.

cond-mat.mes-hall↗

arXiv2019

Non-Hermitian topological phase transitions for quantum spin Hall insulators

The interplay between non-Hermiticity and topology opens an exciting avenue for engineering novel topological matter with unprecedented properties. While previous studies have mainly focused on one-dimensional systems or Chern insulators, here we investigate topological phase transitions to/from quantum spin Hall (QSH) insulators driven by non-Hermiticity. We show that a trivial to QSH insulator phase transition can be induced by solely varying non-Hermitian terms, and there exists exceptional edge arcs in QSH phases. We establish two topological invariants for characterizing the non-Hermitian phase transitions: i) with time-reversal symmetry, the biorthogonal $\mathbb{Z}_2$ invariant based on non-Hermitian Wilson loops, and ii) without time-reversal symmetry, a biorthogonal spin Chern number through biorthogonal decompositions of the Bloch bundle of the occupied bands. These topological invariants can be applied to a wide class of non-Hermitian topological phases beyond Chern classes, and provides a powerful tool for exploring novel non-Hermitian topological matter and their device applications.

cond-mat.mes-hall↗

arXiv2019

Majorana corner modes with solitons in an attractive Hubbard-Hofstadter model of cold atom optical lattices

Higher order topological superconductors hosting Majorana-Kramers pairs (MKPs) as corner modes have recently been proposed in a two-dimensional (2D) quantum spin Hall insulator (QSHI) proximity-coupled to unconventional cuprate or iron-based superconductors. Here, we show that such MKPs can be realized using a conventional s-wave superfluid with a soliton in cold atom systems governed by the Hubbard-Hofstadter model. The MKPs emerge in the presence of interaction at the "corners" defined by the intersections of line solitons and the one-dimensional edges of the system. Our scheme is based on the recently realized cold atom Hubbard-Hofstadter lattice and will pave the way for observing Majorana corner modes and possible higher order topological superfluidity with conventional s-wave superfluids/superconductors.

cond-mat.mes-hall↗

arXiv2019

Application of support vector machine for the fast and accurate reconstruction of nanostructures in optical scatterometry

Nonlinear regression methods, such as local optimization algorithms, are widely used in the extraction of nanostructure profile parameters in optical scatterometry. The success of local optimization algorithms heavily relies on the estimated initial solution. If the initial solution is not appropriately selected, it will either take a long time to converge to the global solution or will result in a local one. Thus, it is of great importance to developing a method to guarantee the capture of a globally optimal solution. In this paper, we propose a method that combines the support vector machine and Levenberg-Marquardt algorithm for the fast and accurate parameters extraction. The SVM technique is introduced to pick out a sub-range in the rough ranges of parameters, in which an arbitrary selected initial solution for the LM algorithm is then able to achieve the global solution with a higher possibility. Simulations and experiments conducted on a one-dimensional Si grating and a deep-etched multilayer grating have demonstrated the feasibility and efficiency of the proposed method.

math.OC↗

arXiv2019

Non-Hermitian Photonics based on Charge-Parity Symmetry

Parity-time ($\mathcal{PT}$) symmetry, originally conceived for non-Hermitian open quantum systems, has opened an excitingly new avenue for the coherent control of light. By tailoring optical gain and loss in integrated photonic structures, $\mathcal{PT}$ symmetric non-Hermitian photonics has found applications in many fields ranging from single mode lasing to novel topological matters. Here we propose a new paradigm towards non-Hermitian photonics based on the charge-parity ($\mathcal{CP}$) symmetry that has the potential to control the flow of light in an unprecedented way. In particular, we consider continuous dielectric chiral materials, where the charge conjugation and parity symmetries are broken individually, but preserved jointly. Surprisingly, the phase transition between real and imaginary spectra across the exceptional point is accompanied by a dramatic change of the photonic band topology from dielectric to hyperbolic. We showcase broad applications of $\mathcal{CP}$ symmetric photonics such as all-angle polarization-dependent negative refraction materials, enhanced spontaneous emission for laser engineering, and non-Hermitian topological photonics. The $\mathcal{CP}$ symmetry opens an unexplored pathway for studying non-Hermitian photonics without optical gain/loss by connecting two previously distinct material properties: chirality and hyperbolicity, therefore providing a powerful tool for engineering many promising applications in photonics and other related fields.

physics.optics↗

arXiv2019

Experimental realization of a non-magnetic one-way spin switch

Controlling magnetism through non-magnetic means is highly desirable for future electronic devices, as such means typically have ultra-low power requirements and can provide coherent control. In recent years, great experimental progress has been made in the field of electrical manipulation of magnetism in numerous material systems. These studies generally do not consider the directionality of the applied non-magnetic potentials and/or magnetism switching. Here, we theoretically conceive and experimentally demonstrate a non-magnetic one-way spin switch device using a spin-orbit coupled Bose-Einstein condensate subjected to a moving spin-independent dipole potential. The physical foundation of this unidirectional device is based on the breakdown of Galilean invariance in the presence of spin-orbit coupling. Such a one-way spin switch opens an avenue for designing novel quantum devices with unique functionalities and may facilitate further experimental investigations of other one-way spintronic and atomtronic devices.

cond-mat.quant-gas↗

arXiv2019

Spin current generation and relaxation in a quenched spin-orbit-coupled Bose-Einstein condensate

Understanding the effects of spin-orbit coupling (SOC) and many-body interactions on spin transport is important in condensed matter physics and spintronics. This topic has been intensively studied for spin carriers such as electrons but barely explored for charge-neutral bosonic quasiparticles (including their condensates), which hold promises for coherent spin transport over macroscopic distances. Here, we explore the effects of synthetic SOC (induced by optical Raman coupling) and atomic interactions on the spin transport in an atomic Bose-Einstein condensate (BEC), where the spin-dipole mode (SDM, actuated by quenching the Raman coupling) of two interacting spin components constitutes an alternating spin current. We experimentally observe that SOC significantly enhances the SDM damping while reducing the thermalization (the reduction of the condensate fraction). We also observe generation of BEC collective excitations such as shape oscillations. Our theory reveals that the SOC-modified interference, immiscibility, and interaction between the spin components can play crucial roles in spin transport.

cond-mat.quant-gas↗

arXiv2018

Majorana Doublets, Flat Bands, and Dirac Nodes in s-Wave Superfluids

Topological superfluids protected by mirror and time-reversal symmetries are exotic states of matter possessing Majorana Kramers pairs (MKPs), yet their realizations have long been hindered by the requirement of unconventional pairing. We propose to realize such a topological superfluid by utilizing $s$-wave pairing and emergent mirror and time-reversal symmetries in two coupled 1D ultracold atomic Fermi gases with spin-orbit coupling. By stacking such systems into 2D, we discover topological and Dirac-nodal superfluids hosting distinct MKP flat bands. We show that the emergent symmetries make the MKPs and their flat bands stable against pairing fluctuations that otherwise annihilate paired Majoranas. Exploiting new experimental developments, our scheme provides a unique platform for exploring MKPs and their applications in quantum computation.

cond-mat.quant-gas↗

arXiv2018

Experimental realization of a superfluid stripe phase in a spin-orbit-coupled Bose-Einstein condensate enabled by momentum-space hopping

In the past few decades, the search for supersolid-like phases has attracted great attention in condensed matter and ultracold atom communities. Here we experimentally demonstrate a route for realizing a superfluid stripe-phase in a spin-orbit coupled Bose-Einstein condensate by employing a weak optical lattice to induce momentum-space hopping between two spin-orbit band minima. We characterize the striped ground state as a function of lattice coupling strength and spin-orbit detuning and find good agreement with mean-field simulations. We observe coherent Rabi oscillations in momentum space between two band minima and demonstrate a long lifetime of the ground state. Our work offers an exciting new and stable experimental platform for exploring superfluid stripe-phases and their exotic excitations, which may shed light on the properties of supersolid-like states.

cond-mat.quant-gas↗

arXiv2018

Topological photonic orbital angular momentum switch

The large number of available orbital angular momentum (OAM) states of photons provides a unique resource for many important applications in quantum information and optical communications. However, conventional OAM switching devices usually rely on precise parameter control and are limited by slow switching rate and low efficiency. Here we propose a robust, fast and efficient photonic OAM switch device based on a topological process, where photons are adiabatically pumped to a target OAM state on demand. Such topological OAM pumping can be realized through manipulating photons in a few degenerate main cavities and involves only a limited number of optical elements. A large change of OAM at $\sim 10^{q}$ can be realized with only $q$ degenerate main cavities and at most $5q$ pumping cycles. The topological photonic OAM switch may become a powerful device for broad applications in many different fields and motivate novel topological design of conventional optical devices.

quant-ph↗