# Zhang, Chuanwei

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/3110

Chuanwei Zhang is a professor of the Physics Department faculty. He is interested in theoretical atomic physics and condensed matter physics. Currently his research activities include:

- Ultra-cold atomic gases
- Physical implementation of quantum information
- Topological superconductors
- Quantum chaos, physics of strongly-correlated multiferroic materials
- High temperature cuprate superconductivity
- Graphene

Learn more about Professor Zhang on his home and Physics Department pages.

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### Recent Submissions

Item Spin Current Generation and Relaxation in a Quenched Spin-Orbit-Coupled Bose-Einstein Condensate(Nature Publishing Group, 2019-01) Li, Chuan-Hsun; Qu, Chunlei; Niffenegger, Robert J.; Wang, Su-Ju; He, Mingyuan; Blasing, David B.; Olson, Abraham J.; Greene, Chris H.; Lyanda-Geller, Yuli; Zhou, Qi; Zhang, Chuanwei; Chen, Yong P.; 0000-0002-3080-8698 (Qu, C); Qu, Chunlei; Zhang, ChuanweiUnderstanding 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.Item Time-Reversal-Invariant Spin-Orbit-Coupled Bilayer Bose-Einstein Condensates(American Physical Society) Maisberger, Matthew; Wang, Lin-Cheng; Sun, Kuei; Xu, Yong; Zhang, Chuanwei; Maisberger, Matthew; Wang, Lin-Cheng; Sun, Kuei; Xu, Yong; Zhang, ChuanweiTime-reversal invariance plays a crucial role for many exotic quantum phases, particularly for topologically nontrivial states, in spin-orbit coupled electronic systems. Recently realized spin-orbit coupled cold-atom systems, however, lack the time-reversal symmetry due to the inevitable presence of an effective transverse Zeeman field. We address this issue by analyzing a realistic scheme to preserve time-reversal symmetry in spin-orbit-coupled ultracold atoms, with the use of Hermite-Gaussian-laser-induced Raman transitions that preserve spin-layer time-reversal symmetry. We find that the system's quantum states form Kramers pairs, resulting in symmetry-protected gap closing of the lowest two bands at arbitrarily large Raman coupling. We also show that Bose gases in this setup exhibit interaction-induced layer-stripe and uniform phases as well as intriguing spin-layer symmetry and spin-layer correlation. © 2018 American Physical Society.Item Self-Adapted Floquet Dynamics of Ultracold Bosons in a Cavity(American Physical Society) Luo, Xi-Wang; Zhang, Chuanwei; Luo, Xi-Wang; Zhang, ChuanweiFloquet dynamics of a quantum system subject to periodic modulations of system parameters provides a powerful tool for engineering new quantum matter with exotic properties. While system dynamics is significantly altered, the periodic modulation itself is usually induced externally and independent of Floquet dynamics. Here we propose a new type of Floquet physics for a Bose-Einstein condensate (BEC) subject to a shaken lattice generated inside a cavity, where the shaken lattice and atomic Floquet bands are mutually dependent, resulting in self-adapted Floquet dynamics. In particular, the shaken lattice induces Floquet quasienergy bands for the BEC, whose backaction leads to a self-adapted dynamical normal-superradiant phase transition for the shaken lattice. Such self-adapted Floquet dynamics shows two surprising and unique features: (i) The normal-superradiant phase transition possesses a hysteresis even without atom interactions. (ii) The dynamical atom-cavity steady state could exist at free energy maxima. The atom interactions strongly affect the phase transition of the BEC from zero to finite momenta. Our results provide a powerful platform for exploring self-adapted Floquet physics, which may open an avenue for engineering novel quantum materials. © 2018 American Physical Society.Item Observation of Floquet Bands in Driven Spin-Orbit-Coupled Fermi Gases(American Physical Society) Huang, L.; Peng, P.; Li, D.; Meng, Z.; Chen, L.; Qu, Chunlei; Wang, P.; Zhang, Chuanwei; Zhang, J.; 4042455 (Zhang, C); Qu, Chunlei; Zhang, ChuanweiPeriodic driving of a quantum system can significantly alter its energy bands and even change the band topology, opening a completely new avenue for engineering novel quantum matter. Although important progress has been made recently in measuring topological properties of Floquet bands in different systems, direct experimental measurement of full Floquet band dispersions and their topology change is still demanding. Here we directly measure Floquet band dispersions in a periodically driven spin-orbit-coupled ultracold Fermi gas using spin-injection radio-frequency spectroscopy. We observe that the Dirac point originating from two-dimensional spin-orbit coupling can be manipulated to emerge at the lowest or highest two dressed bands by fast modulating Raman laser frequencies, demonstrating topological change of Floquet bands. Our work will provide a powerful tool for understanding fundamental Floquet physics as well as engineering exotic topological quantum matter.Item Topological Triply Degenerate Points Induced by Spin-Tensor-Momentum Couplings(American Physical Society) Hu, Haiping; Hou, Junpeng; Zhang, Fan; Zhang, Chuanwei; Hu, Haiping; Hou, Junpeng; Zhang, Fan; Zhang, ChuanweiThe recent discovery of triply degenerate points (TDPs) in topological materials has opened a new perspective toward the realization of novel quasiparticles without counterparts in quantum field theory. The emergence of such protected nodes is often attributed to spin-vector-momentum couplings. We show that the interplay between spin-tensor- and spin-vector-momentum couplings can induce three types of TDPs, classified by different monopole charges (ℭ=±2, ±1, 0). A Zeeman field can lift them into Weyl points with distinct numbers and charges. Different TDPs of the same type are connected by intriguing Fermi arcs at surfaces, and transitions between different types are accompanied by level crossings along high-symmetry lines. We further propose an experimental scheme to realize such TDPs in cold-atom optical lattices. Our results provide a framework for studying spin-tensor-momentum coupling-induced TDPs and other exotic quasiparticles.Item Momentum Space Aharonov-Bohm Interferometry in Rashba Spin-Orbit Coupled Bose-Einstein Condensates(Institute of Physics Publishing) Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Zhang, Chuanwei; 4042455 (Zhang, C); Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Zhang, ChuanweiThe recent experimental realization of synthetic Rashba spin-orbit coupling (SOC) paves a new avenue for exploring topological phases in ultracold atoms. The unequivocal characterization of such topological physics requires a simple scheme for measuring the Berry phase originating from the SOC. Here we propose a scheme to realize momentum space Aharonov-Bohm interferometry in a Rashba spin-orbit-coupled Bose-Einstein condensate through a sudden change of the in-plane Zeeman field. We find that the π Berry phase for the Dirac point of the Rashba SOC is directly revealed by a robust dark interference fringe in the momentum space. An external perpendicular Zeeman field opens a band gap at the Dirac point, which reduces the Berry phase along the Rashba ring, leading to lower brightness of the interference fringe. We develop a variational model with semiclassical equations of motion of essential dynamical quantities for describing the interference process, yielding real and momentum space trajectories and geometric phases agreeing with the real-time simulation of the Gross-Pitaevskii equation. Our study may pave the way for the experimental detection of Berry phases in ultracold atomic systems and further exploration of momentum space interference dynamics.Item Topological Triply Degenerate Points Induced by Spin-Tensor- Momentum Couplings(Amer Physical Soc) Hu, Haiping; Hou, Junpeng; Zhang, Fan; Zhang, Chuanwei; 0000 0000 3722 2361 (Zhang, C); 0000-0003-4623-4200 (Zhang, F); 4042455 (Zhang, C); Hu, Haiping; Hou, Junpeng; Zhang, Fan; Zhang, ChuanweiThe recent discovery of triply degenerate points (TDPs) in topological materials has opened a new perspective toward the realization of novel quasiparticles without counterparts in quantum field theory. The emergence of such protected nodes is often attributed to spin-vector-momentum couplings. We show that the interplay between spin-tensor-and spin-vector-momentum couplings can induce three types of TDPs, classified by different monopole charges (C = ± 2, ± 1, 0). A Zeeman field can lift them into Weyl points with distinct numbers and charges. Different TDPs of the same type are connected by intriguing Fermi arcs at surfaces, and transitions between different types are accompanied by level crossings along high-symmetry lines. We further propose an experimental scheme to realize such TDPs in cold-atom optical lattices. Our results provide a framework for studying spin-tensor-momentum coupling-induced TDPs and other exotic quasiparticles.Item Superfluidity in the Absence of Kinetics in Spin-Orbit-Coupled Optical Lattices(Amer Physical Soc) Hui, Hoi-Yin; Zhang, Yongping; Zhang, Chuanwei; Scarola, V. W.; 0000 0000 3722 2361 (Zhang, C); 4042455 (Zhang, C); Zhang, ChuanweiAt low temperatures bosons typically condense to minimize their single-particle kinetic energy while interactions stabilize superfluidity. Optical lattices with artificial spin-orbit coupling challenge this paradigm, because here kinetic energy can be quenched in an extreme regime where the single-particle band flattens. To probe the fate of superfluidity in the absence of kinetics we construct and numerically solve interaction-only tight-binding models in flatbands. We find that superfluid states arise entirely from interactions operating in quenched kinetic energy bands, thus revealing a distinct and unexpected condensation mechanism. Our results have important implications for the identification of quantum condensed phases of ultracold bosons beyond conventional paradigms.Item Momentum-Space Josephson Effects(Amer Physical Soc) Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Bersano, Thomas; Gokhroo, Vandna; Mossman, Sean; Engels, Peter; Zhang, Chuanwei; 0000 0000 3722 2361 (Zhang, C); 4042455 (Zhang, C); Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Zhang, ChuanweiThe Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two different momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages," but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.Item Spin-Orbit-Driven Transitions between Mott Insulators and Finite-Momentum Superfluids of Bosons in Optical Lattices(Amer Physical Soc, 2018-11-05) Yan, Mi; Qian, Yinyin; Hui, Hoi-Yin; Gong, Ming; Zhang, Chuanwei; Scarola, V. W.; 0000 0000 3722 2361 (Zhang, C); 4042455 (Zhang, C); Qian, Yinyin; Gong, Ming; Zhang, ChuanweiSynthetic spin-orbit coupling in ultracold atomic gases can be taken to extremes rarely found in solids. We study a two-dimensional Hubbard model of bosons in an optical lattice in the presence of spin-orbit coupling strong enough to drive direct transitions from Mott insulators to superfluids. Here we find phase-modulated superfluids with finite momentum that are generated entirely by spin-orbit coupling. We investigate the rich phase patterns of the superfluids, which may be directly probed using time-of-flight imaging of the spin-dependent momentum distribution.Item Spin-Tensor-Momentum-Coupled Bose-Einstein Condensates(American Physical Society, 2017-11-06) Luo, Xi-Wang; Sun, Kuei; Zhang, Chuanwei; Luo, Xi-Wang; Sun, Kuei; Zhang, ChuanweiThe recent experimental realization of spin-orbit coupling for ultracold atomic gases provides a powerful platform for exploring many interesting quantum phenomena. In these studies, spin represents the spin vector (spin 1/2 or spin 1) and orbit represents the linear momentum. Here we propose a scheme to realize a new type of spin-tensor-momentum coupling (STMC) in spin-1 ultracold atomic gases. We study the ground state properties of interacting Bose-Einstein condensates with STMC and find interesting new types of stripe superfluid phases and multicritical points for phase transitions. Furthermore, STMC makes it possible to study quantum states with dynamical stripe orders that display density modulation with a long tunable period and high visibility, paving the way for the direct experimental observation of a new dynamical supersolidlike state. Our scheme for generating STMC can be generalized to other systems and may open the door for exploring novel quantum physics and device applications.Item Adiabatically Tuning Quantized Supercurrents in an Annular Bose-Einstein Condensate(2018-08-20) Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Zhang, Chuanwei; Hou, Junpeng; Luo, Xi-Wang; Sun, Kuei; Zhang, ChuanweiThe ability to generate and tune quantized persistent supercurrents is crucial for building superconducting or atomtronic devices with novel functionalities. In ultracold atoms, previous methods for generating quantized supercurrents are generally based on dynamical processes to prepare atoms in metastable excited states. Here, we show that arbitrary quantized circulation states can be adiabatically prepared and tuned as the ground state of a ring-shaped Bose-Einstein condensate by utilizing spin-orbital-angular-momentum (SOAM) coupling and an external potential. There exists superfluid hysteresis for tuning supercurrents between different quantization values with nonlinear atomic interactions, which is explained by developing a nonlinear Landau-Zener theory. Our work will provide a powerful platform for studying SOAM-coupled ultracold atomic gases and building atomtronic circuits.Item Fulde-Ferrell Superfluids without Spin Imbalance in Driven Optical Lattices(Amer Physical Soc) Zheng, Zhen; Qu, Chunlei; Zou, Xubo; Zhang, Chuanwei; Zhang, ChuanweiSpin-imbalanced ultracold Fermi gases have been widely studied recently as a platform for exploring the long-sought Fulde-Ferrell-Larkin-Ovchinnikov superfluid phases, but so far conclusive evidence has not been found. Here we propose to realize an Fulde-Ferrell (FF) superfluid without spin imbalance in a three-dimensional fermionic cold atom optical lattice, where s- and p-orbital bands of the lattice are coupled by another weak moving optical lattice. Such coupling leads to a spin-independent asymmetric Fermi surface, which, together with the s-wave scattering interaction between two spins, yields an FF type of superfluid pairing. Unlike traditional schemes, our proposal does not rely on the spin imbalance (or an equivalent Zeeman field) to induce the Fermi surface mismatch and provides a completely new route for realizing FF superfluids.Item Dynamical Spin-Density Waves in a Spin-Orbit-Coupled Bose-Einstein Condensate(Amer Physical Soc, 2015-07-31) Li, Yan; Qu, Chunlei; Zhang, Yongsheng; Zhang, Chuanwei; Li, Yan; Qu, Chunlei; Zhang, Yongsheng; Zhang, ChuanweiSynthetic spin-orbit (SO) coupling, an important ingredient for quantum simulation of many exotic condensed matter physics, has recently attracted considerable attention. The static and dynamic properties of a SO-coupled Bose-Einstein condensate (BEC) have been extensively studied in both theory and experiment. Here we numerically investigate the generation and propagation of a dynamical spin-density wave (SDW) in a SO-coupled BEC using a fast moving Gaussian-shaped barrier. We find that the SDW wavelength is sensitive to the barrier's velocity while varies slightly with the barrier's peak potential or width. We qualitatively explain the generation of SDW by considering a rectangular barrier in a one-dimensional system. Our results may motivate future experimental and theoretical investigations of rich dynamics in the SO-coupled BEC induced by a moving barrier.Item Exciton Polaritons in Transition-Metal Dichalcogenides and Their Direct Excitation via Energy Transfer(2015-08-28) Gartstein, Yuri N.; Li, Xiao; Zhang, Chuanwei; 6603852436 (Gartstein, Yuri)Excitons, composite electron-hole quasiparticles, are known to play an important role in optoelectronic phenomena in many semiconducting materials. Recent experiments and theory indicate that the band-gap optics of the newly discovered monolayer transition-metal dichalcogenides (TMDs) is dominated by tightly bound valley excitons. The strong interaction of excitons with long-range electromagnetic fields in these two-dimensional systems can significantly affect their intrinsic properties. Here, we develop a semiclassical framework for intrinsic exciton polaritons in monolayer TMDs that treats their dispersion and radiative decay on the same footing and can incorporate effects of the dielectric environment. It is demonstrated how both inter- and intravalley long-range interactions influence the dispersion and decay of the polaritonic eigenstates. We also show that exciton polaritons can be efficiently excited via resonance energy transfer from quantum emitters such as quantum dots, which may be useful for various applications.Item Majorana Fermions in Quasi-One-Dimensional and Higher-Dimensional Ultracold Optical Lattices(2015-08-14) Qu, Chunlei; Gong, Ming; Xu, Yong; Tewari, Sumanta; Zhang, Chuanwei; Qu, Chunlei; Zhang, ChuanweiWe study Majorana fermions (MFs) in quasi-one dimensional (quasi-1D) and higher-dimensional fermionic optical lattices with a strictly 1D spin-orbit coupling, which has already been realized in cold atom experiments. We show that when the superfluid order parameters are homogeneous and are enforced to be identical along different chains, there are multiple MFs at each end with or without an experimentally tunable in-plane Zeeman field V{y}. For V{y} = 0 the multiple MFs are topologically protected by a chiral symmetry; however, for V{y} ≠ 0 the existence of multiple MFs is related to the peculiar spectrum properties of the system despite the broken chiral symmetry. In the generalization to higher dimensions, the multiple MFs form a zero-energy flat band. Furthermore, when the superfluid order parameters are solved self-consistently, the multiple MFs are usually destroyed because of the inhomogeneous order parameters of either Bardeen-Cooper-Schrieffer (V{y} = 0) type or Fulde-Ferrell (V{y} ≠ 0). Our results are useful to guide the experimentalists on searching for MFs in ultracold spin-orbit coupled fermionic superfluids.Item Spin-Orbital-Angular-Momentum Coupling in Bose-Einstein Condensates(Amer Physical Soc, 2015-06-22) Sun, Kuei; Qu, Chunlei; Zhang, Chuanwei; Sun, Kuei; Qu, Chunlei; Zhang, ChuanweiSpin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultracold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultracold atoms using higher-order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground-state phase diagrams of SOAM-coupled Bose-Einstein condensates on a ring trap and explore their applications in gravitational force detection. Our results may provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling, such as collective excitations.Item Tunable Spin-Orbit Coupling via Strong Driving in Ultracold-Atom Systems(American Physical Society, 2015-03-24) Jim©nez-Garc©a, K.; Leblanc, L. J.; Williams, R. A.; Beeler, M. C.; Qu, Chunlei; Gong, Ming; Zhang, Chuanwei.; Spielman, I. B.; Jim©nez-Garc©a, K.; Leblanc, L. J.; Williams, R. A.; Beeler, M. C.; Qu, Chunlei; Gong, Ming; Zhang, Chuanwei.; Spielman, I. B.Spin-orbit coupling is an essential ingredient in topological materials, conventional and quantum-gas-based alike. Engineered spin-orbit coupling in ultracold-atom systems - unique in their experimental control and measurement opportunities - provides a major opportunity to investigate and understand topological phenomena. Here we experimentally demonstrate and theoretically analyze a technique for controlling spin-orbit coupling in a two-component Bose-Einstein condensate using amplitude-modulated Raman coupling.Item Interacting Spin-Orbit-Coupled Spin-1 Bose-Einstein Condensates(American Physical Society, 2016-02-10) Sun, Kuei; Qu, Chunlei; Xu, Yong; Zhang, Y.; Zhang, Chaunwei; H-3571-2011 (Zhang, C); Sun, Kuei; Qu, Chunlei; Xu, Yong; Zhang, ChaunweiThe recent experimental realization of spin-orbit (SO) coupling for spin-1 ultracold atoms opens an interesting avenue for exploring SO-coupling-related physics in large-spin systems, which is generally unattainable in electronic materials. In this paper, we study the effects of interactions between atoms on the ground states and collective excitations of SO-coupled spin-1 Bose-Einstein condensates (BECs) in the presence of a spin-tensor potential. We find that ferromagnetic interaction between atoms can induce a stripe phase exhibiting in-phase or out-of-phase modulating patterns between spin-tensor and zero-spin-component density waves. We characterize the phase transitions between different phases using the spin-tensor density as well as the collective dipole motion of the BEC. We show that there exists a double maxon-roton structure in the Bogoliubov-excitation spectrum, attributed to the three band minima of the SO-coupled spin-1 BEC.Item Spin-Momentum Coupled Bose-Einstein Condensates with Lattice Band Pseudospins(Nature Publishing Group, 2016-02-29) Khamehchi, M. A.; Qu, Chunlei; Mossman, M. E.; Zhang, Chuanwei; Engels, P.; H-3571-2011 (Zhang, C); Qu, Chunlei; Zhang, ChuanweiThe quantum emulation of spin-momentum coupling, a crucial ingredient for the emergence of topological phases, is currently drawing considerable interest. In previous quantum gas experiments, typically two atomic hyperfine states were chosen as pseudospins. Here, we report the observation of a spin-momentum coupling achieved by loading a Bose-Einstein condensate into periodically driven optical lattices. The s and p bands of a static lattice, which act as pseudospins, are coupled through an additional moving lattice that induces a momentum-dependent coupling between the two pseudospins, resulting in s-p hybrid Floquet-Bloch bands. We investigate the band structures by measuring the quasimomentum of the Bose-Einstein condensate for different velocities and strengths of the moving lattice, and compare our measurements to theoretical predictions. The realization of spin-momentum coupling with lattice bands as pseudospins paves the way for engineering novel quantum matter using hybrid orbital bands.