Browsing by Author "Qu, Chunlei"
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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 Floquet Fulde-Ferrell-Larkin-Ovchinnikov Superfluids and Majorana Fermions in a Shaken Fermionic Optical Lattice(Amer Physical Soc, 2015-06-22) Zheng, Zhen; Qu, Chunlei; Zou, Xubo; Zhang, Chuanwei; Zheng, Zhen; Qu, Chunlei; Zhang, ChuanweiFulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids, Cooper pairings with finite momentum, and Majorana fermions (MFs), quasiparticles with non-Abelian exchange statistics, are two topics under intensive investigation in the past several decades, but unambiguous experimental evidence for them has not been found yet. Here we show that the recent experimentally realized shaken optical lattice provides a pathway to realize FFLO superfluids and MFs. By tuning the shaking frequency and amplitude, various couplings between the s and p orbitals of the lattice (called the pseudospins) can be generated. We show that the combination of the s- and p-band dispersion inversion, the engineered pseudospin coupling, and the on-site attractive interaction naturally allows the observation of FFLO superfluids as well as MFs in different parameter regions.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 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 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 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 Quantum Phases of Bose-Einstein Condensates with Synthetic Spin-Orbital-Angular-Momentum Coupling(2015-05) Qu, Chunlei; Sun, Kuei; Zhang, Chuanwei; H-3571-2011 (Zhang, C); Zhang, ChuanweiThe experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves the way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for an experimental investigation of SOAM-coupled BECs.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 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.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.