Browsing by Author "Xu, Yong"
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Item Anisotropic Weyl Fermions from the Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid(American Physical Society, 2014-04-04) Xu, Yong; Chu, Rui-Lin; Zhang, Chuanwei; Zhang, ChuanweiWeyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single-particle energy dispersions in certain solid state materials (named Weyl semimetals), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional spin-orbit-coupled Fulde-Ferrell superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of Fulde-Ferrell fermionic superfluids, which are detectable in experiments.Item Berezinskii-Kosterlitz-Thouless Phase Transition in 2D Spin-Orbit-Coupled Fulde-Herrell Superfluids(American Physical Society, 2015-03-17) Xu, Yong; Zhang, Chuanwei; Zhang, ChuanweiThe experimental observation of traditional Zeeman-field induced Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids has been hindered by various challenges, in particular, the requirement of low dimensional systems. In 2D, finite temperature phase fluctuations lead to an extremely small Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for FFLO superfluids, raising serious concerns regarding their experimental observability. Recently, it was shown that FFLO superfluids can be realized using a Rashba spin-orbit coupled Fermi gas subject to Zeeman fields, which may also support topological excitations such as Majorana fermions in 2D. Here we address the finite temperature BKT transition issue in this system, which may exhibit gapped, gapless, topological, and gapless topological FF phases. We find a large BKT transition temperature due to large effective superfluid densities, making it possible to observe 2D FF superfluids at finite temperature. In addition, we show that gapless FF superfluids can be stable due to their positive superfluid densities. These findings pave the way for the experimental observation of 2D gapped and gapless FF superfluids and their associated topological excitations at finite temperature.Item Dark Solitons with Majorana Fermions in Spin-Orbit-Coupled Fermi Gases(American Physical Society, 2014-09-26) Xu, Yong; Mao, Li; Wu, Biao; Zhang, Chuanwei; Zhang, ChuanweiWe show that a single dark soliton can exist in a spin-orbit-coupled Fermi gas with a high spin imbalance, where spin-orbit coupling favors uniform superfluids over nonuniform Fulde-Ferrell-Larkin-Ovchinnikov states, leading to dark soliton excitations in highly imbalanced gases. Above a critical spin imbalance, two topological Majorana fermions without interactions can coexist inside a dark soliton, paving a way for manipulating Majorana fermions through controlling solitons. At the topological transition point, the atom density contrast across the soliton suddenly vanishes, suggesting a signature for identifying topological solitons.Item Gap Solitons in Spin-Orbit-Coupled Bose-Einstein Condensates in Optical Lattices(American Physical Society, 2015-04-23) Zhang, Yongping; Xu, Yong; Busch, ThomasWhile different ways to realize spin-orbit coupling in Bose-Einstein condensates exist, not all are currently experimentally implementable. Here we present a detailed study of gap solitons in a Bose-Einstein condensate with experimentally realizable spin-orbit coupling and discuss two cases relating to a spin-dependent parity symmetry. In the parity symmetric case, two families of fundamental gap solitons in the second linear energy gap are demonstrated with the opposite sign of the parity, with one family having single-humped densities and the other double-humped ones. In the case of broken parity symmetry, the fundamental modes manifest spin polarization. Both families possess an opposite sign of the spin polarization.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 Structured Weyl Points in Spin-Orbit Coupled Fermionic Superfluids(American Physical Society, 2015-12-30) Xu, Yong; Zhang, Fan; Zhang, Chuanwei; Xu, Yong; Zhang, Fan; Zhang, ChuanweiWe demonstrate that a Weyl point, widely examined in 3D Weyl semimetals and superfluids, can develop a pair of nondegenerate gapless spheres. Such a bouquet of two spheres is characterized by three distinct topological invariants of manifolds with full energy gaps, i.e., the Chern number of a 0D point inside one developed sphere, the winding number of a 1D loop around the original Weyl point, and the Chern number of a 2D surface enclosing the whole bouquet. We show that such structured Weyl points can be realized in the superfluid quasiparticle spectrum of a 3D degenerate Fermi gas subject to spin-orbit couplings and Zeeman fields, which supports Fulde-Ferrell superfluids as the ground state.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.