# Browsing by Author "Hou, Junpeng"

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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, ChuanweiShow more The 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.Show more Item Exotic Phases and Quantum Dynamics in Spin-Orbit Coupled Bose-Einstein Condensates(2021-07-21) Hou, Junpeng; Zhang, ChuanweiShow more In this dissertation, we study a range of exotic quantum phases and dynamics in BoseEinstein condensates (BECs) with spin-orbit (SO) coupling. Firstly, inspired by the recent experimental progresses in engineering supersolid stripe states in a SO-coupled BEC, we conjecture a new quantum state of matter called “superfluid-qausicrystal”, in which quasicrystalline orders form spontaneously in the ground states of the BECs. We propose a realistic setup to prepare and observe this exotic quantum states and confirm its existence through both variational ansatz analysis and direct simulation of mean-field Gross-Pitaevskii equation (GPE). Moreover, we have identified rich phase diagram including superfluid-qausicrystals, supersoilds and plane-wave phases. By utilizing the double-well band structure of a SO-coupled BEC, we propose and investigate a momentum-pace Josephson junction (MSJJ). We show that, under an extra pair of Raman lasers, the two band minimums in momentum space can be weakly coupled. In such a MSJJ, Josephson currents can be induced not only by applying the equivalent of “voltages”, but also by tuning tunneling phases. We characterize the MSJJs through both full mean-field analysis and a concise two-level model and demonstrate the important role of interactions between atoms. Besides, we experimentally demonstrate that this can be a route for realizing a striped BEC, providing an accessible platform to investigate the excitation spectra and other important properties of supersolid-like states. We observe coherent Rabi oscillations in momentum space between two band minima and demonstrate a long lifetime of the ground state, which find good agreements with mean-field simulations. Finally, we theoretically conceive and experimentally demonstrate a non-magnetic one-way spin switch device using a SO-coupled BEC subjected to a moving spin-independent repulsive dipole potential. We show that the physical foundation of this unidirectional device is based on the breakdown of Galilean invariance in the presence of SO coupling. The experimental observation, the numerical simulations and the theoretic interpretations are consistent with the others.Show more 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, ChuanweiShow more The 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.Show more 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, ChuanweiShow more The 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.Show more Item Symmetry-Protected Localized States at Defects in Non-Hermitian Systems(American Physical Society, 2019-06-12) Wu, Ya-Jie; Hou, Junpeng; Wu, Ya-Jie; Hou, JunpengShow more Understanding how local potentials affect system eigenmodes is crucial for experimental studies of nontrivial bulk topology. Recent studies have discovered many exotic and highly nontrivial topological states in non-Hermitian systems. As such, it would be interesting to see how non-Hermitian systems respond to local perturbations. In this work we consider chiral and particle-hole-symmetric non-Hermitian systems on a bipartite lattice, including the Su-Schrieer-Heeger model and photonic graphene, and find that a disordered local potential could induce bound states evolving from the bulk. When the local potential on a single site becomes infinite, which renders a lattice vacancy, chiral-symmetry-protected zero-energy mode and particle-hole-symmetry-protected bound states with purely imaginary eigenvalues emerge near the vacancy. These modes are robust against any symmetry-preserved perturbations. Our work generalizes the symmetry-protected localized states to non-Hermitian systems. © 2019 American Physical Society.Show more 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, ChuanweiShow more The 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.Show more 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, ChuanweiShow more The 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.Show more