Arxiv Selection Nov 2018
Nov 1-Nov 7 Ahmet Keles, Nov 8-Nov 14 Haiping Hu, Nov 15-Nov 21 Biao Huang, Nov 22- Nov 28 Xuguang Yue
Nov. 12
arXiv:1811.01977
Classification of Crystalline Topological Insulators and Superconductors with Point Group Symmetries
Eyal Cornfeld, Adam Chapman
Crystalline topological phases have recently attracted a lot of experimental and theoretical attention. Key advances include the complete elementary band representation analyses of crystalline matter by symmetry indicators and the discovery of higher order hinge and corner states. However, current classification schemes of such phases are either implicit or limited in scope. We present a new scheme for the explicit classification of crystalline topological insulators and superconductors. These phases are protected by crystallographic point group symmetries and are characterized by bulk topological invariants. The classification paradigm generalizes the Clifford algebra extension process of each Altland-Zirnbauer symmetry class and utilizes algebras which incorporate the point group symmetry. Explicit results for all point group symmetries of 3 dimensional crystals are presented as well as for all symmorphic layer groups of 2 dimensional crystals. We discuss future extensions for treatment of magnetic crystals and defected or higher dimensional systems as well as weak and fragile invariants.
Nov. 13
arXiv:1811.04474
Emergent supersymmetry in a chain of Majorana Cooper pair boxes
Hiromi Ebisu, Eran Sagi, Yuval Oreg
The charging energy of a small superconducting island containing Majorana zero modes - a Majorana Cooper-pair box - induces interactions between the Majorana zero modes. In this manuscript, we investigate a chain of Majorana Cooper-pair boxes, and theoretically demonstrate the emergence of supersymmetry in the strong charging energy regime. A mapping between the Majorana zero modes and spin-1 degrees of freedom results in an effective Blume-Capel model, known for exhibiting an emergent supersymmetry at a non-trivial critical point with central charge c=7/10. We corroborate our findings by mapping the chain to a supersymmetric low energy field theory, exhibiting the same central charge at criticality. The microscopic model we propose consists of local tunneling of Majorana zero modes and local charging energy terms, which can be controlled by gate potentials, thus making its realization more feasible.
Nov. 14
arXiv:1811.05327
Quench Dynamics in a Trapped Bose-Einstein Condensate with Spin-Orbit Coupling
Sheng Liu, Yongsheng Zhang
We consider the phase transition dynamics of a trapped Bose-Einstein condensate subject to Raman-type spin-orbit coupling (SOC). By tuning the coupling strength the condensate is taken through a second order phase transition into an immiscible phase. We observe the domain wall defects produced by a finite speed quench is described by the Kibble-Zurek mechanism (KZM), and quantify a power law behavior for the scaling of domain number and formation time with the quench speed.
Nov. 7
arXiv:1811.02718
Two-dimensional composite solitons in a spin-orbit-coupled Fermi gas in free space
Pablo Díaz, David Laroze, Andrés Ávila, Boris A. Malomed
We address a possibility of creating soliton states in oblate binary-fermionic clouds in the framework of the density-functional theory, which includes the spin-orbit coupling (SOC) and nonlinear attraction between spin-up and down-polarized components of the spinor wave function. In the limit when the inter-component attraction is much stronger than the effective intra-component Pauli repulsion, the resulting model also represents a system of Gross-Pitaevskii equations for a binary Bose-Einstein condensate including the SOC effect. We show that the model gives rise to two-dimensional quiescent composite solitons in free space. A stability region is identified for solitons of the mixed-mode type (which feature mixtures of zero-vorticity and vortical terms in both components), while solitons of the other type, semi-vortices (with the vorticity carried by one component) are unstable. Due to breaking of the Galilean invariance by SOC, the system supports moving solitons with velocities up to a specific critical value. Collisions between moving solitons are briefly considered too. The collisions lead, in particular, to a quasi-elastic rebound, or an inelastic outcome, which features partial merger of the solitons.
Nov. 6
arXiv:1811.02613
Observation of stable stripes in a dipolar quantum gas
Luca Tanzi, Eleonora Lucioni, Francesca Famà, Jacopo Catani, Andrea Fioretti, Carlo Gabbanini, Giovanni Modugno
In dipolar quantum gases, the competition of dipole-dipole and contact interactions leads to two seemingly disconnected phenomena: a tunable rotonic spectrum, with an associated rotonic instability, and self-bound droplets stabilized by quantum fluctuations. In this work we connect these phenomena using a Bose-Einstein condensate of highly magnetic atoms. We find that the rotonic instability leads to a long-lived stripe regime, stabilized by quantum fluctuations. These stripes exist for a narrow range of contact interactions close to the rotonic instability, and are significantly more coherent than the arrays of droplets appearing at weaker contact strengths. We speculate that stripes exist in the absence of a self-binding mechanism. They are a promising candidate for the study of supersolidity in dipolar systems.
arXiv:1811.02434
Tensor Berry connections and their topological invariants
Giandomenico Palumbo, Nathan Goldman
The Berry connection plays a central role in our description of the geometric phase and topological phenomena. In condensed matter, it describes the parallel transport of Bloch states and acts as an effective "electromagnetic" vector potential defined in momentum space. Inspired by developments in mathematical physics, where higher-form (Kalb-Ramond) gauge fields were introduced, we hereby explore the existence of "tensor Berry connections" in quantum matter. Our approach consists in a general construction of effective gauge fields, which we ultimately relate to the components of Bloch states. We apply this formalism to various models of topological matter, and we investigate the topological invariants that result from generalized Berry connections. For instance, we introduce the 2D Zak phase of a tensor Berry connection, which we then relate to the more conventional first Chern number; we also reinterpret the winding number characterizing 3D topological insulators to a Dixmier-Douady invariant, which is associated with the curvature of a tensor connection. Besides, our approach identifies the Berry connection of tensor monopoles, which are found in 4D Weyl-type systems [Phys. Rev. Lett. 121, 170401 (2018)]. Our work sheds light on the emergence of gauge fields in condensed-matter physics, with direct consequences on the search for novel topological states in solid-state and quantum-engineered systems.
Nov. 3
arXiv:1811.01236
Higher-order topological superconductivity: possible realization in Fermi gases and Sr2RuO4
Zhigang Wu, Zhongbo Yan, Wen Huang
We propose to realize second-order topological superconductivity in bilayer spin-polarized Fermi gas superfluids. We focus on systems with intra-layer chiral p-wave pairing and with tunable inter-layer hopping and inter-layer interactions. Under appropriate circumstances, an inter-layer even-parity s- or d-wave pairing may coexist with the intra-layer p-wave. Such mixed-parity phases do not carry one-dimensional gapless Majorana modes on the boundary, but could support Majorana zero modes at the corners of the system geometry and at the terminations of certain one-dimensional defects. We show how such topological phases and the Majorana zero modes therein can be manipulated in a multitude of ways by tuning the inter-layer pairing and hopping. Generalized to spinful systems, we further propose that the putative p-wave superconductor Sr2RuO4, when placed under uniaxial strains, may also realize the desired topological phase.
Nov. 2
arXiv:1811.00963
The quantized Hall conductance of a single atomic wire: A proposal based on synthetic dimensions
G. Salerno, H. M. Price, M. Lebrat, S. Häusler, T. Esslinger, L. Corman, J.-P. Brantut, N. Goldman
We propose a method by which the quantization of the Hall conductance can be directly measured in the transport of a one-dimensional atomic gas. Our approach builds on two main ingredients: (1) a constriction optical potential, which generates a mesoscopic channel connected to two reservoirs, and (2) a time-periodic modulation of the channel, specifically designed to generate motion along an additional synthetic dimension. This fictitious dimension is spanned by the harmonic-oscillator modes associated with the tightly-confined channel, and hence, the corresponding "lattice sites" are intimately related to the energy of the system. We analyze the quantum transport properties of this hybrid two-dimensional system, highlighting the appealing features offered by the synthetic dimension. In particular, we demonstrate how the energetic nature of the synthetic dimension, combined with the quasi-energy spectrum of the periodically-driven channel, allows for the direct and unambiguous observation of the quantized Hall effect in a two-reservoir geometry. Our work illustrates how topological properties of matter can be accessed in a minimal one-dimensional setup, with direct and practical experimental consequences.
Nov. 1
arXiv:1811.00563
Dropping an impurity into a Chern insulator: a polaron view on topological matter
A. Camacho-Guardian, N. Goldman, P. Massignan, G. M. Bruun
We investigate the properties of an impurity particle interacting with a Fermi gas in a Chern-insulating state. The interaction leads to the formation of an exotic polaron, which consists of a coherent superposition of the topologically-trivial impurity and the surrounding topological cloud. We characterize this intriguing topologically-composite object by calculating its transverse (Hall) conductivity, using diagrammatic as well as variational methods. The "polaronic Hall conductivity" is shown to be non-zero whenever the surrounding cloud is prepared in a non-trivial Chern insulating state, which we attribute to the transverse drag exerted by the dressing cloud on the impurity. In this way, the polaron partially inherits the topological properties of the Chern insulator through genuine interaction effects. This is also analysed at the microscopic level of wave functions, by identifying a "composite Berry curvature" for the polaron, which closely mimics the Berry curvature of the Chern insulator's band structure. Finally, we discuss how this interplay between topology and many-body correlations can be studied in cold-atom experiments, using available technologies.
arXiv:1811.00481
Boiling a Unitary Fermi Liquid
Zhenjie Yan, Parth B. Patel, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein
We study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime and the short-range correlations of Fermi polarons at low temperatures T. In this regime we observe a characteristic T2 dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high T the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, ∝T−1/2. In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins, and yield the compressibility along with the effective mass of Fermi polarons.
arXiv:1811.00446
Spin-polarized droplets in the unitary Fermi gas
Piotr Magierski, Buğra Tüzemen, Gabriel Wlazłowski
We demonstrate the existence of a new type of spatially localized excitations in the unitary Fermi gas: spin polarized droplets with a peculiar internal structure involving the abrupt change of the pairing phase at the surface of the droplet. It resembles the structure of the Josephson-π junction occurring when a slice of a ferromagnet is sandwiched between two superconductors. The stability of the impurity is enhanced by the mutual interplay between the polarization effects and the pairing field, resulting in an exceptionally long-lived state. The prospects for its realization in experiment are discussed.
