Apr 2016

Apr 4-Apr 8 Bo Liu, Apr 11-Apr 15 Max, Apr 18-Apr 22 Haiyuan Zou, Apr 25-Apr 29 Ahmet Kel

Apr 25-29

arXiv:1604.08280 [pdf, other]
Universal Symmetry-Protected Resonances in a Spinful Luttinger Liquid
Yichen Hu, C. L. Kane
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)

We study the problem of resonant tunneling through a quantum dot in a spinful Luttinger liquid. For a range of repulsive interactions, we find that for symmetric barriers there exist resonances with a universal peak conductance $2g^* e^2/h$ that are controlled by a non-trivial intermediate fixed point. This fixed point is also a quantum critical point separating symmetry-protected topological phases. By tuning the system through resonance, all SPT phases can be accessed. For a particular interaction strength with Luttinger parameters $g_\rho=1/3$ and $g_\sigma=1$, we show that the problem is equivalent to a two channel $SU(3)$ Kondo problem($SU(3)_2$ CFT). At the Toulouse limit, both problems can be mapped to a quantum Brownian motion model on a Kagome lattice, which in turn is related to the quantum Brownian motion on a honeycomb lattice and the three-channel $SU(2)$ Kondo problem($SU(2)_3$ CFT). "Level-rank duality" in the quantum Brownian motion model relating $SU(2)_k$ CFT to $SU(k)_2$ CFT is also explored. Utilizing the boundary conformal field theory, the on-resonance conductance of our resonant tunneling problem is calculated as well as the scaling dimension of the leading relevant operator. This allows us to compute the scaling behavior of the resonance line-shape as a function of temperature.

arXiv:1604.08400 [pdf, other]
Quantum Skyrmions in Two-Dimensional Chiral Magnets
Rina Takashima, Hiroaki Ishizuka, Leon Balents
Comments: 8 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We study the quantum mechanics of magnetic skyrmions in the vicinity of the skyrmion-crystal to ferromagnet phase boundary in two-dimensional magnets. We show that the skyrmion excitation has an energy dispersion that splits into multiple bands due to the combination of magnus force and the underlying lattice. Condensation of the skyrmions can give rise to an intermediate phase between the skyrmion crystal and ferromagnet: a quantum liquid, in which skyrmions are not spatially localized. We show that the critical behavior depends on the spin size $S$ and the topological number of the skyrmion. Experimental signatures of quantum skyrmions in inelastic neutron scattering measurements are also discussed.

arXiv:1604.08522 [pdf, ps, other]
Spin evolution of cold atomic gases in SU(2)$\otimes $U(1) fields
I. V. Tokatly, E. Ya. Sherman
Comments: 8 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We consider response function and spin evolution in spin-orbit coupled cold atomic gases in a synthetic gauge magnetic field influencing solely the orbital motion of atoms. We demonstrate that various regimes of spin-orbit coupling strength, magnetic field, and disorder can be treated within a single approach based on the representation of atomic motion in terms of auxiliary collective classical trajectories. Our approach allows for a unified description of fermionic and bosonic gases.

arXiv:1604.08151 [pdf, other]
Topological phases protected by point group symmetry
Hao Song, Sheng-Jie Huang, Liang Fu, Michael Hermele
We consider symmetry protected topological (SPT) phases with crystalline point group symmetry, dubbed point group SPT (pgSPT) phases. We show that such phases can be understood in terms of lower-dimensional topological phases with on-site symmetry, and can be constructed as stacks and arrays of these lower-dimensional states. This provides the basis for a general framework to classify and characterize bosonic and fermionic pgSPT phases, that can be applied for arbitrary crystalline point group symmetry and in arbitrary spatial dimension. We develop and illustrate this framework by means of a few examples, focusing on three-dimensional states. We classify bosonic pgSPT phases and fermionic topological crystalline superconductors with ZP2 (reflection) symmetry, electronic topological crystalline insulators (TCIs) with U(1)×ZP2 symmetry, and bosonic pgSPT phases with C2v symmetry, which is generated by two perpendicular mirror reflections. We also study surface properties, with a focus on gapped, topologically ordered surface states. For electronic TCIs we find a Z8×Z2classification, where the Z8 corresponds to known states obtained from non-interacting electrons, and the Z2 corresponds to a "strongly correlated" TCI that requires strong interactions in the bulk. Our approach may also point the way toward a general theory of symmetry enriched topological (SET) phases with crystalline point group symmetry.

arXiv:1604.08141 [pdf, other]
Quantum quenches to the attractive one-dimensional Bose gas: exact results
Lorenzo Piroli, Pasquale Calabrese, Fabian H. L. EsslerWe study quantum quenches to the one-dimensional Bose gas with attractive interactions in the case when the initial state is an ideal one-dimensional Bose condensate. We focus on properties of the stationary state reached at late times after the quench. This displays a finite density of multi-particle bound states, whose rapidity distribution is determined exactly by means of the quench action method. We discuss the relevance of the multi-particle bound states for the physical properties of the system, computing in particular the stationary value of the local pair correlation function g2.

arXiv:1604.07857 [pdf, ps, other]
Berry Fermi Liquid Theory
Jing-Yuan Chen, Dam Thanh Son
We develop an extension of the Landau Fermi liquid theory to systems of interacting fermions with non-trivial Berry curvature. We propose a kinetic equation and a constitutive relation for the electromagnetic current that together encode the linear response of such systems to external electromagnetic perturbations, to leading and next-to-leading orders in the expansion over the frequency and wave number of the perturbations. We analyze the Feynman diagrams in a large class of interacting quantum field theories and show that, after summing up all orders in perturbation theory, the current-current correlator exactly matches with the result obtained from the kinetic theory.

arXiv:1604.07512 [pdf, ps, other]

Interaction-Driven Spontaneous Quantum Hall Effect on Kagome Lattice
W. Zhu, S. S. Gong, T. S. Zeng, L. Fu, D. N. Sheng
Non-interacting topological states of matter can be realized in band insulators with intrinsic spin-orbital couplings as a result of the nontrivial band topology. In recent years, the possibility of realizing novel interaction-driven topological phase has attracted a lot of research activities, which may significantly extend the classes of topological states of matter. Here, we report a new finding of an interaction-driven spontaneous quantum Hall effect (QHE) (Chern insulator) emerging in an extended fermion-Hubbard model on kagome lattice. By means of the state-of-the-art density-matrix renormalization group, we expose universal properties of the QHE including time-reversal symmetry spontaneous breaking and quantized Hall conductance. By accessing the ground state in large systems, we demonstrate the robustness of the QHE against finite-size effects. Moreover, we map out a phase diagram and identify two competing charge density wave phases by varying interactions, where transitions to the QHE phase are determined to be of the first order. Our study provides a "proof-of-the-principle" demonstration of interaction-driven QHE without requirement of external magnetic field or magnetic doping.


arXiv:1604.07459 [pdf, other]

A Five Dimensional Generalization of the Topological Weyl Semimetal
Biao Lian, Shou-Cheng Zhang
We present a simple solution to enhance the separation ability of deterministic lateral displacement (DLD) systems by expanding the two-dimensional nature of these devices and driving the particles into size-dependent, fully three-dimensional trajectories. Specifically, we drive the particles through an array of long cylindrical posts, such that they not only move in the plane perpendicular to the posts as in traditional two-dimensional DLD systems (in-plane motion), but also along the axial direction of the solid posts (out-of-plane motion). We show that the (projected) in-plane motion of the particles is completely analogous to that observed in 2D-DLD systems. In fact, a theoretical model originally developed for force-driven, two-dimensional DLD systems accurately describes the experimental results. More importantly, we analyze the particles out-of-plane motion and observe that, for certain orientations of the driving force, significant differences in the out-of-plane displacement depending on particle size. Therefore, taking advantage of both the in-plane and out-of-plane motion of the particles, it is possible to achieve the simultaneous fractionation of a polydisperse suspension into multiple streams.


arXiv:1604.07458 [pdf, ps, other]

Second-order hydrodynamics for fermionic cold atoms: Detailed analysis of transport coefficients and relaxation times
Yuta Kikuchi, Kyosuke Tsumura, Teiji KunihiroWe give a detailed derivation of the second-order (local) hydrodynamics for Boltzmann equation with an external force by using the renormalization group method. In this method, we solve the Boltzmann equation faithfully to extract the hydrodynamics without recourse to any ansatz. Our method leads to microscopic expressions of not only all the transport coefficients that are of the same form as those in Chapman-Enskog method but also those of the viscous relaxation times τi that admit physically natural interpretations. As an example, we apply our microscopic expressions to calculate the transport coefficients and the relaxation times of the cold fermionic atoms in a quantitative way, where the transition probability in the collision term is given explicitly in terms of the s-wave scattering length as. We thereby discuss the quantum statistical effects, temperature dependence, and scattering-length dependence of the first-order transport coefficients and the viscous relaxation times: It is shown that as the temperature is lowered, the transport coefficients and the relaxation times increase rapidly because Pauli principle acts effectively. On the other hand, as as is increased, these quantities decrease and become vanishingly small at unitarity because of the strong coupling. The numerical calculation shows that the relation τπ=η/P, which is derived in the relaxation-time approximation and used in most of literature without almost any foundation, turns out to be satisfied quite well, while the similar relation for the relaxation time τJ of the heat conductivity is satisfied only approximately with a considerable error.


arXiv:1604.06807 [pdf, other]

Composite fermi liquids in the lowest Landau level
Chong Wang, T. SenthilWe study composite fermi liquid (CFL) states in the lowest Landau level (LLL) limit at a generic filling ν=1n. We begin with the old observation that, in compressible states, the composite fermion in the lowest Landau level should be viewed as a charge-neutral particle carrying vorticity. This leads to the absence of a Chern-Simons term in the effective theory of the CFL. We argue here that instead a Berry curvature should be enclosed by the fermi surface of composite fermions, with the total Berry phase fixed by the filling fraction ϕB=−2πν. We illustrate this point with the CFL of fermions at filling fractions ν=1/2q and (single and two-component) bosons at ν=1/(2q+1). The Berry phase leads to sharp consequences in the transport properties including thermal and spin Hall conductances, which in the RPA approximation are distinct from the standard Halperin-Lee-Read predictions. We emphasize that these results only rely on the LLL limit, and do not require particle-hole symmetry, which is present microscopically only for fermions at ν=1/2. Nevertheless, we show that the existing LLL theory of the composite fermi liquid for bosons at ν=1 does have an emergent particle-hole symmetry. We interpret this particle-hole symmetry as a transformation between the empty state at ν=0 and the boson integer quantum hall state at ν=2. This understanding enables us to define particle-hole conjugates of various bosonic quantum Hall states which we illustrate with the bosonic Jain and Pfaffian states. The bosonic particle-hole symmetry can be realized exactly on the surface of a three-dimensional boson topological insulator. We also show that with the particle-hole and spin SU(2) rotation symmetries, there is no gapped topological phase for bosons at ν=1.















Apr 22
arXiv:1604.06292 [pdf, other]
A two-leg Su-Schrieffer-Heeger chain with glide reflection symmetry
Shao-Liang Zhang, Qi Zhou
Comments: 16 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

The Su-Schrieffer-Heeger (SSH) model lays the foundation of many important concepts in quantum topological matters. Since it tells one that topological states may be distinguished by abelian geometric phases, a question naturally arises as to what happens if one assembles two topologically distinct states. Here, we show that a spin-dependent double-well optical lattice allows one to couple two topologically distinct SSH chains in the bulk and realise a glided-two-leg SSH model that respects the glide reflection symmetry. Such model gives rise to intriguing quantum phenomena beyond the paradigm of a traditional SSH model. It is characterised by Wilson line that requires non-abelian Berry connections, and the interplay between the glide symmetry and interaction automatically leads to charge fractionalisation without jointing two lattice potentials at an interface. Our work demonstrates the power of ultracold atoms to create new theoretical models for studying topological matters.

Apr 21
arXiv:1604.06082 [pdf, other]
Topological bound states of a quantum walk with cold atoms
Samuel Mugel, Alessio Celi, Pietro Massignan, János K. Asbóth, Maciej Lewenstein, Carlos Lobo
Comments: 17 pages, 16 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We suggest a method for engineering a quantum walk, with cold atoms as walkers, which presents topologically non-trivial properties. We derive the phase diagram, and show that we are able to produce a boundary between topologically distinct phases using the finite beam width of the applied lasers. A topologically protected bound state can then be observed, which is pinned to the interface and is robust to perturbations. We show that it is possible to identify this bound state by averaging over spin sensitive measures of the atom's position, based on the spin distribution that these states display. Interestingly, there exists a parameter regime in which our system maps on to the Creutz ladder.

Apr 20
arXiv:1604.05669 [pdf, other]
Particle Physics and Condensed Matter: The Saga Continues
Frank Wilczek
Comments: 18 pages, 3 figures. Invited presentation of concluding remarks at Nobel Symposium 156 on New Forms of Matter, Topological Insulators and Superconductors, June 13-15 2014, H\"ogberga G{\aa}rd, Stockholm
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); High Energy Physics - Theory (hep-th)

Ideas from quantum field theory and topology have proved remarkably fertile in suggesting new phenomena in the quantum physics of condensed matter. Here I'll supply some broad, unifying context, both conceptual and historical, for the abundance of results reported at the Nobel Symposium on "New Forms of Matter, Topological Insulators and Superconductors". Since they distill some most basic ideas in their simplest forms, these concluding remarks might also serve, for non-specialists, as an introduction.

Apr 19
arXiv:1604.04630 [pdf, other]
Disorder-Induced Entanglement in Spin Ice Pyrochlores
Lucile Savary, Leon Balents
Comments: 6+2 pages, 2+1 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We propose that in a certain class of magnetic materials, known as non-Kramers 'spin ice,' disorder induces quantum entanglement. Instead of driving glassy behavior, disorder provokes quantum superpositions of spins throughout the system, and engenders an associated emergent gauge structure and set of fractional excitations. More precisely, disorder transforms a classical phase governed by a large entropy, classical spin ice, into a quantum spin liquid governed by entanglement. As the degree of disorder is increased, the system transitions between (i) a "regular" Coulombic spin liquid, (ii) a phase known as "Mott glass," which contains rare gapless regions in real space, but whose behavior on long length scales is only modified quantitatively, and (iii) a true glassy phase for random distributions with large width or large mean amplitude.


Apr 18
arXiv:1604.04365 [pdf, other]
Low-energy Spin Dynamics of the Honeycomb Spin Liquid Beyond the Kitaev Limit
Xue-Yang Song, Yi-Zhuang You, Leon Balents
Comments: 9 pages, 6 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We investigate the generic features of the low energy dynamical spin structure factor of the Kitaev honeycomb quantum spin liquid perturbed away from its exact soluble limit by different types of spin exchange couplings, including Heisenberg, symmetric off-diagonal and Dzyaloshinskii-Moriya interactions. We formulate the generic expansion of the spin operator in terms of fractionalized Majorana fermion operators according to the symmetry enriched topological order of the Kitaev spin liquid, described by its projective symmetry group. We find that the dynamical spin structure factor displays power-law scaling bounded by Dirac cones in the vicinity of the Γ, K and K′ points of the Brillouin zone, rather than the spin gap found for the exactly soluble point.


Apr 8

1. arXiv:1604.01953 (cross-list from nlin.PS) [pdf, other]
Oscillatory instabilities of gap solitons in a repulsive Bose-Einstein condensate
Pavel P. Kizin, Dmitry A. Zezyulin, Georgy L. Alfimov
Subjects: Pattern Formation and Solitons (nlin.PS); Quantum Gases (cond-mat.quant-gas)

The paper is devoted to numerical study of stability of nonlinear localized modes ("gap solitons") for the spatially one-dimensional Gross-Pitaevskii equation (1D GPE) with periodic potential and repulsive interparticle interactions. We use the Evans function approach combined with the exterior algebra formulation in order to detect and describe weak oscillatory instabilities. We show that the simplest ("fundamental") gap solitons in the first and in the second spectral gaps can undergo oscillatory instabilities for certain values of the frequency parameter (i.e., the chemical potential). The number of unstable eigenvalues and the associated instability rates are described. Several stable and unstable more complex (non-fundamental) gap solitons are also discussed. The results obtained from the Evans function approach are independently confirmed using the direct numerical integration of the GPE.


Apr 7
1. arXiv:1604.01730 [pdf, ps, other]
Homogeneous one-dimensional Bose-Einstein Condensate in the Bogoliubov's Regime
Elías Castellanos
Comments: 5 pages
Subjects: Quantum Gases (cond-mat.quant-gas)

We analyze the corrections caused by finite size effects upon the ground state properties of a homogeneous one-dimensional Bose-Einstein condensate. We assume from the very beginning that the Bogoliubov's formalism is valid and consequently we show that in order to obtain a well defined ground state properties, finite size effects of the system must be taken into account. Indeed, the formalism described in the present work allows to recover the usual properties related to the ground state of a homogeneous one-dimensional Bose-Einstein condensate but corrected by finite size effects of the system. Finally, this scenario allows us to analyze the sensitivity of the system when the Bogoliubov's regime is valid and when finite size effects are present. These facts open the possibility to apply these ideas to more realistic scenarios, e.g., low-dimensional trapped Bose-Einstein condensates.

2. arXiv:1604.01749 (cross-list from cond-mat.mes-hall) [pdf, other]
Quantum Wigner molecules in semiconductor quantum dots and cold-atom optical traps and their mathematical symmetries
Constantine Yannouleas, Uzi Landman

Strong repelling interactions between a few fermions or bosons confined in two-dimensional circular traps lead to particle localization and formation of quantum Wigner molecules (QWMs) possessing definite point-group space symmetries. These point-group symmetries are "hidden" (or emergent), namely they cannot be traced in the circular single-particle densities (SPDs) associated with the exact many-body wave functions, but they are manifested as characteristic signatures in the ro-vibrational spectra. An example, among many, are the few-body QWM states under a high magnetic field or at fast rotation, which are precursor states for the fractional quantum Hall effect. The hidden geometric symmetries can be directly revealed by using spin-resolved conditional probability distributions, which are extracted from configuration-interaction (CI), exact-diagonalization wave functions. The hidden symmetries can also be revealed in the CI SPDs by reducing the symmetry of the trap (from circular to elliptic to quasi-linear). In addition the hidden symmetries are directly connected to the explicitly broken-symmetry (BS) solutions of mean-field approaches, such as unrestricted Hartree-Fock (UHF). A companion step of restoration of the broken symmetries via projection operators applied on the BS-UHF solutions produces wave functions directly comparable to the CI ones, and sheds further light into the role played by the emergence of hidden symmetries in the exact many-body wave functions. Illustrative examples of the importance of hidden symmetries in the many-body problem of few electrons in semiconductor quantum dots and of few ultracold atoms in optical traps (where unprecedented control of the interparticle interaction has been experimentally achieved recently) will be presented.


Apr 6
1. arXiv:1604.01052 [pdf, ps, other]
Ferromagnetism in a Repulsive Atomic Fermi Gas with Correlated Disorder
S. Pilati, E. Fratini
Comments: 5+1 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate the zero-temperature ferromagnetic behavior of a two-component repulsive Fermi gas in the presence of a correlated random field that represents an optical speckle pattern. The density is tuned so that the (noninteracting) Fermi energy is close to the mobility edge of the Anderson localization transition. We employ quantum Monte Carlo simulations to determine various ground-state properties, including the equation of state, the magnetic susceptibility, and the energy of an impurity immersed in a polarized Fermi gas (repulsive polaron). In the weakly interacting limit, the magnetic susceptibility is found to be suppressed by disorder. However, it rapidly increases with the interaction strength, and it diverges at a much weaker interaction strength compared to the clean gas. Both the transition from the paramagnetic phase to the partially ferromagnetic phase, and the one from the partially to the fully ferromagnetic phase are strongly favored by disorder, indicating a case of order induced by disorder.


Apr 5

1. arXiv:1604.00658 [pdf, ps, other]
Kohn-Sham approach to Fermi gas superfluidity: the bilayer of fermionic polar molecules
Francesco Ancilotto
Comments: 10 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

By using a well established 'ab initio' theoretical approach developed in the past to quantitatively study the superconductivity of condensed matter systems, which is based on the Kohn-Sham Density Functional theory, I study the superfluid properties and the BCS-BEC crossover of two parallel bi-dimensional layers of fermionic dipolar molecules, where the pairing mechanism leading to superfluidity is provided by the inter-layer coupling between dipoles. The finite temperature superfluid properties of both the homogeneous system and one were the fermions in each layer are confined by a square optical lattice are studied at half filling conditions, and for different values of the strength of the confining optical potential. The T=0 results for the homogeneous system are found to be in excellent agreement with Diffusion Monte Carlo results. The superfluid transition temperature in the BCS region is found to increase, for a given inter-layer coupling, with the strength of the confining optical potential. A transition occurs at sufficiently small interlayer distances, where the fermions becomes localized within the optical lattice sites in a square geometry with an increased effective lattice constant, forming a system of localized composite bosons. This transition should be signalled by a sudden drop in the superfluid fraction of the system.