Jan 30 - Feb 3, Xiaopeng Li
Feb 3rd
1. arXiv:1202.0497 [pdf, other]
Title:Incommensurate nematic fluctuations in two dimensional metals
Author: Tobias Holder, Walter Metzner
To assess the strength of nematic fluctuations with a finite wave vector in a two-dimensional metal, we compute the static d-wave polarization function for tight-binding electrons on a square lattice. At Van Hove filling and zero temperature the function diverges logarithmically at q=0. Away from Van Hove filling the ground state polarization function exhibits finite peaks at finite wave vectors. A nematic instability driven by a sufficiently strong attraction in the d-wave charge channel thus leads naturally to a spatially modulated nematic state, with a modulation vector that increases in length with the distance from Van Hove filling. Above Van Hove filling, for a Fermi surface crossing the magnetic Brillouin zone boundary, the modulation vector connects antiferromagnetic hot spots.
2. arXiv:1202.0383 [pdf, ps, other]
Title: Quantum Entanglement of Tensor Networks with Symmetry Projections
Author: Masashi Orii, Hiroshi Ueda, Isao Maruyama
We investigate the global-symmetry projections applied to the tensor network states from the view point of the entanglement entropy and the mutual information. The projections to the translational invariant space and to the total-$S^z$-zero space give logarithmically increasing mutual information with respect to the system size. In the anti-ferromagnetic $S=1/2$ Heisenberg chain and lattice, the optimized energies become accurate numerically by using variational states of the projected tensor network states, because the projections reflecting symmetries of the ground states generate quantum entanglement.
3. arXiv:1202.0369 [pdf, ps, other]
Title: Neutron Scattering Study on Commensurate and Incommensurate Antiferromagnetic Phases in UPd2Si2 under Uniaxial Stress
Author: T. Nakada, M. Yokoyama, C. Tabata, H. Igarashi, H. Hidaka, H. Amitsuka, K. Tenya, T. J. Sato
The nature of competition between incommensurate (IC) and commensurate (C) antiferromagnetic (AF) orders in UPd2Si2 was investigated by performing elastic neutron scattering experiments under uniaxial stress sigma. It is found that applying sigma along tetragonal [010] direction reduces the IC-AF order, and then stabilizes the C-AF order. The transition temperature from IC- to C-AF phases T_Nl is enhanced from 109 K (sigma=0) to 112.5 K (0.8 GPa), while the onset of IC-AF transition T_Nh is unchanged from 132 K under sigma. In addition, c-axis component q_z of the IC-AF modulation at 115 K also increases from 0.736 (sigma=0) to 0.747 (0.8 GPa). The magnitude of C-AF moment at 5 K is estimated to be 2.2 mu_B/U in the entire sigma range presently investigated (sigma <= 0.8 GPa). These features are similar to those obtained from the investigations using hydrostatic pressure p, indicating that applications of p and sigma||[010] commonly induce the crystal strains which inherently affect a delicate balance of frustrated magnetic interactions between uranium 5f moments.
4. **arXiv:1202.0291** [**pdf**, **other**]
Title: Fractionalized topological insulators from frustrated spin models in three dimensions
Author: Subhro Bhattacharjee, Yong Baek Kim, Sung-Sik Lee, Dung-Hai Lee
We present a theory of three dimensional fractionalized topological insulators in the form of U(1) spin liquids with gapped fermionic spinons in the bulk and topologically protected gapless spinon surface states. Starting from a spin-1/2 model on a pyrochlore lattice, with frustrated antiferromagnetic and ferromagnetic exchange interactions, we show that decomposition of the latter interactions, within slave-fermion representation of the spins, can naturally give rise to an emergent spin-orbit coupling for the spinons. This stabilizes a fractionalized topological insulators which also have bulk bond spin-nematic order. Finally, we describe the low energy properties of these states.
Feb 2nd
1. arXiv:1202.0060 [pdf, other]
Title: Imaging topologically protected transport with quantum degenerate gases
Author: Brian Dellabetta, Taylor L. Hughes, Matthew J. Gilbert, Benjamin L. Lev
Ultracold and quantum degenerate gases held near conductive surfaces can serve as sensitive, high resolution, and wide-area probes of electronic current flow. Previous work has imaged transport around grain boundaries in a gold wire by using ultracold and Bose-Einstein condensed atoms held microns from the surface with an atom chip trap. We show that atom chip microscopy may be applied to useful purpose in the context of materials exhibiting topologically protected surface transport. Current flow through lithographically tailored surface defects in topological insulators (TI)---both idealized and with the band-structure and conductivity typical of Bi_2Se_3---is numerically calculated. We propose that imaging current flow patterns enables the differentiation of an ideal TI from one with a finite bulk--to--surface conductivity ratio, and specifically, that the determination of this ratio may be possible by imaging transport around trenches etched into the TI's surface.
Feb 1st
1. **arXiv:1201.6674** [**pdf**, **ps**, **other**]
Title: Dynamics of correlations in shallow optical lattices
Author: Stefan S. Natu, Erich J. Mueller
We explore how correlations evolve in a gas of lattice bosons when the lattice depth is rapidly reduced. We find a simple closed form expression for the static structure factor in the limit of vanishing interactions. The corresponding real-space density correlation function shows multiple spatial oscillations which linearly disperse as a function of time. By perturbatively including the effects of the interactions we calculate how the boson quasi-momentum evolves following the quench.
2. arXiv:1201.6630 [pdf, other]
Title: The Peierls substitution in an engineered lattice potential
Author: K. Jiménez-García, L. J. LeBlanc, R. A. Williams, M. C. Beeler, A. R. Perry, I. B. Spielman
Artificial gauge fields open new possibilities to realize quantum many-body systems with ultracold atoms, by engineering Hamiltonians usually associated with electronic systems. In the presence of a periodic potential, artificial gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here, we describe a one-dimensional lattice derived purely from effective Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency magnetic fields. In this lattice, the tunneling matrix element is generally complex. We control both the amplitude and the phase of this tunneling parameter, experimentally realizing the Peierls substitution for ultracold neutral atoms.
3. **arXiv:1201.6586** [**pdf**, **ps**, **other**]
Title: Universality in Four-Boson Systems
Author: T. Frederico, A. Delfino, M. R. Hadizadeh, L. Tomio, M. T. Yamashita
We report recent advances in our studies on universality in four-boson systems. In particular, we are reporting results considering the solution of Faddeev-Yakubovsky equations, within a renormalized zero-range interaction, which are confirming a previous conjecture on four-body scale dependence. We also discuss applications of the results in cold-atom systems.
4. **arXiv:1201.6431** [**pdf**, **ps**, **other**]
Title: Topological invariants for interacting topological insulators with inversion symmetry
Author: Zhong Wang, Xiao-Liang Qi, Shou-Cheng Zhang
For interacting Z_2 topological insulators with inversion symmetry, we propose a simple topological invariant expressed in terms of the parity eigenvalues of the interacting Green's function at time-reversal invariant momenta. We derive this result from our previous formula involving the integral over the frequency-momenta space. This formula greatly simplifies the explicit calculation of Z_2 topological invariants in inversion symmetric insulators with strong interactions.
5. **arXiv:1201.6400** [**pdf**, **ps**, **other**]
Title: Universal dynamical phase diagram of lattice spin models and strongly correlated ultracold atoms in optical lattices
Author: E. Demler, A. Maltsev, A. Prokofiev
We study semiclassical dynamics of anisotropic Heisenberg models in two and three dimensions. Such models describe lattice spin systems and hard core bosons in optical lattices. We solve numerically Landau-Lifshitz type equations on a lattice and show that in the phase diagram of magnetization and interaction anisotropy, one can identify several distinct regimes of dynamics. These regions can be distinguished based on the character of one dimensional solitonic excitations, and stability of such solitons to transverse modulation. Small amplitude and long wavelength perturbations can be analyzed analytically using mapping of non-linear hydrodynamic equations to KdV type equations. Numerically we find that properties of solitons and dynamics in general remain similar to our analytical results even for large amplitude and short distance inhomogeneities, which allows us to obtain a universal dynamical phase diagram. As a concrete example we study dynamical evolution of the system starting from a state with magnetization step and show that formation of oscillatory regions and their stability to transverse modulation can be understood from the properties of solitons. In regimes unstable to transverse modulation we observe formation of lump type solutions with modulation in all directions. We discuss implications of our results for experiments with ultracold atoms.
Jan 31st
1. **arXiv:1201.6235** [**pdf**, **ps**, **other**]
Title: Boltzmann equation simulation for a trapped Fermi gas of atoms
Author: Olga Goulko, Frédéric Chevy, Carlos Lobo
The dynamics of an interacting Fermi gas of atoms at sufficiently high temperatures can be efficiently studied via a numerical simulation of the Boltzmann equation. In this work we describe in detail the setup we used recently to study the oscillations of two spin-polarised fermionic clouds in a trap. We focus here on the evaluation of interparticle interactions. We compare different ways of choosing the phase space coordinates of a pair of atoms after a successful collision and demonstrate that the exact microscopic setup has no influence on the macroscopic outcome.
2. arXiv:1201.5929 [pdf, other]
Title: Topological Quantum Phase Transition in 5$d$ Transition Metal Oxide Na$_2$IrO$_3$
Author: Choong H. Kim, Heung Sik Kim, Hogyun Jeong, Hosub Jin, Jaejun Yu
We predict a quantum phase transition from normal-to-topological insulators in 5$d$ transition metal oxide Na$_2$IrO$_3$, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na$_2$IrO$_3$ model structures show that the inter-layer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na$_2$IrO$_3$ is suggested to be a candidate material which can have both non-trivial topology of bands and strong electron correlations.
Jan 30th
1. arXiv:1201.5874 [pdf, other]
Title: Anomalous quantum Hall states in an optical lattice
Author: S. R. Hassan, Sandeep Goyal, R. Shankar, David Sénéchal
We analyze the Physics of cold atoms in honeycomb optical lattices with on-site repulsion and spin-orbit couplings that break time reversal symmetry. Such systems, at half filling and large on-site repulsion, have been proposed as a possible realization of the Kitaev model. The spin-orbit couplings break the spin degeneracy and, if strong-enough, lead to four non-overlapping bands in the non-interacting limit. These bands carry non-zero Chern number and therefore the non-interacting system has non-zero angular momentum and chiral edge states at 1/4 and 3/4 filling. We have investigated the effect of interactions using the variational cluster perturbation theory and conclude that the chiral edge states exist in finite range of interaction and hopping parameter space.
2. arXiv:1201.5857 [pdf, other]
Title: Observation of a superfluid Hall effect
Author: L. J. LeBlanc, K. Jimenez-Garcia, R. A. Williams, M. C. Beeler, A. R. Perry, W. D. Phillips, I. B Spielman
Measurement techniques based upon the Hall effect are invaluable tools in condensed matter physics. When an electric current flows perpendicular to a magnetic field, a Hall voltage develops in the direction transverse to both the current and the field. In semiconductors, this behaviour is routinely used to measure the density and charge of the current carriers (electrons in conduction bands or holes in valence bands) -- internal properties of the system that are not accessible from measurements of the conventional resistance. For strongly interacting electron systems, whose behaviour can be very different from the free electron gas, the Hall effect's sensitivity to internal properties makes it a powerful tool; indeed, the quantum Hall effects are named after the tool by which they are most distinctly measured instead of the physics from which the phenomena originate. Here we report the first observation of a Hall effect in an ultracold gas of neutral atoms, revealed by measuring a Bose-Einstein condensate's transport properties perpendicular to a synthetic magnetic field. Our observations in this vortex-free superfluid are in good agreement with hydrodynamic predictions, demonstrating that the system's global irrotationality influences this superfluid Hall signal.
3. arXiv:1201.5663 [pdf, ps, other]
Title: Topological superfluid in one-dimensional spin-orbit coupled atomic Fermi gases
Author: Xia-Ji Liu, Hui Hu
Jan 23 - Jan 27, Zixu Zhang
Jan 27
1. arXiv:1201.5526 [pdf, ps, other]
Inhomogeneous spin diffusion in traps with cold atoms
H. Heiselberg
The spin diffusion and damped oscillations are studied in the collision of two spin polarized clouds of cold atoms with resonant interactions. The strong density dependence of the diffusion coefficient leads to inhomogeneous spin diffusion that changes from central to surface spin flow as the temperature increases. The inhomogeneity and the smaller finite trap size significantly reduce the spin diffusion rate at low temperatures. The resulting spin diffusion rates, spin drag and initial damped oscillations are compatible with measurements at low to high temperatures for resonant attractive interactions but are incompatible with a metastable ferromagnetic phase.
2. arXiv:1201.5576 [pdf, ps, other]
Universal probes for antiferromagnetic correlations and entropy in cold fermions on optical lattices
E. V. Gorelik, D. Rost, T. Paiva, R. Scalettar, A. Klümper, N. Blümer
We determine antiferromagnetic (AF) signatures in the half-filled Hubbard model at strong coupling for a cubic lattice and in lower dimensions. Upon cooling, the transition from the charge-excitation regime to the AF Heisenberg regime is signaled by a universal minimum of the double occupancy at entropy S/(N k_B)=log(2) per particle and a linear increase of the next-nearest neighbor (NNN) spin correlation function below. This crossover, driven by a gain in kinetic exchange energy, appears as the essential AF physics relevant for current cold-atom experiments. The detection of long-range AF order requires measurements of spin correlations at or beyond NNN distances.
Jan 26
1. arXiv:1201.5332 [pdf, other]
Collective excitations across the BCS-BEC crossover induced by a synthetic Rashba spin-orbit coupling
Jayantha P. Vyasanakere, Vijay B. Shenoy
Synthetic non-Abelian gauge fields in cold atom systems produce a Rashba spin-orbit interaction described by a vector $\blam = (\lambda_x, \lambda_y, \lambda_z)$. It was recently shown [Phys. Rev. B 84, 014512 (2011)] that on increasing $\lambda = |\blam|$, fermions at a finite density $\rho\approx\kf^3$ evolve to a BEC like state even in the presence of a weak attractive interaction (described by a scattering length $\as$). The BEC obtained at large spin-orbit coupling ($\lambda \gg k_F$) is a condensate of rashbons -- novel bosonic bound pairs of fermions whose properties are determined solely by the gauge field. Here we study the collective excitations of such superfluids by constructing a Gaussian theory using functional integral methods. We derive explicit expressions for superfluid phase stiffness, sound speed and mass of the Anderson-Higgs boson that are valid for any $\blam$ and scattering length. We find that at finite $\lambda$, the phase stiffness is always lower than that set by the density of particles, consistent with earlier work[arXiv:1110.3565] which attributed this to the lack of Galilean invariance of the system at finite $\lambda$. We show that there is an emergent Galilean invariance at large $\lambda$, and the phase stiffness is determined by the rashbon density and mass, consistent with Leggett's theorem. We further demonstrate that the rashbon BEC state is a superfluid of anisotropic rashbons interacting via a contact interaction characterized by a rashbon-rashbon scattering length $a_R$. We show that $a_R$ goes as $\lambda^{-1}$ and is essentially {\em independent} of the scattering length between the fermions as long as it is nonzero. Analytical results are presented for a rashbon BEC obtained in a spherical gauge field with $\lambda_x = \lambda_y = \lambda_z = \frac{\lambda}{\sqrt{3}}$.
Jan 25
1. arXiv:1201.4974 [pdf, ps, other]
Exotic vortex lattices in two-species Bose-Einstein condensates
Pekko Kuopanportti, Jukka A. M. Huhtamäki, Mikko Möttönen
We numerically investigate vortex lattices in rotating two-component Bose-Einstein condensates in which the two components have unequal atomic masses and interact attractively with each other. For sufficiently strong attraction, the system is found to exhibit exotic ground-state structures in a harmonic trap, such as lattices having a square geometry or consisting of two-quantum vortices. The obtained states satisfy the Feynman relation, and they can be realized with current experimental techniques.
2. arXiv:1201.5015 [pdf, ps, other]
Spatio-temporal Fermionization of Strongly Interacting 1D Bosons
V. Guarrera, D. Muth, R. Labouvie, A. Vogler, G. Barontini, M. Fleischhauer, H. Ott
Building on the recent experimental achievements obtained with scanning electron microscopy on ultracold atoms, we study one-dimensional Bose gases in the crossover between the weakly (quasi-condensate) and the strongly interacting (Tonks-Girardeau) regime. We measure the temporal two-particle correlation function and compare it with calculations performed using the Time Evolving Block Decimation (TEBD) algorithm. Clear antibunching is observed when entering the strongly interacting regime. The onset of fermionization is also reflected in the density distribution, which we measure \emph{in situ} to extract the relevant parameters and to identify the different regimes. Our results show very good agreement between experiment and theory and give new insight into the dynamics of strongly correlated many-body systems.
Jan 24
1. arXiv:1201.4393 [pdf, ps, other]
A continuous transition between fractional quantum Hall and superfluid states
Maissam Barkeshli, John McGreevy
We develop a theory of a direct, continuous quantum phase transition between a bosonic Laughlin fractional quantum Hall (FQH) state and a superfluid, generalizing the Mott insulator to superfluid phase diagram of bosons to allow for the breaking of time-reversal symmetry. The direct transition can be protected by a spatial symmetry, and the critical theory is a pair of Dirac fermion fields coupled to an emergent Chern-Simons gauge field. The transition may be achieved in optical traps of ultracold atoms by starting with a $\nu = 1/2$ bosonic Laughlin state and tuning an appropriate periodic potential to change the topology of the composite fermion band structure.
Jan 23
1. arXiv:1201.4174 [pdf, other]
An order parameter for symmetry-protected phases in one dimension
Jutho Haegeman, David Perez-Garcia, Ignacio Cirac, Norbert Schuch
We introduce an order parameter for symmetry-protected phases in one dimension which allows to directly identify those phases. The order parameter consists of string-like operators and swaps, but differs from conventional string order operators in that it only depends on the symmetry but not on the state. We verify our framework through numerical simulations for the SO(3) invariant spin-1 bilinear-biquadratic model which exhibits a dimerized and a Haldane phase, and find that the order parameter not only works very well for the dimerized and the Haldane phase, but it also returns a distinct signature for gapless phases. Finally, we discuss possible ways to measure the order parameter in experiments with cold atoms.
2. arXiv:1201.4310 [pdf, ps, other]
Efimov physics beyond universality
Richard Schmidt, Steffen Patrick Rath, Wilhelm Zwerger
We calculate the spectrum of three-body Efimov bound states in the context of ultracold gases, where the two-body scattering length a is tuned via a Feshbach resonance. It is shown that the finite range in the Feshbach coupling makes the introduction of an adjustable three-body parameter obsolete. Depending on whether the resonance is open or closed channel dominated, the length which sets the scale in the energy level diagram as a function of 1/a is either the van der Waals length l_vdw or the intrinsic length r* associated with the Feshbach coupling, with a continuous crossover in between. Our results explain a number of experimental observations on Efimov states, in particular the apparent `universality' of the ratio between the scattering length where the first Efimov state appears and the van der Waals length.
Jan 16 - Jan 20, Bin Wang
Jan 20
1. arXiv:1201.4117 [pdf, ps, other]
Title: Half-Vortex Unbinding and Ising Transition in Constrained Superfluids
Authors: Lars Bonnes, Stefan Wessel
We analyze the thermodynamics of the atomic and (nematic) pair superfluids appearing in the attractive two-dimensional Bose-Hubbard model with a three-body hard-core constraint that has been derived as an effective model for cold atoms subject to strong three-body losses in optical lattices. We show that the thermal disintegration of the pair superfluidity is governed by the proliferation of fractional half-vortices leading to a Berezinskii-Kosterlitz-Thousless transition with unusual jump in the helicity modulus. In addition to the (conventional) Berezinskii-Kosterlitz-Thousless transition out of the atomic superfluid, we furthermore identify a direct thermal phase transition separating the pair and the atomic superfluid phases, and show that this transition is continuous with critical scaling exponents consistent with those of the two-dimensional Ising universality class. We exhibit a direct connection between the partial loss of quasi long-range order at the Ising transition between the two superfluids and the parity selection in the atomic winding number fluctuations that distinguish the atomic from the pair superfluid.
2. arXiv:1201.4035 [pdf, ps, other]
Title: Density and spin response of a strongly-interacting Fermi gas in the attractive and quasi-repulsive regime
Authors: F. Palestini, P. Pieri, G. C. Strinati
Recent experimental advances in ultra-cold Fermi gases allow for exploring response functions under different dynamical conditions. In particular, the issue of obtaining a "quasi-repulsive" regime starting from a Fermi gas with an attractive inter-particle interaction while avoiding the formation of the two-body bound state is currently debated. Here, we provide a calculation of the density and spin response for a wide range of temperature and coupling both in the attractive and quasi-repulsive regime, whereby the system is assumed to evolve non-adiabatically toward the "upper branch" of the Fermi gas. A comparison is made with the available experimental data for these two quantities.
3. arXiv:1201.3975 [pdf, ps, other]
Title: Variational Monte Carlo with the Multi-Scale Entanglement Renormalization Ansatz
Authors: Andrew J. Ferris, Guifre Vidal
Tensor network states are powerful variational ans\"atze for ground states of quantum many-body systems on a lattice. Monte Carlo sampling techniques have been proposed as a strategy to reduce the computational cost of contractions in tensor network approaches. Here we put forward a variational Monte Carlo approach for the multi-scale entanglement renormalization ansatz (MERA), which is a unitary tensor network. Two major adjustments are required compared to previous proposals with non-unitary tensor networks. First, instead of sampling over configurations of the original lattice, made of L sites, we sample over configurations of an effective lattice, which is made of just log(L) sites. Second, the optimization of unitary tensors must account for their unitary character while being robust to statistical noise, which we accomplish with a modified steepest descent method within the set of unitary tensors. We demonstrate the performance of the variational Monte Carlo MERA approach in the relatively simple context of a finite quantum spin chain at criticality, and discuss future, more challenging applications, including two dimensional systems.
4. arXiv:1201.3974 [pdf, ps, other]
Title: Perfect Sampling with Unitary Tensor Networks
Authors: Andrew J. Ferris, Guifre Vidal
Tensor network states are powerful variational ans\"atze for many-body ground states of quantum lattice models. The use of Monte Carlo sampling techniques in tensor network approaches significantly reduces the cost of tensor contractions, potentially leading to a substantial increase in computational efficiency. Previous proposals are based on a Markov chain Monte Carlo scheme generated by locally updating configurations and, as such, must deal with equilibration and autocorrelation times, which result in a reduction of efficiency. Here we propose a perfect sampling scheme, with vanishing equilibration and autocorrelation times, for unitary tensor networks -- namely tensor networks based on efficiently contractible, unitary quantum circuits, such as unitary versions of the matrix product state (MPS) and tree tensor network (TTN), and the multi-scale entanglement renormalization ansatz (MERA). Configurations are directly sampled according to their probabilities in the wavefunction, without resorting to a Markov chain process. We also describe a partial sampling scheme that can result in a dramatic (basis-dependent) reduction of sampling error.
Jan 19
1. arXiv:1201.3837 [pdf, ps, other]
Title: Emergence of Quintet Superfluidity in the Chain of Partially Polarized Spin-3/2 Ultracold Atom
Authors: G. Barcza, E. Szirmai, Ö. Legeza, J. Sólyom
The system of ultracold atoms with hyperfine spin $F=3/2$ might be unstable against the formation of quintet pairs if the interaction is attractive in the quintet channel. We have investigated the behavior of correlation functions in a model including only s-wave interactions at quarter filling by large-scale density matrix renormalization group simulations. The correlations of quintet pairs remain short-ranged even for strong attractive interaction in the paramagnetic ground state. They become, however, quasi-long-ranged when the system is partially polarized, in a broad range around two thirds of saturation. We also discuss possible experimental realizations and detection of quintet pairing in the system of ultracold atoms.
2. arXiv:1201.3620 (cross-list from quant-ph) [pdf, other]
Title: Quantum Simulation of Cooperative Jahn-Teller Systems with Linear Ion Crystals
Authors: Diego Porras, Peter A. Ivanov, Ferdinand Schmidt-Kaler
The Jahn-Teller effect explains distortions and non-degenerate energy levels in molecular and solid-state physics via a coupling of effective spins to collective bosons. Here we propose and theoretically analyze the quantum simulation of a many-body Jahn-Teller model with linear ion crystals subjected to magnetic field gradients. We show that the system undergoes a quantum magnetic structural phase transition which leads to a reordering of particle positions and the formation of a spin-phonon quasi-condensate in mesoscopic ion chains.
3. arXiv:1201.3757 (cross-list from quant-ph) [pdf, ps, other]
Title: From Majorana Fermions to Topological Order
Authors: Barbara M. Terhal, Fabian Hassler, David P. DiVincenzo
We consider a system consisting of a 2D network of links between Majorana fermions on superconducting islands. We show that the fermionic Hamiltonian modeling this system is topologically-ordered in a region of parameter space. In particular we show that Kitaev's toric code emerges in fourth-order perturbation theory. By using a Jordan-Wigner transformation we can map the model onto a family of signed 2D Ising models in a transverse field where the signs (FM or AFM) are determined by additional gauge bits. Our mapping allows an understanding of the non-perturbative regime and the phase transition to a non-topological phase. We discuss the physics behind a possible implementation of this model and argue how it can be used for topological quantum computation by adiabatic changes in the Hamiltonian.
Jan 18
1. arXiv:1201.3560 [pdf, ps, other]
Title: Polarons in the radio-frequency spectrum of a quasi-two-dimensional Fermi gas
Authors: Y. Zhang, W. Ong, I. Arakelyan, J. E. Thomas
We measure radio-frequency spectra for a two-component mixture of a $^6$Li atomic Fermi gas in the quasi-two-dimensional regime. Near the Feshbach resonance, where the transverse Fermi energy is large compared to the confinement-induced dimer binding energies for the initial and final states, we find that the observed resonances do not correspond to transitions between confinement-induced dimers. The spectrum appears to be well-described by transitions between noninteracting polaron states in two dimensions.
Jan 17
1. arXiv:1201.3344 [pdf, other]
Title: Coexistence of spin-1/2 and spin-1 Dirac-Weyl fermions in the edge-centered honeycomb lattice
Authors: Zhihao Lan, Nathan Goldman, Patrik Ohberg
We investigate the properties of an edge-centered honeycomb lattice, and show that this lattice features both spin-1/2 and spin-1 Dirac-Weyl fermions at different filling fractions f (f=1/5,4/5 for spin-1/2 and f=1/2 for spin-1). This five-band system is the simplest lattice that can support simultaneously the two different paradigmatic Dirac-Weyl fermions with half-integer spin and integer spin which also includes a flat band. We further explore the effects of several perturbations on this system, such as a uniform magnetic field, an intrinsic spin-orbit coupling, and charge density waves. Our results demonstrate how these perturbations makes it possible to probe the similarities and differences between the two kinds of relativistic fermions, or even to isolate them individually. We comment also on the possibility to realize such a system using cold atoms.
2. arXiv:1201.3253 [pdf, other]
Title: Probing Atomic Majorana Fermions in Optical Lattices
Authors: Christina V. Kraus, Sebastian Diehl, Peter Zoller, Mikhail A. Baranov
We introduce a one-dimensional system of fermionic atoms in an optical lattice whose phase diagram includes topological states of different symmetry classes. These states can be identified by their zero-energy edge modes which are Majorana fermions. We propose several universal methods of detecting the Majorana edge states, based on their genuine features: zero-energy, localized character of the wave functions, and induced non-local fermionic correlations.
Jan 16
1. arXiv:1201.2856 [pdf, other]
Title: Phase diagram of the rotating two-component Fermi gas including vortices
Authors: Harmen J. Warringa
We determine the conditions under which superfluidity with and without quantized vortices appears in a weakly interacting two-component atomic Fermi gas that is trapped in a rotating cylindrical symmetric harmonic potential. We compute the phase diagram as a function of rotation frequency, scattering length, temperature, total number of trapped atoms, and population imbalance.
Jan 9 - Jan 13, Johannes Schachenmayer
Jan 13
1. arXiv:1201.2615 [pdf, ps, other]
Mean-field description of dipolar bosons in triple-well potentials
David Peter, Krzysztof Pawlowski, Tilman Pfau, Kazimierz Rzazewski
We investigate the ground state properties of a polarized dipolar Bose-Einstein condensate trapped in a triple-well potential. By solving the dipolar Gross-Pitaevskii equation numerically for different geometries we identify states which reveal the non-local character of the interaction. Depending on the strength of the contact and dipolar interaction we depict the stable and unstable regions in parameter space.
2. arXiv:1201.2573 [pdf, ps, other]
A novel route to Bose-Einstein condensation of two-electron atoms
Purbasha Halder, Chih-Yun Yang, Andreas Hemmerich
We present a novel route to Bose-Einstein condensation devised for two-electron atoms, which do not admit practicable cooling techniques based upon narrow intercombination lines. A dipole trap for $^{40}$Ca atoms in the singlet ground state is loaded from a moderately cold source of metastable triplet atoms via spatially and energetically selective optical pumping permitting four orders of magnitude increase of the phase space density. Further cooling to quantum degeneracy is achieved by forced evaporation optimized to minimize three-body losses. In a combined loading and evaporation cycle of less than three seconds we are able to condense 3000 atoms.
3. arXiv:1201.2440 [pdf, ps, other]
Self-consistent field theory of polarized BEC: dispersion of collective excitation
P. A. Andreev, L. S. Kuz'menkov
We propose the construction of a set of quantum hydrodynamics equations for the Bose-Einstein condensate (BEC) where atoms have electric dipole moment. The contribution of the dipole-dipole interactions (DDI) to the Euler equation is estimated. Quantum equations for the evolution of medium polarization are constructed. The mathematical method we developed allows studying the effects of interactions on the evolution of polarization. The developed method may be applied to various physical systems in which dynamics is affected by DDI. Derivation of Gross-Pitaevskii equation for polarized particles is discussed using point of view of quantum hydrodynamics. We shown that Gross-Pitaevskii equation appear at condition there all dipoles has the same direction which does not change in time. Comparison of equation of electric dipole interaction with equation of magnetization evolution is described. A problem of elementary excitations in BEC, either affected or not affected by the uniform external electric field, is considered using our method. We show that the evolution of polarization in BEC leads to the formation of a novel type of elementary excitations. Detailed description of dispersion of elementary excitations is presented. We consider also the process of wave generation in polarized BEC by means of monoenergetic beam of neutral polarized particles. We compute the possibilities of the generation of Bogoliubov's modes and polarization modes by the dipole beam.
4. arXiv:1201.2492 [pdf, ps, other]
Equilibration in long-range quantum spin systems from a BBGKY perspective
Rytis Paškauskas, Michael Kastner
The time evolution of $\ell$-spin reduced density operators is studied for a class of Heisenberg-type quantum spin models with long-range interactions. In the framework of the quantum Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy, we introduce an unconventional representation, different from the usual cluster expansion, which casts the hierarchy into the form of a second-order recursion. This structure suggests a scaling of the expansion coefficients and the corresponding time scales in powers of $N^{1/2}$ with the system size $N$, implying a separation of time scales in the large system limit. For special parameter values and initial conditions, we can show analytically that closing the BBGKY hierarchy by neglecting $\ell$-spin correlations does never lead to equilibration, but gives rise to quasi-periodic time evolution with at most $\ell/2$ independent frequencies. Moreover, for the same special parameter values and in the large-$N$ limit, we solve the complete recursion relation (the full BBGKY hierarchy), observing a superexponential decay to equilibrium in rescaled time $\tau=tN^{-1/2}$.
5. arXiv:1112.5082 [pdf, ps, other]
Exploring spin-orbital models with dipolar fermions in zig-zag optical lattices
G. Sun, G. Jackeli, L. Santos, T. Vekua
Ultra-cold dipolar spinor fermions in zig-zag type optical lattices can mimic spin-orbital models relevant in solid-state systems, as transition-metal oxides with partially filled d-levels, with the interesting advantage of reviving the quantum nature of orbital fluctuations. We discuss two different physical systems in which these models may be simulated, showing that the interplay between lattice geometry and spin-orbital quantum dynamics produces a wealth of novel quantum phases.
Jan 12
1. arXiv:1201.2396 [pdf, ps, other]
Association of Efimov trimers from three-atoms continuum
Olga Machtey, Zav Shotan, Noam Gross, Lev Khaykovich
We develop an experimental technique for rf-association of Efimov trimers from three-atoms continuum. We apply it to probe the lowest Efimov energy level in bosonic lithium in the region where strong deviations from the universal behavior are expected, and provide quantitative study of this effect. Position of the Efimov resonance at the atom-dimer threshold, measured with a different experimental technique, concurs with the rf-association results.
2. arXiv:1201.2238 [pdf, ps, other]
BKT Transition and Majorana Fermions in 2D Spin-orbit Coupled Fermi Superfluids
Gang Chen, Ming Gong, Suotang Jia, Chuanwei Zhang
Recent experimental breakthrough in realizing spin-orbit coupling (SOC) for cold atoms has spurred considerable interest in the physics of 2D spin-orbit coupled Fermi superfluids, especially topological Majorana fermions which were predicted to exist at zero temperature. However, in realistic experiments, phase fluctuations and the resulting finite temperature Berezinskii-Kosterlitz-Thouless (BKT) transition are crucial for understanding general 2D physics as well as observing Majorana fermions. Here we show that the BKT transition temperature first decreases (in the small SOC region) then increases (in the large SOC region) with increasing SOC, although superfluid pairing and quasiparticle energy gaps always increase. Similar features are observed for the vortex lattice melting temperature, below which free vortices emerge. Majorana fermions exist inside these free vortices within a suitable parameter region, which is also obtained.
3. arXiv:1201.2326 [pdf, ps, other]
Momentum-space calculation of four-boson recombination
A. Deltuva
The system of four identical bosons with large two-boson scattering length is described using momentum-space integral equations for the four-particle transition operators. The creation of Efimov trimers via ultracold four-boson recombination is studied. The universal behavior of the recombination rate is demonstrated.
Jan 11
1. arXiv:1201.2138 [pdf, ps, other]
Soliton-phonon scattering problem in 1D nonlinear Schrödinger systems with general nonlinearity
Daisuke A. Takahashi
A scattering problem (or more precisely, a transmission-reflection problem) of linearized excitations in the presence of a dark soliton is considered in a one-dimensional nonlinear Schr\"odinger system with a general nonlinearity: $ \mathrm{i}\partial_t \phi = -\partial_x^2 \phi + F(|\phi|^2)\phi $. If the system is interpreted as a Bose-Einstein condensate, the linearized excitation is a Bogoliubov phonon, and the linearized equation is the Bogoliubov equation. We exactly prove that the perfect transmission of the zero-energy phonon is suppressed at a critical state determined by Barashenkov's stability criterion [Phys. Rev. Lett. 77, (1996) 1193.], and near the critical state, the energy-dependence of the reflection coefficient shows a saddle-node type scaling law. Our result gives an exact example of scaling laws of saddle-node bifurcation in time-reversible Hamiltonian systems. As a by-product of the proof, we also give all exact zero-energy solutions of Bogoliubov equation and their finite energy extension.
2. arXiv:1201.2112 [pdf, other]
Majorana Modes in Driven-Dissipative Atomic Superfluids With Zero Chern Number
C.-E. Bardyn, M. A. Baranov, E. Rico, A. Imamoglu, P. Zoller, S. Diehl
We investigate dissipation-induced p-wave paired states of fermions in two dimensions and show that dissipation can break the bulk-edge correspondence present in Hamiltonian systems in a way that leads to the appearance of spatially separated Majorana zero modes in a phase with vanishing Chern number. We construct an explicit model of a dissipative vortex that traps a single of these modes and establish its topological origin by mapping it to a one-dimensional wire where we observe a non-equilibrium topological phase transition characterized by an abrupt change of a topological invariant (winding number). Engineered dissipation opens up possibilities for experimentally realizing such states with no Hamiltonian counterpart.
3. arXiv:1201.2019 [pdf, other]
Nonequilibrium Damping of Collective Motion of Homogeneous Cold Fermi Condensates with Feshbach Resonances
Chi-Yong Lin, Da-Shin Lee, Ray J. Rivers
Collisionless damping of a condensate of cold Fermi atoms, whose scattering is controlled by a Feshbach resonance, is explored throughout the BCS and BEC regimes when small perturbations on its phase and amplitude modes are turned on to drive the system slightly out of equilibrium. Using a one-loop effective action, we first recreate the known result that for a broad resonance the amplitude of the condensate decays as $t^{-1/2}$ at late times in the BCS regime whereas it decays as $t^{-3/2}$ in the BEC regime. We then examine the case of an idealized narrow resonance, and find that this collective mode decays as $t^{-3/2}$ throughout both the BCS and BEC regimes. Although this seems to contradict earlier results that damping is identical for both broad and narrow resonances, the breakdown of the narrow resonance limit restores this universal behaviour. More measureably, the phase perturbation may give a shift on the saturated value to which the collective amplitude mode decays, which vanishes only in the deep BCS regime when the phase and amplitude modes are decoupled.
4. arXiv:1201.1958 [pdf, other]
Bose Gases Near Unitarity
Weiran Li, Tin-Lun Ho
We study the properties of strongly interacting Bose gases at the density and temperature regime when the three-body recombination rate is substantially reduced. In this regime, one can have a Bose gas with all particles in scattering states (i.e. the "upper branch") with little loss even at unitarity over the duration of the experiment. We show that because of bosonic enhancement, pair formation is shifted to the atomic side of the original resonance (where scattering length $a_s<0$), opposite to the fermionic case. In a trap, a repulsive Bose gas remains mechanically stable when brought across resonance to the atomic side until it reaches a critical scattering length $a_{s}^{\ast}<0$. For $a_s<a_{s}^{\ast}$, the density consists of a core of upper branch bosons surrounded by an outer layer of equilibrium phase. The conditions of low three-body recombination requires that the particle number $N<\alpha (T/\omega)^{5/2}$ in a harmonic trap with frequency $\omega$, where $\alpha$ is a constant.
5. arXiv:1201.2066 [pdf, ps, other]
Mott transition of fermionic mixtures with mass imbalance in optical lattices
T.-L. Dao, M. Ferrero, P. S. Cornaglia, M. Capone
We investigate the effect of mass imbalance in binary Fermi mixtures loaded in optical lattices. Using dynamical mean-field theory, we study the transition from a fluid to a Mott insulator driven by the repulsive interactions. For almost every value of the parameters we find that the light species with smaller bare mass is more affected by correlations than the heavy one, so that their effective masses become closer than their bare masses before a Mott transition occurs. The strength of the critical repulsion decreases monotonically as the mass imbalance grows so that the minimum is realized when one of the species is localized. The evolution of the spectral functions testifies that a continuous loss of coherence and a destruction of the Fermi liquid occur as the imbalance grows. The two species display distinct properties and experimentally-observable deviations from the behavior of a balanced Fermi mixture.
6. arXiv:1201.1932 [pdf, ps, other]
A quantum phase transition in a quantum external field: The formation of a Schroedinger magnet
Marek M. Rams, Michael Zwolak, Bogdan Damski
We study an Ising lattice undergoing a quantum phase transition in a quantum magnetic field. Such a field can be emulated by coupling the lattice to a central spin initially in a superposition state. We show that - by adiabatically driving such a system - one can prepare a quantum superposition of any two ground states of the Ising lattice. In particular, one can end up with the Ising lattice in a superposition of ferromagnetic and paramagnetic phases - a scenario with no analogue in prior studies of quantum phase transitions. Remarkably, the resulting magnetization of the lattice encodes the position of the critical point and universal critical exponents, as well as the ground state fidelity.
Jan 10
1. arXiv:1201.1885 [pdf, other]
Exact time evolution of space- and time-dependent correlation functions after an interaction quench in the 1D Bose gas
Jorn Mossel, Jean-Sébastien Caux
We consider the non-equilibrium dynamics of the interacting Lieb-Liniger gas after instantaneously switching the interactions off. The subsequent time evolution of the space- and time-dependent correlation functions is computed exactly. Different relaxation behavior is observed for different correlation functions. The long time average is compared with the predictions of several statistical ensembles. The generalized Gibbs ensemble restricted to a fixed number of particles is shown to give correct results at large times for all length scales.
2. arXiv:1201.1607 [pdf, ps, other]
Spin-orbit coupled Fermi liquid theory with magnetic dipolar interaction
Yi Li, Congjun Wu
We investigate the Fermi liquid properties of the ultra-cold magnetic dipolar Fermi gases in the simplest case of two-component. The magnetic dipolar interaction is invariant under the simultaneous spin-orbit rotation, but not under either spin or orbit rotation separately, thus the corresponding Fermi liquid theory is intrinsically spin-orbit coupled. The Landau interaction matrix is diagonalized in terms of the partial-wave channels of the total angular momentum $J$. The leading thermodynamic instabilities lie in the channels of ferromagnetism hybridized with the ferro-nematic order with $J=1^+$ and the spin-current mode with $J=1^-$, where $\pm$ represents even and odd parities, respectively. An exotic propagating collective mode is identified as spin-orbit coupled Fermi surface oscillations in which the spin distribution on the Fermi surface is topologically non-trivial.
3. arXiv:1201.1779 [pdf, ps, other]
Manipulating dipolar and spin-exchange interactions in spin-1 Bose-Einstein condensates
Bo-Yuan Ning, S. Yi, Jun Zhuang, J. Q. You, Wenxian Zhang
Effects of dipolar and spin-exchange interactions are entangled in spin-1 Bose-Einstein condensates, due to their coexistence. We propose to independently manipulate the magnetic dipolar and the spin-exchange interactions by applying generalized WAHUHA sequences of rf pulses and by appllying periodic dynamical decoupling sequences of optical Feshbach resonance pulses, respectively. While suppressing one interaction, we can make the other interaction dominate the spin dynamics in the condensates. Furthermore, by suppressing both interactions, this method can be harnessed to realize spinor-condensate-based magnetometers with a higher sensitivity.
Jan 9
1. arXiv:1201.1479 [pdf, other]
Bloch-Zener oscillations across a merging transition of Dirac points
Lih-King Lim, Jean-Noël Fuchs, Gilles Montambaux
Bloch oscillations are a powerful tool to investigate spectra with Dirac points. By varying band parameters, Dirac points can be manipulated and merged at a topological transition towards a gapped phase. Under a constant force, a Fermi sea initially in the lower band performs Bloch oscillations and may Zener tunnel to the upper band mostly at the location of the Dirac points. The tunneling probability is computed from the low energy universal Hamiltonian describing the vicinity of the merging. The agreement with a recent experiment on cold atoms in an optical lattice is very good.
2. arXiv:1201.1471 [pdf, other]
Half-quantum vortex state in a spin-orbit coupled Bose-Einstein condensate
B. Ramachandhran, Bogdan Opanchuk, Xia-Ji Liu, Han Pu, Peter D. Drummond, Hui Hu
We investigate theoretically the condensate state and collective excitations of a two-component Bose gas in two-dimensional harmonic traps subject to isotropic Rashba spin-orbit coupling. In the weakly interacting regime when the inter-species interaction is larger than the intra-species interaction ($g_{\uparrow\downarrow}>g$), we find that the condensate ground state has a half-quantum-angular-momentum vortex configuration with spatial rotational symmetry and skyrmion-type spin texture. Upon increasing the interatomic interaction beyond a threshold $g_{c}$, the ground state starts to involve higher-order angular momentum components and thus breaks the rotational symmetry. In the case of $g_{\uparrow\downarrow}<g$, the condensate becomes unstable towards the superposition of two degenerate half-quantum vortex states. Both instabilities (at $g>g_{c}$ and $g_{\uparrow\downarrow}<g$) can be determined by solving the Bogoliubov equations for collective density oscillations of the half-quantum vortex state, and by analyzing the softening of mode frequencies. We present the phase diagram as functions of the interatomic interactions and the spin-orbit coupling. In addition, we directly simulate the time-dependent Gross-Pitaevskii equation to examine the dynamical properties of the system. Finally, we investigate the stability of the half-quantum vortex state against both the trap anisotropy and anisotropy in the spin-orbit coupling term.
Jan 2 - Jan 6, Saubhik Sarkar
Jan 6
1. arXiv:1201.1176 [pdf, ps, other]
Origin of the Three-body Parameter Universality in Efimov Physics
Jia Wang, J. P. D'Incao, B. D. Esry, Chris H. GreeneIn recent years extensive theoretical and experimental studies of universal few-body physics have led to advances in our understanding of universal Efimov physics [1]. The Efimov effect, once considered a mysterious and esoteric effect, is today a reality that many experiments in ultracold quantum gases have successfully observed and continued to explore [2-14]. Whereas theory was the driving force behind our understanding of Efimov physics for decades, recent experiments have contributed an unexpected discovery. Specifically, measurements have found that the so-called three-body parameter determining several properties of the system is universal, even though fundamental assumptions in the theory of the Efimov effect suggest that it should be a variable property that depends on the precise details of the short-range two- and three-body interactions. The present Letter resolves this apparent contradiction by elucidating unanticipated implications of the two-body interactions. Our study shows that the three-body parameter universality emerges because a universal effective barrier in the three-body potentials prevents the three particles from simultaneously getting close to each other. Our results also show limitations on this universality, as it is more likely to occur for neutral atoms and less likely to extend to light nuclei.
2. arXiv:1201.1055 [pdf, other]
Superdiffusive nonequilibrium motion of an impurity in a Fermi sea
Hyungwon Kim, David A. HuseWe treat the nonequilibrium motion of a single impurity atom in a low-temperature single-species Fermi sea, interacting via a contact interaction. In the nonequilibrium regime, the impurity does a superdiffusive geometric random walk where the typical distance traveled grows with time as $\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\geq 2$. For nonzero temperature $T$, this crosses over to diffusive motion at long times with diffusivity $D\sim T^{-(d-1)/2}$. These results apply also to a nonzero concentration of impurity atoms as long as they remain dilute and nondegenerate.
3. arXiv:1201.1008 [pdf, other]
Preparation and spectroscopy of a metastable Mott insulator state with attractive interactions
Manfred J. Mark, Elmar Haller, Katharina Lauber, Johann G. Danzl, Alexander Janisch, Hans Peter Büchler, Andrew J. Daley, Hanns-Christoph NägerlWe prepare and study a metastable attractive Mott insulator state formed with bosonic atoms in a three-dimensional optical lattice. Starting from a Mott insulator with Cs atoms at weak repulsive interactions, we use a magnetic Feshbach resonance to tune the interactions to large attractive values and produce a metastable state pinned by attractive interactions with a lifetime on the order of 10 seconds. We probe the (de-)excitation spectrum via lattice modulation spectroscopy, measuring the interaction dependence of two- and three-body bound state energies. As a result of increased on-site three-body loss we observe resonance broadening and suppression of tunneling processes that produce three-body occupation.
Jan 5
1.arXiv:1201.0956 [pdf, ps, other]
Quasi 1D Bose-Einstein condensate flow past a nonlinear barrier
F. Kh. Abdullaev, R. M. Galimzyanov, Kh. N. IsmatullaevThe problem of flow of a quasi 1D repulsive BEC past through a nonlinear barrier is investigated. Steady state solutions for BEC moving through this barrier have been found. Critical velocities are calculated for the broad barrier using the hydrodynamical approach. For the case of short range nonlinear barrier below a critical velocity there exist two steady solutions, stable and unstable ones. An unstable solution is shown to decay into a gray soliton moving with the velocity less than the flow velocity and the stable solution plus some radiation. Steady flow profiles are obtained in an analytic form.
2.arXiv:1201.0980 [pdf, other]
Supersolid phases of cold atom assemblies
Massimo BoninsegniWe review recent theoretical results for soft-core Bose systems, and describe the low-temperature supersolid "droplet crystal" phase, predicted for a broad class of soft-core interactions. We identify the conditions on the inter-particle interaction that render such intriguing phase possible, and outline proposals for its observation. We argue this to be the prototypical supersolid, at least in the context of assemblies of ultracold atoms.
Jan 4
1. arXiv:1201.0718 [pdf, other]
Novel techniques to cool and rotate Bose-Einstein condensates in time-averaged adiabatic potentials
M. Gildemeister, B. E. Sherlock, C. J. FootWe report two novel techniques for cooling and rotating Bose-Einstein condensates in a dilute rubidium vapour that highlight the control and versatility afforded over cold atom systems by time-averaged adiabatic potentials (TAAPs). The intrinsic loss channel of the TAAP has been successfully employed to evaporatively cool a sample of trapped atoms to quantum degeneracy. The speed and efficiency of this process compares well with that of conventional forced rf-evaporation. In an independent experiment, we imparted angular momentum to a cloud of atoms forming a Bose-Einstein condensate by introducing a rotating elliptical deformation to the TAAP geometry. Triangular lattices of up to 60 vortices were created. All findings reported herein result from straightforward adjustments of the magnetic fields that give rise to the TAAP.
2. arXiv:1201.0718 [pdf, other]
Boson and fermion dynamics in quasi-one-dimensional flat band lattices
M. Hyrkäs, V. Apaja, M. ManninenThe difference between boson and fermion dynamics in quasi-one-dimensional lattices is studied with exact simulations of particle motion and by calculating the persistent current in small quantum rings. We consider three different lattices which in the tight binding model exhibit flat bands. The physical realization is considered to be an optical lattice with bosonic or fermionic atoms. The atoms are assumed to interact with a repulsive short range interaction. The different statistics of bosons and fermions causes different dynamics. Spinless fermions are easily trapped in the flat band states due to the Pauli exclusion principle, which prevents them from interacting, while boson are able to push each other out from the flat band states.
3. arXiv:1201.0444 [pdf, ps, other]
Statistical physics of Bose-condensed light in a dye microcavity
Jan Klaers, Julian Schmitt, Tobias Damm, Frank Vewinger, Martin Weitz We theoretically analyze the temperature behavior of paraxial light in thermal equilibrium with a dye-filled optical microcavity. At low temperatures the photon gas undergoes Bose-Einstein condensation (BEC), and the photon number in the cavity ground state becomes macroscopic with respect to the total photon number. Owing to a grandcanonical excitation exchange between the photon gas and the dye molecule reservoir, a regime with unusually large fluctuations of the condensate number is predicted for this system that is not observed in present atomic physics BEC experiments.
4. arXiv:1201.0177 [pdf, ps, other]
Who is the Lord of the Rings: Majorana, Dirac or Lifshitz? The Spin-Orbit-Zeeman Saga in Ultra-cold Fermions
Kangjun Seo, Li Han, C. A. R. Sá de Melo We discuss the emergence of rings of zero-energy excitations in momentum space for superfluid phases of ultra-cold fermions when spin-orbit, Zeeman fields and interactions are varied. We show that phases containing rings of nodes possess non-trivial topological invariants, and that phase transitions between distinct topological phases belong to the Lifshitz class. Upon crossing phase boundaries, existing massless Dirac fermions in the gapless phase anihilate to produce bulk zero-mode Majorana fermions at phase boundaries and then become massive Dirac fermions in the gapped phase. We characterize these tunable topological phase transitions via several spectroscopic properties, including excitation spectrum, spectral function and momentum distribution. Since the emergence or disappearance of rings leads to topological transitions in momentum space, we conclude that Lifshitz is the lord of the rings.
5. arXiv:1112.6136 (cross-list from physics.optics) [pdf, ps, other]
Controlled manipulation of light by cooperative response of atoms in an optical lattice
Stewart D. Jenkins, Janne Ruostekoski
We show that a cooperative atom response in an optical lattice to resonant incident light can be employed for precise control and manipulation of light on a subwavelength scale. Specific collective excitation modes of the system that result from strong light-mediated dipole-dipole interactions can be addressed by tailoring the spatial phase-profile of the incident light. We demonstrate how the collective response can be used to produce optical excitations at well-isolated sites on the lattice.
