Sep 23-Sep 27, Jin-Long Yu
Sep 27
1. arXiv:1309.7027 [pdf, other]
Non-equilibrium Functional Renormalization for Driven-Dissipative Bose-Einstein Condensation
L. M. Sieberer, S. D. Huber, E. Altman, S. Diehl
We present a comprehensive analysis of critical behavior in the driven-dissipative Bose condensation transition in three spatial dimensions. Starting point is a microscopic description of the system in terms of a many-body quantum master equation, where coherent and driven-dissipative dynamics occur on an equal footing. An equivalent Keldysh real time functional integral reformulation opens up the problem to a practical evaluation using the tools of quantum field theory. In particular, we develop a functional renormalization group approach to quantitatively explore the universality class of this stationary non-equilibrium system. Key results comprise the emergence of an asymptotic thermalization of the distribution function, while manifest non-equilibrium properties are witnessed in the response properties in terms of a new, independent critical exponent. Thus the driven-dissipative microscopic nature is seen to bear observable consequences on the largest length scales. The absence of two symmetries present in closed equilibrium systems - underlying particle number conservation and detailed balance, respectively - is identified as the root of this new non-equilibrium critical behavior. Our results are relevant for broad ranges of open quantum systems on the interface of quantum optics and many-body physics, from exciton-polariton condensates to cold atomic gases.
2. arXiv:1309.7012 [pdf, ps, other]
Vortex Ring Dynamics in Trapped Bose-Einstein Condensates
Matthew D. Reichl, Erich J. Mueller
We use the time-dependent Gross-Pitaevskii equation to study the motion of a vortex ring produced by phase imprinting on an elongated cloud of cold atoms. Our approach models the experiments of Yefsah et. al. [Nature 499, 426] on 6Li in the BEC regime where the fermions are tightly bound into bosonic dimers. We find ring oscillation periods which are much larger than the period of the axial harmonic trap. Our results lend further strength to Bulgac et. al.'s arguments [arXiv: 1306.4266] that the "heavy solitons" seen in those experiments are actually vortex rings. We numerically calculate the periods of oscillation for the vortex rings as a function of interaction strength, trap aspect ratio, and minimum vortex ring radius.
3. arXiv:1309.6922 [pdf, ps, other]
Two-body and Three-body Contacts for Identical Bosons near Unitarity
D. Hudson Smith, Eric Braaten, Daekyoung Kang, Lucas Platter
In a recent experiment with ultracold trapped Rb-85 atoms, Makotyn et al. have studied a quantum-degenerate Bose gas in the unitary limit where its scattering length is infinitely large. We show that the observed momentum distributions are compatible with a universal relation that expresses the high-momentum tail in terms of the 2-body and the 3-body contacts. We determine the 2- and 3-body contact densities for the unitary Bose gas with number density n to be approximately 20n^{4/3} and 2n^{5/3}, respectively. We also show that the observed atom loss rate is compatible with that from 3-atom inelastic collisions, which gives a contribution proportional to the 3-body contact, but the loss rate is not compatible with that from 2-atom inelastic collisions, which gives a contribution proportional to the 2-body contact. We point out that the contacts could be measured independently by using the virial theorem near and at unitarity.
4. arXiv:1309.6746 [pdf, other]
Transition states and thermal collapse of dipolar Bose-Einstein condensates
Andrej Junginger, Manuel Kreibich, Jörg Main, Günter Wunner
We investigate thermally excited, dipolar Bose-Einstein condensates. Quasi-particle excitations of the atomic cloud cause density fluctuations which can induce the collapse of the condensate if the inter-particle interaction is attractive. Within a variational approach, we identify the collectively excited stationary states of the gas which form transition states on the way to the BEC's collapse. We analyze transition states with different $m$-fold rotational symmetry and identify the one which mediates the collapse. The latter's symmetry depends on the trap aspect ratio of the external trapping potential which determines the shape of the BEC. Moreover, we present the collapse dynamics of the BEC and calculate the corresponding decay rate using transition state theory. We observe that the thermally induced collapse mechanism is important near the critical scattering length, where the lifetime of the condensate can be significantly reduced. Our results are valid for an arbitrary strength of the dipole-dipole interaction. Specific applications are discussed for the elements $^{52}$Cr, $^{164}$Dy and $^{168}$Er with which dipolar BECs have been experimentally realized.
Sep 26
1. arXiv:1309.6573 [pdf, other]
Pairing effects in the non-degenerate limit of the two-dimensional Fermi gas
Marcus Barth, Johannes Hofmann
The spectral function of a spin-balanced two-dimensional Fermi gas with short-range interactions is calculated by means of a quantum cluster expansion. Good qualitative agreement is found with a recent experiment by [M. Feld et al., Nature 480, 75 (2011)]. The effects of pairing are clearly visible in the density of states, which displays a pseudogap structure due to the formation of a two-body bound state. In addition, the momentum distribution and the radiofrequency spectrum are derived, which are in excellent agreement with exact universal results. It is demonstrated that in the limit of high temperature, quasiparticle excitations are well-defined, allowing for a kinetic description of the gas.
2. arXiv:1309.6436 [pdf, ps, other]
Dissipative preparation of phase- and number-squeezed states with ultracold atoms
Roland Cristopher F. Caballar, Sebastian Diehl, Harri Mäkelä, Markus Oberthaler, Gentaro Watanabe
We develop a dissipative quantum state preparation scheme for the creation of phase- and number-squeezed states. It utilizes ultracold atoms in a double-well configuration immersed in a background BEC acting as a dissipative quantum reservoir. We derive a master equation starting from microscopic physics, and show that squeezing develops on a time scale proportional to 1/N, where $N$ is the number of particles in the double well. This scaling, caused by bosonic enhancement, allows us to make the time scale for the creation of squeezed states very short. The lifetime of squeezed states is limited by dephasing arising from the intrinsic structure of the setup. However, the dephasing can be avoided by stroboscopically switching the driving off and on. We show that this approach leads to robust stationary squeezed states. Finally, we provide the necessary ingredients for a potential experimental implementation by specifying a parameter regime for rubidium atoms that leads to squeezed states.
Sep 25
1. arXiv:1309.6218 [pdf, other]
Quantum turbulence by vortex stirring in a spinor Bose-Einstein condensate
B. Villaseñor, R. Zamora-Zamora, D. Bernal, V. Romero-Rochín
We introduce a novel mechanism to develop a turbulent flow in a spinor Bose-Einstein condensate, consisting in the stirring of a single line vortex by means of an external magnetic field. We find that density and velocity fluctuations have white-noise power spectra at large frequencies and that Kolmogorov 5/3 law is obeyed in the turbulent region. As the stirring is turned off, the flow decays to an agitated non-equilibrium state that shows an energy bottleneck crossover at small length scales. We demonstrate our findings by numerically solving two-state spinor coupled 3D Gross-Pitaevskii equations. We suggest that this mechanism may be experimentally implemented in spinor ultracold gases confined by optical traps.
2. arXiv:1309.6205 [pdf, other]
Finite temperature dynamics of vortices in Bose-Einstein condensates
S. Gautam, Arko Roy, Subroto Mukerjee
We study the dynamics of a single and a pair of vortices in quasi two-dimensional Bose-Einstein condensates at finite temperatures. We use the stochastic Gross-Pitaevskii equation, which is the Langevin equation for the Bose-Einstein condensate, to this end. For a pair of vortices, we study the dynamics of both the vortex-vortex and vortex-antivortex pairs, which are generated by rotating the trap and moving the Gaussian obstacle potential, respectively. Due to thermal fluctuations, the constituent vortices are not symmetrically generated with respect to each other at finite temperatures. This initial asymmetry coupled with the presence of random thermal fluctuations in the system can lead to different decay rates for the component vortices of the pair, especially in the case of two corotating vortices.
3. arXiv:1309.6107 [pdf, ps, other]
Dynamics of the modified Kibble-Żurek mechanism in antiferromagnetic spin-1 condensates
Emilia Witkowska, Jacek Dziarmaga, Tomasz Świsłocki, Michał Matuszewski
We investigate the dynamics and outcome of a quantum phase transition from an antiferromagnetic to phase separated ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We explicitly demonstrate double universality in dynamics within experiments with various quench time. Furthermore, we show that spin domains created in the nonequilibrium transition constitute a set of mutually incoherent quasicondensates. The quasicondensates appear to be positioned in a semi-regular fashion, which is a result of the conservation of local magnetization during the post-selection dynamics.
Sep 24
1. arXiv:1309.5683 [pdf, other]
Fate of Topology in Spin-1 Spinor Bose-Einstein Condensate
Yun-Tak Oh, Panjin Kim, Jin-Hong Park, Jung Hoon Han
One of the excitements generated by the cold atom systems is the possibility to realize, and explore, varied topological phases stemming from multi-component nature of the condensate. Popular examples are the antiferromagnetic (AFM) and the ferromagnetic (FM) phases in the three-component atomic condensate with effective spin-1, to which different topological manifolds can be assigned. It follows, from consideration of homotopy, that different sorts of topological defects will be stable in each manifold. For instance, Skyrmionic texture is believed to be a stable topological object in two-dimensional AFM spin-1 condensate. Countering such common perceptions, here we show on the basis of a new wave function decomposition scheme that there is no physical parameter regime wherein the temporal dynamics of spin-1 condensate can be described solely within AFM or FM manifold. Initial state of definite topological number prepared entirely within one particular phase must immediately evolve into a mixed state. Accordingly, the very notion of topology and topological stability within the sub-manifold of AFM or FM become invalid. Numerical simulation reveals the linear Zeeman effect to be an efficient catalyst to extract the alternate component from an initial topological object prepared entirely within one particular sub-manifold, serving as a potential new tool for "topology engineering" in multi-component Bose-Einstein condensates.
2. arXiv:1309.5635 [pdf, other]
Universal Conductivity in a Two-dimensional Superfluid-to-Insulator Quantum Critical System
Kun Chen, Longxiang Liu, Youjin Deng, Lode Pollet, Nikolay Prokof'ev
We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional $J$-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, $\sigma(\infty)=0.359(4)\sigma_Q$ with $\sigma_Q$ the conductivity quantum. The universal conductivity is the schoolbook example of where the AdS/CFT correspondence from string theory can be tested and made to use. The shape of our $\sigma(i\omega_n)- \sigma(\infty)$ function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particle-like nature of charge transport. We find that the holographic gauge/gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.
3. arXiv:1309.5622 [pdf, other]
Excitation properties and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas
Ryo Hanai, Takashi Kashimura, Ryota Watanabe, Daisuke Inotani, Yoji Ohashi
We investigate single-particle properties of a mass-imbalanced Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. In the presence of mass imbalance, we point out that the ordinary $T$-matrix approximation, which has been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically gives a double-valued solution in terms of the superfluid phase transition temperature $T_{\rm c}$ in the crossover region. To overcome this serious problem, we include higher order strong-coupling corrections beyond the $T$-matrix level. Using this extended $T$-matrix theory, we calculate single-particle excitations in the normal state above $T_{\rm c}$. The so-called pseudogap phenomena originating from pairing fluctuations are shown to be different between the light mass component and heavy mass component, which becomes more remarkable at higher temperatures. Since Fermi condensates with hetero-Cooper pairs have recently been discussed in various fields, such as exciton (polariton) condensates, as well as color superconductivity, our results would be useful for the further development of Fermi superfluid physics, beyond the conventional superfluid state with homo-Cooper pairs.
4. arXiv:1309.5935 [pdf, other]
Self-interference of a toroidal Bose-Einstein condensate
L. A. Toikka
The self-interference of a toroidal Bose-Einstein condensate at T = 0 is considered. We start by explaining the observed features in a recent proposal for creating ring dark solitons [L. A. Toikka, O. K\"{a}rki, K.-A. Suominen arXiv:1309.0732] using the method of images. The Wigner function is calculated to study the nonclassical features in more detail, and finally, we demonstrate the self-interference effects due to the toroidal geometry alone - another example of the quantum wave-nature of matter.
Sep 23
1. arXiv:1309.5330 [pdf, other]
Efficient Continuous-Duty Bitter-Type Electromagnets for Cold Atom Experiments
Dylan Sabulsky, Colin V. Parker, Nathan D. Gemelke, Cheng Chin
We present the design, construction and characterization of Bitter-type electromagnets which can generate high magnetic fields under continuous operation with efficient heat removal for cold atom experiments. The electromagnets are constructed from a stack of alternating layers consisting of copper arcs and insulating polyester spacers. Efficient cooling of the copper is achieved via parallel rectangular water cooling channels between copper layers with low resistance to flow; a high ratio of the water-cooled surface area to the volume of copper ensures a short length scale ~1 mm to extract dissipated heat. High copper fraction per layer ensures high magnetic field generated per unit energy dissipated. The ensemble is highly scalable and compressed to create a watertight seal without epoxy. From our measurements, a peak field of 770 G is generated 14 mm away from a single electromagnet with a current of 400 A and a total power dissipation of 1.6 kW. With cooling water flowing at 3.8 l/min, the coil temperature only increases by 7 degrees Celsius under continuous operation.
2. arXiv:1309.5121 [pdf, other]
Quantum dynamics with an ensemble of Hamiltonians
Armin Rahmani
We review recent progress in the nonequilibrium dynamics of thermally isolated many-body quantum systems, evolving with an ensemble of Hamiltonians as opposed to deterministic evolution with a single time-dependent Hamiltonian. Such questions arise in (i) quantum dynamics of disordered systems, where different realizations of disorder give rise to an ensemble of real-time quantum evolutions. (ii) quantum evolution with noisy Hamiltonians (temporal disorder), which leads to stochastic Schrodinger equations, and, (iii) in the broader context of quantum optimal control, where one needs to analyze an ensemble of permissible protocols in order to find one that optimizes a given figure of merit. The theme of ensemble quantum evolution appears in several emerging new directions in noneqilibrium quantum dynamics of thermally isolated many-body systems, which include many-body localization, noise-driven systems, and shortcuts to adiabaticity.
3. arXiv:1309.5335 [pdf, other]
Landau Levels in Lattices with Long Range Hopping
Hakan Atakişi, M.Ö. Oktel
In the presence of a periodic potential Landau levels (LLs) are broadened, forming a barrier for accurate simulation of fractional quantum Hall effect using cold atoms in optical lattices. Recently, it has been shown that the degeneracy of the lowest Landau level (LLL) can be restored in a tight binding lattice, if a particular form of long range hopping is introduced [E. Kapit and E. J. Mueller, Phys. Rev. Lett. 105, 215303 (2010)]. In this paper, we investigate three problems related to such quantum Hall parent Hamiltonians in lattices. First, we show that there are infinitely many long range hopping models in which a massively degenerate manifold is formed by lattice discretizations of wavefunctions in the continuum LLL. We then give a general method to construct such models, which is applicable to not only the LLL but also higher LLs. We use this method to give an analytic expression for the hoppings that restore the LLL, and an integral expression for the next LL. We also consider whether the space spanned by discretized LL wavefunctions is as large as the space spanned by continuum wavefunctions and find the constraints on magnetic field for this condition to be satisfied. Finally, using these constraints and first Chern numbers, we identify the bands of the Hofstadter butterfly that correspond to continuum LLs.
Sep 16-Sep 20,Stephan Langer
Sep 20
1. arXiv:1309.4963 [pdf, ps, other]
Tensor renormalization group study of classical XY model on the square lattice
J. F. Yu, Z. Y. Xie, Y. Meurice, Yuzhi Liu, A. Denbleyker, Haiyuan Zou, M. P. Qin, J. Chen, T. Xiang
Using the tensor renormalization group method based on the higher-order singular value decom- position, we have studied the thermodynamic properties of the continuous XY model on the square lattice. The temperature dependence of the free energy, the internal energy and the specific heat agree with the Monte Carlo calculations. From the field dependence of the magnetic susceptibility, we find the Kosterlitz-Thouless transition temperature to be 0.8921 \pm 0.0019, consistent with the Monte Carlo as well as the high temperature series expansion results. At the transition temperature, the critical exponent \delta is estimated as 14.5, close to the analytic value by Kosterlitz.
2. arXiv:1309.4880 [pdf, other]
Markov chains for tensor network states
S. Iblisdir
Markov chains for probability distributions related to matrix product states and 1D Hamiltonians are introduced. With appropriate 'inverse temperature' schedules, these chains can be combined into a random approximation scheme for ground states of such Hamiltonians. Numerical experiments suggest that a linear, i.e. fast, schedule is possible in non-trivial cases. A natural extension of these chains to 2D settings is next presented and tested. The obtained results compare well with Euclidean evolution. The proposed Markov chains are easy to implement and are inherently sign problem free (even for fermionic degrees of freedom).
3. arXiv:1309.5000 [pdf, other]
Optimally focused cold atom systems obtained using density-density correlations
Andika Putra, Daniel L. Campbell, Ryan M. Price, Subhadeep De, I. B. Spielman
Resonant absorption imaging is a common technique for detecting the two-dimensional column density of ultracold atom systems. In many cases, the system's thickness along the imaging direction greatly exceeds the imaging system's depth of field, making the identification of the optimally focused configuration difficult. Here we describe a systematic technique for bringing Bose-Einstein condensates (BEC) and other cold-atom systems into an optimal focus even when the ratio of the thickness to the depth of field is large: a factor of 8 in this demonstration with a BEC.
Sep 19
Sep 9-Sep 13, Bo Liu
Sep 13
1. arXiv:1309.3224 [pdf, ps, other]
Nambu-Goldstone Modes in Segregated Bose-Einstein Condensates
Hiromitsu Takeuchi, Kenichi Kasamatsu
Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates are studied theoretically. In a uniform system, a flat domain wall is stabilized and then the translational invariance normal to the wall is spontaneously broken in addition to the breaking of two U(1) symmetries in the presence of two complex order parameters. We clarify properties of the low-energy excitations and identify that there exist two Nambu-Goldstone modes: in-phase phonon with a linear dispersion and ripplon with a fractional dispersion. The signature of the characteristic dispersion can be verified in segregated condensates in a harmonic potential.
2.arXiv:1309.2941 [pdf, other]
The dynamics of quantum criticality: Quantum Monte Carlo and holography
William Witczak-Krempa, Erik Sorensen, Subir Sachdev
Understanding the real time dynamics of systems near quantum critical points at non-zero temperatures constitutes an important yet challenging problem, especially in two spatial dimensions where interactions are strong. We present detailed quantum Monte Carlo results for two separate realizations of the superfluid-insulator transition of bosons on a lattice: their low-frequency conductivities are found to have the same universal dependence on imaginary frequency and temperature. We then use the structure of the real time dynamics of conformal field theories described by the holographic gauge/gravity duality to make progress on the difficult problem of analytically continuing the Monte Carlo data to real time. Our method yields quantitative and experimentally testable results on the frequency-dependent conductivity at the quantum critical point, and on the spectrum of quasinormal modes in the vicinity of the superfluid-insulator quantum phase transition. Extensions to other observables and universality classes are discussed.
Sep 12
1. arXiv:1309.2674 [pdf, ps, other]
Artificial gauge potentials induced by evanescent waves
Malgorzata Mochol, Krzysztof Sacha
We show that artificial gauge potentials for ultra-cold atoms can be created by means of evanescent waves. Theoretical description of adiabatic motion of atoms in the presence of an external electromagnetic field involves artificial vector and scalar potentials which are the stronger the larger gradient of the external field amplitude. The evanescent wave possesses a large gradient of the amplitude and a gradient of phase which are the most important features in the generation of the synthetic gauge potentials. We consider an evanescent wave created by a single plane wave as well as by a realistic laser beam.
Sep 11
1. arXiv:1309.2358[pdf, other]
Effect of defects on the phonons and the effective spin-spin interactions of an ultracold Penning trap quantum simulator
M. McAneny, B. Yoshimura, J. K. Freericks
We generalize the analysis of the normal modes for a rotating ionic Coulomb crystal in a Penning trap to allow for inhomogeneities in the system. Our formal developments are completely general, but we choose to examine a crystal of Be+ ions with BeH+ defects to compare with current experimental efforts. We examine the classical phonon modes (both transverse and planar) and we determine the effective spin-spin interactions when the system is driven by an axial spin-dependent optical dipole force. We examine situations with up to approximately 15% defects. We find that most properties are not strongly influenced by the defects, indicating that the presence of a small number of defects will not significantly affect experiments.
Sep 10
1.arXiv:1309.1938 [pdf, other]
Thermometry of cold atoms in optical lattices via artificial gauge fields
Tommaso Roscilde
Artificial gauge fields are a unique way of manipulating the motional state of cold atoms. Here we propose the use of artificial gauge fields -- obtained e.g. via lattice shaking -- to perform primary noise thermometry of cold atoms in optical lattices - not requiring any form of prior calibration. The proposed thermometric scheme relies on fundamental fluctuation-dissipation relations, connecting the global response to the variation of the applied gauge field and the fluctuation of quantities related to the momentum distribution (such as the average kinetic energy or the average current). We demonstrate gauge-field thermometry for several physical situations, including free fermions and strongly interacting bosons. The proposed approach is extremely robust to quantum fluctuations - even in the vicinity of a quantum phase transition - when it relies on the thermal fluctuations of an emerging classical field, associated with the onset of Bose condensation or chiral order
2. arXiv:1309.1925 [pdf, other]
Universal Trimers induced by Spin-Orbit Coupling in Ultracold Fermi Gases
Zhe-Yu Shi, Xiaoling Cui, Hui Zhai
In this letter we address the issue how synthetic spin-orbit (SO) coupling can strongly affect three-body physics in ultracold atomic gases. We consider a system which consists of three fermionic atoms, including two spinless heavy atoms and one spin-1/2 light atom subjected to an isotropic SO coupling. We find that SO coupling can induce universal three-body bound states with negative s-wave scattering length at a smaller mass ratio, where no trimer bound state can exist if in the absence of SO coupling. The energies of these trimers are independent of high-energy cutoff, and therefore they are universal ones. Moreover, the resulting atom-dimer resonance can be effectively controlled by SO coupling strength. Our results can be applied to systems like ${}^6$Li and ${}^{40}$K mixture.
Sep 9
1.arXiv:1309.2036 [pdf, ps, other]
The geometry of the tangent bundle and the relativistic kinetic theory of gases
Olivier Sarbach, Thomas Zannias
This article discusses the relativistic kinetic theory for a simple collisionless gas from a geometric perspective. We start by reviewing the rich geometrical structure of the tangent bundle TM of a given spacetime manifold, including the splitting of the tangent spaces of TM into horizontal and vertical subspaces and the natural metric and symplectic structure it induces on TM. Based on these structures we introduce the Liouville vector field L and a suitable Hamiltonian function H on TM. The Liouville vector field turns out to be the Hamiltonian vector field associated to H. On the other hand, H also defines the mass shells as Lorentzian submanifolds of the tangent bundle. A simple collisionless gas is described by a distribution function on a particular mass shell, satisfying the Liouville equation. Together with the Liouville vector field the distribution function can be thought of as defining a fictitious incompressible fluid on the mass shells, with associated conserved current density. Finally, we discuss the relationship between symmetries of the spacetime manifold and symmetries of the distribution function. Taking advantage of the natural metric on TM, we show that groups of isometries G of the spacetime manifold lift naturally to groups of isometries on the tangent bundle. Motivated by this property, we define a distribution function to be G-invariant whenever it is invariant under the lifted isometries.
As a first application of our formalism we derive the most general spherically symmetric distribution function on any spherically symmetric spacetime and write the Einstein-Liouville equations as effective field equations on the two-dimensional radial manifold. As a second application we derive the most general collisionless distribution function on a Kerr black hole spacetime background.
Sep 2-Sep 6, Xiaopeng Li
Sep 6
1. arXiv:1309.1214 [pdf, ps, other]
Time-Reversal-Symmetry-Broken State in the BCS Formalism for a Multi-Band Superconductor
Brendan J. Wilson, Mukunda P. Das
In three-band BCS superconductors with repulsive interband interactions, frustration between the bands can lead to an inherently complex gap function, arising out of a phase difference between the bands in the range 0 and {\pi}. Since the complex conjugate of this state is also a solution, the ground state is degenerate, and there appears a time-reversal-symmetry-broken state. In this paper we investigate the existence of this state as a function of interband coupling strength and show how a new phase transition appears between the TRSB and conventional BCS states.
2.arXiv:1309.1171 [pdf, other]Three dimensional quantum spin liquid in a hyperhoneycomb iridate model and phase diagram in an infinite-D approximation
Itamar Kimchi, James G. Analytis, Ashvin Vishwanath
We present a possible structure for magnetic insulators with stoichiometry A2IrO3 in which Ir4+ S=1/2 moments form a three dimensional hyperhoneycomb lattice, retaining the honeycomb's three-fold coordination. The 90 degree Ir-O-Ir bonds of the ideal structure are predicted to generate Kitaev type spin exchange. The resulting model is exactly solvable, and exhibits a 3D Z2 quantum spin liquid (QSL) with emergent Majorana fermions and flux loop excitations. The fermions may be gapped or gapless along a nodal contour, depending on a bond-anisotropy parameter. Unlike 2D QSLs, this 3D fractionalized phase is stable to finite temperature, until T_c ~ J_K/100. On including Heisenberg couplings, exact solubility is lost. However, we exploit the fact that the shortest loop on this lattice has a long length ell=10, to construct a $\ell \rightarrow \infty$ approximation - the Bethe lattice. We solve the Kitaev-Hesenberg spin model on this lattice, directly in the thermodynamic limit, using tensor product states and the infinite system time-evolving-block-decimation (iTEBD) algorithm. We compute the phase diagram which includes both magnetically ordered and gapped quantum spin liquid phases. The latter are identified via an entanglement fingerprint.
Sep 5
1. arXiv:1309.1139 [pdf, ps, other]
The roton-assisted chiral p-wave superfluid in a quasi-two-dimensional dipolar Bose-Fermi quantum gas mixture
Ben Kain, Hong Y. Ling
The chiral p-wave (p_x \pm ip_y) superfluid has attracted significant attention in recent years, mainly because its vortex core supports a Majorana fermion which, due to its non-Abelian statistics, can be explored for implementing topological quantum computation. Mixing dipolar bosons with fermions in quasi-two-dimensional (2D) space offers the opportunity to use the roton minimum as a tool for engineering the phonon-induced attractive interaction between fermions. We study, within the Hartree-Fock-Bogoliubov approach, the p-wave superfluid pairings in a quasi-2D dipolar Bose-Fermi mixture. We show that enhancing the induced interaction by lowering the roton minimum can affect the stability property of the mixture as well as the effective mass of the fermions in an important way. We also show that one can tune the system to operate in stable regions where chiral p-wave superfluid pairings can be resonantly enhanced by lowering the energy cost of the phonons near the roton minimum.
2. arXiv:1309.0828 [pdf, ps, other]
Quenching to unitarity: Quantum dynamics in a 3D Bose gas
A. G. Sykes, J. P. Corson, J. P. D'Incao, A. P. Koller, C. H. Greene, A. M. Rey, K. R. A. Hazzard, J. L. Bohn
We study the dynamics of a dilute Bose gas at zero temperature following a sudden quench of the scattering length from a noninteracting Bose condensate to unitarity (infinite scattering length). We apply three complementary approaches to understand the momentum distribution and loss rates. First, using a time-dependent variational ansatz for the many-body state, we calculate the dynamics of the momentum distribution. Second, we demonstrate that, at short times and large momenta compared to those set by the density, the physics can be well understood within a simple, analytic two-body model. We derive a quantitative prediction for the evolution of Tan's contact, which increases linearly at short times. We also study the three-body losses at finite densities. Consistent with experiments, we observe lifetimes which are long compared to the dynamics of large momentum modes.
3. arXiv:1309.0851 (cross-list from quant-ph) [pdf, other]
Pure state thermodynamics with matrix product states
Silvano Garnerone
We extend the formalism of pure state thermodynamics to matrix product states. In pure state thermodynamics finite temperature properties of quantum systems are derived without the need of statistical mechanics ensembles, but instead using typical properties of random pure states. We show that this formalism can be useful from the computational point of view when combined with tensor network algorithms. In particular, a recently introduced Monte Carlo algorithm is considered which samples matrix product states at random for the estimation of finite temperature observables. Here we characterize this algorithm as an $(\epsilon, \delta)$-approximation scheme and we analytically show that sampling one single state is sufficient to obtain a very good estimation of finite temperature expectation values. These results provide a substantial computational improvement with respect to similar algorithms for one-dimensional quantum systems based on uniformly distributed pure states. The analytical calculations are numerically supported simulating finite temperature interacting spin systems of size up to 100 qubits.
4. arXiv:1309.0816 (cross-list from quant-ph) [pdf, other]
Correlations in thermal quantum states
M. Kliesch, C. Gogolin, M. J. Kastoryano, A. Riera, J. EisertCharacterizing how correlations behave in interacting lattice systems is one of the main aims of many-body physics. In this work, we show that for any locally bounded Hamiltonian on a spin lattice, there exists a critical temperature above which correlations in the thermal state between any two observables decay exponentially with the distance separating their supports. Our proof is based on a cluster expansion, which allows to approximate thermal states by matrix-product operators. We obtain a stability theorem for thermal quantum states against distant Hamiltonian perturbations. As a consequence, above the critical temperature, local expectation values can be computed with a cost independent of the size of the system. The stability theorem also provides a definition of temperature as a local quantity.
Sep 4
1. arXiv:1309.0523 [pdf, other]
Formation and detection of a chiral orbital Bose liquid in an optical lattice
Xiaopeng Li, Arun Paramekanti, Andreas Hemmerich, W. Vincent Liu
Recent experiments on $p$-orbital atomic bosons have suggested the emergence of a spectacular ultracold superfluid with staggered orbital currents in optical lattices. This raises fundamental questions like the effects of collective thermal fluctuations, and how to directly observe such chiral order. Here, we show via Monte Carlo simulations that thermal fluctuations destroy this superfluid in an unexpected two-step process, unveiling an intermediate normal phase with spontaneously broken time-reversal symmetry, dubbed "chiral Bose liquid". For integer fillings ($n\geq 2$) in the chiral Mott regime, thermal fluctuations are captured by an effective orbital Ising model, and Onsager's powerful exact solution is adopted to determine the transition from this intermediate liquid to the para-orbital normal phase at high temperature. A suitable lattice quench is designed to convert the staggered angular momentum, previously thought by experts difficult to directly probe, into coherent orbital oscillations, providing a smoking-gun signature of chiral order.
2.arXiv:1309.0514 [pdf, ps, other]String order in dipole-blockaded quantum liquids
Hendrik Weimer
We study the quantum melting of quasi-one-dimensional lattice models in which the dominant energy scale is given by a repulsive dipolar interaction. By constructing an effective low-energy theory, we show that the melting of crystalline phases can occur into two distinct liquid phases, having the same algebraic decay of density-density correlations, but showing a different non-local correlation function expressing string order. We present possible experimental realizations using ultracold atoms and molecules, introducing an implementation based on resonantly driven Rydberg atoms that offers additional benefits compared to a weak admixture of the Rydberg state.
3.arXiv:1309.0507 [pdf, ps, other]Time-reversal symmetry breaking near a superconducting multicritical point in graphene
Bitan Roy, Vladimir Juricic
The competition between a singlet s-wave and a triplet f-wave pairings in graphene is addressed in the framework of an $\epsilon$-expansion near four space-(imaginary)time dimensions. In pristine graphene, these two pairings can be favored by strong onsite and next-nearest-neighbor attractive interactions, respectively. We show when their strengths are comparable, an exotic time-reversal-symmetry breaking f+is superconducting state emerges at low temperatures. Furthermore, strain in graphene, which brings a finite number of states at the zero energy, can be conducive to the formation of such paired state, even for weak pairing interactions. Various critical exponents, universal quantities, and the scaling of the pairing amplitudes in axial magnetic field are reported. Existence of the Majorana states and possibility of realizing various local orders in the vortex phase are discussed.
Sep 3
1. arXiv:1309.0272 [pdf, other]
Pair correlations in the two-dimensional Fermi gas
V. Ngampruetikorn, J. Levinsen, Meera M. Parish
We consider the two-dimensional Fermi gas at finite temperature with attractive short-range interactions. Using the virial expansion, which provides a controlled approach at high temperatures, we determine the spectral function and Tan's contact for the normal state. Our calculated spectra are in qualitative agreement with recent photoemission measurements [B. Feld et al., Nature 480, 75 (2011)], thus suggesting that the observed pairing gap is a feature of the high-temperature gas rather than being evidence of a pseudogap phase just above the superfluid transition temperature. We further argue that the strong pair correlations result from the fact that the crossover to bosonic dimers occurs at weaker interactions than previously assumed.
2. arXiv:1309.0012 (cross-list from hep-ph) [pdf, other]
Temporal Chiral Spiral in Strong Magnetic Fields
Tomoya Hayata, Yoshimasa Hidaka, Arata Yamamoto
Vacuum properties of quantum chromodynamics in strong magnetic and finite electric fields are investigated. We show that when a uniform electric field is instantaneously applied in the parallel direction to a strong magnetic field, it induces temporal oscillation of the chiral and pion condensates. This is a temporal analog to the chiral spiral. The oscillation originates with the propagation of the collective mode, which is protected by the axial anomaly and thus nondissipative.Comments
Sep 2
1.arXiv:1308.6831 [pdf, other]
Lattice construction of pseudopotential Hamiltonians for Fractional Chern Insulators
Ching Hua Lee, Xiao-Liang Qi
Fractional Chern insulators are new realizations of fractional quantum Hall states in lattice systems without orbital magnetic field. These states can be mapped onto conventional fractional quantum Hall states through the Wannier state representation (Phys. Rev. Lett. 107, 126803 (2011)). In this paper, we use the Wannier state representation to construct the pseudopotential Hamiltonians for fractional Chern insulators, which are interaction Hamiltonians with certain ideal model wavefunctions as exact ground states. We show that these pseudopotential Hamiltonians can be approximated by short-ranged interactions in fractional Chern insulators, and that their range will be minimized by an optimal gauge choice for the Wannier states. As illustrative examples, we explicitly write down the form of the lowest pseudopotential for several fractional Chern insulator models including the lattice Dirac model and the checkerboard model with Chern number 1, and the d-wave model and the triangular lattice model with Chern number 2. The proposed pseudopotential Hamiltonians have the 1/3 Laughlin state as their groundstate when the Chern number C=1, and a topological nematic (330) state as their groundstate when C=2. Also included are the results of an interpolation between the d-wave model and two decoupled layers of lattice Dirac models, which explicitly demonstrate the relation between C=2 fractional Chern insulators and bilayer fractional quantum Hall states. The proposed states can be verified by future numerical works, and in particular provide a model Hamiltonian for the topological nematic states that have not been realized numerically.
