July 23 - July 27, Bin Wang
July 27
1. arXiv:1207.6151 [pdf, ps, other]
Theory of topological quantum phase transitions in 3D noncentrosymmetric systems
Bohm-Jung Yang, Mohammad Saeed Bahramy, Ryotaro Arita, Hiroki Isobe, Naoto Nagaosa
We have constructed a general theory for a new class of topological quantum phase transitions in 3D systems with broken inversion symmetry. Although the consideration of the system's codimension predicts the appearance of a stable metallic phase between the normal and topological insulators, when the band touching occurs along the direction with high crystalline symmetry, a direct topological phase transition between two insulators can occur. At the quantum critical point (QCP), the energy dispersion becomes quadratic along one direction while the dispersions along the other two orthogonal directions are linear, which manifests the zero chirality of the band touching point. Due to the anisotropic dispersion at QCP, various thermodynamic and transport properties show unusual temperature dependence and anisotropic behaviors.
2. arXiv:1207.6291 (cross-list from physics.atom-ph) [pdf, other]
Bound lattice complexes through distance selective two-body loss
C. Ates, B. Olmos, W. Li, I. Lesanovsky
We show that in a gas of ultra cold atoms distance selective two-body loss can be engineered via the resonant laser excitation of atom pairs to interacting electronic states. In an optical lattice this leads to a dissipative Master equation dynamics with Lindblad jump operators that annihilate atom pairs with a specific interparticle distance. In conjunction with coherent hopping between lattice sites this unusual dissipation mechanism leads to the formation of coherent long-lived complexes that can even exhibit an internal level structure which is strongly coupled to their external motion. We analyze this counterintuitive phenomenon in detail in a system of hard-core bosons. While current research has established that dissipation in general can lead to the emergence of coherent features in many-body systems our work shows that strong non-local dissipation can effectuate a binding mechanism for particles.
July 25
1.arXiv:1207.5892 [pdf, other]
Zero-energy states in rotating trapped Bose-Einstein condensates
Tapio Simula
We solve the quasiparticle excitation spectrum of a rotating three-dimensional Bose-Einstein condensate whose ground state is pierced by seven singly quantized vortex filaments. We find the elementary excitation spectrum of this system to harbour a quasiparticle with zero excitation energy. This quasiparticle is degenerate with the rotating ground state and may be described as a massless Nambu--Goldstone mode or a Majorana boson. Macroscopic population in such a state corresponds to a Bose--Einstein condensate of Tkachenko vortex wave quanta.
2. arXiv:1207.5903 [pdf, other]
Temporal Quantum Control with Graphene
Alejandro Manjavacas, Sukosin Thongrattanasiri, Darrick E. Chang, F. Javier García de Abajo
We introduce a novel strategy for controlling the temporal evolution of a quantum system at the nanoscale. Our method relies on the use of graphene plasmons, which can be electrically tuned in frequency by external gates. Quantum emitters (e.g., quantum dots) placed in the vicinity of a graphene nanostructure are subject to the strong interaction with the plasmons of this material, thus undergoing time variations in their mutual interaction and quantum evolution that are dictated by the externally applied gating voltages. This scheme opens a new path towards the realization of quantum-optics devices in the robust solid-state environment of graphene.
July 24
1. arXiv:1207.5760 [pdf, ps, other]
Phenomenology of a semi-Dirac semi-Weyl semi-metal
Swapnonil Banerjee, Warren E. Pickett
We extend the study of fermionic particle-hole symmetric semi-Dirac (alternatively, semi-Weyo) dispersion of quasiparticles, $\varepsilon_K = \pm \sqrt{(k_x^2/2m)^2 + (vk_y)^2)} = \pm \varepsilon_0 \sqrt{K_x^4 + K_y^2}$ in dimensionless units, discovered computationally in oxide heterostructures by Pardo and collaborators. This unique system a highly anisotropic sister phase of both (symmetric) graphene and what has become known as a Weyl semimetal, with $<v_y^2>^{1/2} \approx v$ independent of energy, and $<v_x^2>^{1/2} \propto m^{-1/2}\sqrt{\varepsilon}$ being very strongly dependent on energy ($\varepsilon$) and depending only on the effective mass $m$. Each of these systems is distinguished by bands touching (alternatively, crossing) at a point Fermi surface, with one consequence being that for this semi-Dirac system the ratio $|\chi_{orb}/\chi_{sp}|$ of orbital to spin susceptibilities diverges at low doping. We extend the study of the low-energy behavior of the semi-Dirac system, finding the plasmon frequency to be highly anisotropic while the Hall coefficient scales with carrier density in the usual manner. The Faraday rotation behavior is also reported. For Klein tunneling for normal incidence on an arbitrarily oriented barrier, the kinetic energy mixes both linear (massless) and quadratic (massive) contributions depending on orientation. Analogous to graphene, perfect transmission occurs under resonant conditions, except for the specific orientation that eliminates massless dispersion. Comparisons of the semi-Dirac system are made throughout with both other types of point Fermi surface systems.
2. arXiv:1207.5587 [pdf, other]
Pseudopotential Formalism for Fractional Chern Insulators
Ching Hua Lee, Ronny Thomale, Xiao-Liang Qi
Recently, generalizations of fractional quantum Hall (FQH) states known as fractional quantum anomalous Hall or, equivalently, fractional Chern insulators states have been realized in lattice models. Ideal wavefunctions such as the Laughlin wavefunction, as well as the corresponding trial Hamiltonians for which the former are exact ground states, have been vital to characterizing FQH phases. The Wannier function representation of fractional Chern insulators proposed in [X.-L. Qi, Phys. Rev. Lett., 126803] defines an approach to generalize these concepts to fractional Chern insulators. In this paper, we apply the Wannier function representation to develop a systematic pseudopotential formalism for fractional Chern insulators. The family of pseudopotential Hamiltonians is defined as the set of projectors onto asymptotic relative angular momentum components which forms an orthogonal basis of two-body Hamiltonians with magnetic translation symmetry. This approach serves both as an expansion tool for interactions and as a definition of positive semidefinite Hamiltonians for which the ideal fractional Chern insulator wavefunctions are exact nullspace modes. We compare the short-range two-body pseudopotential expansion of various fractional Chern insulator models at one-third filling in phase regimes where a Laughlin-type ground state is expected to be realized. We also discuss the effect of inhomogeneous Berry curvature which leads to components of the Hamiltonian that cannot be expanded into pseudopotentials, and elaborate on their role in determining low energy theories for fractional Chern insulators. Finally, we generalize our Chern pseudopotential approach to interactions involving more than two bodies with the goal of facilitating the identification of non-Abelian fractional Chern insulators.
3. arXiv:1207.5534 [pdf, other]
Nearly flat Andreev bound states of Massless Dirac electrons
Mahmoud Lababidi, Erhai Zhao
Massless Dirac electrons on the surface of a three-dimensional topological insulator (TI) can be turned into exotic excitations by proximity coupling to superconductors. For example, Majorana fermions with a linear dispersion, $E\sim k$, exist in a short $\pi$ Josephson junction on the TI surface. We show that in these systems, the Andreev bound states spectrum turns into a nearly flat band at zero energy when the chemical potential is sufficiently away from the Dirac point. The flat dispersion is well approximated by $E\sim k^N$, where $N$ scales with the chemical potential. Similar evolution from linear to flat dispersion also occurs for the subgap spectrum of a periodic superconducting proximity structure, such as a TI surface in contact with a stripe superconductor.
July 23
1. arXiv:1207.5304 [pdf, ps, other]
Gauge theory of topological phases of matter
Juerg Froehlich, Philipp Werner
We study the response of quantum many-body systems to coupling some of their degrees of freedom to small, slowly varying external gauge fields, which arise when global symmetries are gauged. This analysis leads to a "gauge theory of states of matter" generalizing the well known Landau theory of order parameters. We illustrate the power of our approach by deriving and interpreting the gauge-invariant (local) effective actions of superconductors, 2D electron gases exhibiting the quantized Hall- and spin-Hall effect, 3D topological insulators and axion electrodynamics.
2. arXiv:1207.5234 [pdf, ps, other]
Charge Density Waves and Axion Strings from Weyl Semimetals
Zhong Wang, Shou-Cheng Zhang
We study dynamical instability and chiral symmetry breaking in three dimensional Weyl semimetals, which turns Weyl semimetals into "axion insulators". Charge density waves (CDW) is found to be the natural consequence of the chiral symmetry breaking. The Goldstone mode of this charge density wave state is identified as the axion, which couples to electromagnetic field in the topological $\theta{\bf E}\cdot{\bf B}$ form. Our main finding is that the "axion strings" can be realized as the (screw or edge) dislocations in the charge density wave, which provides a simple physical picture for the elusive axion strings. These axion strings carry gapless chiral modes, therefore they have important implications for dissipationless transport properties of Weyl semimetals with broken symmetry.
3. arXiv:1207.5137 [pdf, ps, other]
Increasing Quantum Degeneracy by Heating a Superfluid
D.J. Papoular, G. Ferrari, L.P. Pitaevskii, S. Stringari
We consider a uniform superfluid confined in two compartments connected by a superleak and initially held at equal temperatures. If one of the two compartments is heated, a fraction of the superfluid will flow through the superleak. We show that, under certain thermodynamic conditions, the atoms flow from the hotter to the colder compartment, contrary to what happens in the fountain effect observed in superfluid Helium. This flow causes quantum degeneracy to increase in the colder compartment. In superfluid Helium, this novel thermomechanical effect takes place in the phonon regime of very low temperatures. In dilute quantum gases, it occurs at all temperatures below Tc . The increase in quantum degeneracy reachable through the adiabatic displacement of the wall separating the two compartments is also discussed.
July 22
1. arXiv:1207.4795 [pdf, other]
Superfluidity of Bosons in Kagome Lattices with Frustration
Yi-Zhuang You, Zhu Chen, Xiao-Qi Sun, Hui Zhai
In this letter we consider spinless bosons in Kagome lattice with nearest-neighbor hopping and on-site interaction, and the sign of hopping is inverted by insetting a {\pi} flux in each triangle of Kagome lattice so that the lowest single particle band is perfectly flat. We show that in the high density limit, despite of the infinite degenerate of the single particle ground state, interaction will select out the Bloch state at the K point of Brillouin zone for boson condensation at the lowest temperature. As temperature increases, we show that the single boson superfluid order can be easily destroyed, while an exotic triple-boson paired superfluid order will remain. This trion superfluid exist in a broad temperature window until the temperature is increased to the same order of hopping and then the system turns into normal phases. Finally we show that time-of-flight measurement of momentum distribution can be used to distinguish these three phases.
2. arXiv:1207.4775 (cross-list from physics.atom-ph) [pdf, other]
Coherent Backscattering of Ultra-cold Atoms
Fred Jendrzejewski (LCF), Kilian Müller (LCF), Jérémie Richard (LCF), Aditya Date (LCF), Thomas Plisson (LCF), Philippe Bouyer (LP2N), Alain Aspect (LCF), Vincent Josse (LCF)
We report on the direct observation of coherent backscattering (CBS) of ultra-cold atoms, in a quasi two dimensional configuration. Launching atoms with a well defined momentum in a laser speckle disordered potential, we follow the progressive build up of the momentum scattering pattern, consisting of a ring associated with multiple elastic scattering, and the CBS peak in the backward direction. Monitoring the depletion of the initial momentum component and the formation of the angular ring profile allow us to determine microscopic transport quantities. The time resolved evolution of the CBS peak is studied and is found a fair agreement with predictions, at long times as well as at short times. The observation of CBS can be considered a direct signature of coherence in quantum transport of particles in disordered media. It is responsible for the so called weak localization phenomenon, which is the precursor of Anderson localization.
July 16 - July 20, Johannes Schachenmayer
July 20
1. arXiv:1207.4777 [pdf, other]
Emergent thermodynamics in a quenched quantum many-body system
Ross Dorner, John Goold, Cecilia Cormick, Mauro Paternostro, Vlatko Vedral
We study the statistics of the work done, the fluctuation relations and the irreversible entropy production in a quantum many-body system subject to the sudden quench of a control parameter. By treating the quench as a thermodynamic transformation we show that the emergence of irreversibility in the nonequilibrium dynamics of closed many-body quantum systems can be accurately characterized. We demonstrate our ideas by considering a transverse quantum Ising model that is taken out of equilibrium by the instantaneous switching of the transverse field.
2. arXiv:1207.4741 [pdf, other]
Steady-state many-body entanglement of hot reactive fermions
Michael Foss-Feig, Andrew J. Daley, James K. Thompson, Ana Maria Rey
Entanglement is typically created via systematic intervention in the time evolution of an initially unentangled state, which can be achieved by coherent control, carefully tailored non-demolition measurements, or dissipation in the presence of properly engineered reservoirs. In this paper we show that two-component Fermi gases at ~\mu K temperatures naturally evolve, in the presence of reactive two-body collisions, into states with highly entangled (Dicke-type) spin wavefunctions. The entanglement is a steady-state property that emerges---without any intervention---from uncorrelated initial states, and could be used to improve the accuracy of spectroscopy in experiments with fermionic alkaline earth atoms or fermionic groundstate molecules.
3. arXiv:1207.4547 [pdf, ps, other]
Topological antiferromagnetic phase in a correlated Bernevig-Hughes-Zhang model
Tsuneya Yoshida, Robert Peters, Satoshi Fujimoto, Norio Kawakami
Topological properties of antiferromagnetic phases are studied for a correlated topological band insulator by applying the dynamical mean field theory to an extended Bernevig-Hughes-Zhang model including the Hubbard interaction. The calculation of the magnetic moment and the spin Chern number confirms the existence of a non-trivial antiferromagnetic (AF) phase beyond the Hartree-Fock theory. In particular, we uncover the intriguing fact that the topologically non-trivial AF phase is essentially stabilized by correlation effects but not by the Hartree shifts alone. This counterintuitive effect is demonstrated, through a comparison with the Hartree-Fock results, and should apply for generic topological insulators with strong correlations.
4. arXiv:1207.4479 [pdf, other]
Topological Flat Bands from Dipolar Spin Systems
Norman Y. Yao, Chris R. Laumann, Alexey V. Gorshkov, Steven D. Bennett, Eugene Demler, Peter Zoller, Mikhail D. Lukin
We propose and analyze a physical system that naturally admits two-dimensional topological nearly flat bands. Our approach utilizes an array of three-level dipoles (effective S = 1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hardcore bosons, namely, the dressed spin-flips. These gauge fields result in topological band structures, whose bandgap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at half-filling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultra-cold polar molecules or spins in the solid state is considered.
Jan 19
1. arXiv:1207.4206 [pdf, ps, other]
Non-Fermi Liquid behavior at the Orbital Ordering Quantum Critical Point in the Two-Orbital Mode
Ka Wai Lo, Wei-Cheng Lee, Philip W. Phillips
The critical behavior of a two-orbital model with degenerate $d_{xz}$ and $d_{yz}$ orbitals is investigated by multidimensional bosonization. We find that the corresponding bosonic theory has an overdamped collective mode with dynamical exponent $z=3$, which appears to be a general feature of a two-orbital model and becomes the dominant fluctuation in the vicinity of the orbital-ordering quantum critical point. Since the very existence of this $z=3$ overdamped collective mode induces non-Fermi liquid behavior near the quantum critical point, we conclude that a two-orbital model generally has a sizable area in the phase diagram showing non-Fermi liquid behavior. Furthermore, we show that the bosonic theory resembles the continuous model near the d-wave Pomeranchuk instability, suggesting that orbital order in a two-orbital model is identical to nematic order in a continuous model. Our results can be applied to systems with degenerate $d_{xz}$ and $d_{yz}$ orbitals such as iron-based superconductors and bilayer strontium ruthenates Sr$_3$Ru$_2$O$_7$.
2. arXiv:1207.4188 [pdf, other]
Renyi Entropy of the Interacting Fermi Liquid
Jeremy McMinis, Norm M. Tubman
We perform quantum Monte Carlo calculations to determine how the Renyi entropies, $S_n$, of the interacting Fermi liquid depend on Renyi order, $n$, and scale as a function of system size, $L$. Using the swap operator and an accurate Slater-Jastrow wave function, we compute Renyi entropies for spinless fermions interacting via the Coulomb and modified P\"{o}schl-Teller potentials across a range of correlation strengths. Our results show that interactions increase the Renyi entropies and increase the prefactor of their scaling laws. The relationships between Renyi entropies of different order $n$ are also modified. Additionally, we investigate the effect of the swap operator on the Fermi liquid wave function to determine the source of the $L\log L$ scaling form.
Jan 18
1. arXiv:1207.3957 [pdf, other]
Entanglement entropy for the long range Ising chain
Thomas Koffel, M. Lewenstein, Luca Tagliacozzo
We consider the Ising model in a transverse field with long-range antiferromagnetic interactions that decay as a power law with their distance. We study both the phase diagram and the entanglement properties as a function of the exponent of the interaction. The phase diagram can be used as a guide for future experiments with trapped ions. We find two gapped phases, one dominated by the transverse field, exhibiting quasi long range order, and one dominated by the long range interaction, with long range N\'eel ordered ground states. We determine the location of the quantum critical points separating those two phases. We determine their critical exponents and central-charges. In the phase with quasi long range order the ground states exhibit exotic corrections to the area law for the entanglement entropy coexisting with gapped entanglement spectra.
July 17
1. arXiv:1207.3777 [pdf, ps, other]
Time-evolution and dynamical phase transitions at a critical time in a system of one dimensional bosons after a quantum quench
Aditi Mitra
The time-evolution of one-dimensional bosons after a simultaneous interaction and commensurate lattice quench is studied using a renormalization group approach. As a consequence of the quench, the effective scaling dimension of the lattice potential is found to be time-dependent which has a crucial effect on the time-evolution of the system. For certain quench protocols a critical time is found at which the lattice potential goes from being irrelevant to relevant in the renormalization group sense leading to a dynamical phase transition where an order-parameter is exactly zero before the critical time, and nonzero after this time. Explicit results are presented for the time-evolution of the renormalized interaction parameter when the lattice potential is irrelevant at all times. When the lattice potential becomes relevant, results are presented for the time-evolution of the gap which may be tested experimentally via quantities such as the optical conductivity.
2. arXiv:1207.3463 [pdf, ps, other]
Topological phase transition induced by random substitution
Stanislav Chadov, Janos Kiss, Claudia Felser, Kristina Chadova, Diemo Ködderitzsch, Jan Minár, Hubert Ebert
The transition from topologically nontrivial to a trivial state is studied by first-principles calculations on bulk zinc-blende type (Hg$_{1-x}$Zn$_x$)(Te$_{1-x}$S$_x$) disordered alloy series. The random chemical disorder was treated by means of the Coherent Potential Approximation. We found that although the phase transition occurs at the strongest disorder regime (${x\approx 0.5}$), it is still manifested by well-defined Bloch states forming a clear Dirac cone at the Fermi energy of the bulk disordered material. The computed residual resistivity tensor confirm the topologically-nontrivial state of the HgTe-rich (${x<0.5}$), and the trivial state of the ZnS-rich alloy series (${x>0.5}$) by exhibiting the quantized behavior of the off-diagonal spin-projected component, independently on the concentration $x$.
July 16
1. arXiv:1207.3224 [pdf, ps, other]
Theory of current-driven motion of Skyrmions and spirals in helical magnets
Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa
We study theoretically the dynamics of the spin textures, i.e., Skyrmion crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets under external current. By numerically solving the Landau-Lifshitz-Gilbert equation, it is found that (i) the critical current density of the motion is much lower for SkX compared with SS in agreement with the recent experiment, (ii) there is no intrinsic pinning effect for SkX and the deformation of the internal structure of Skyrmion reduces the pinning effect dramatically, (iii) the Bragg intensity of SkX shows strong time-dependence as can be observed by neutron scattering experiment.
July 9 - July 13, Saubhik Sarkar
July 13
1. arXiv:1207.3002 [pdf, other]
Damping of phase fluctuations in superfluid Bose gases
Philipp Lange, Peter Kopietz, Andreas Kreisel
Using Popov's hydrodynamic approach we derive an effective Euclidean action for the long-wavelength phase fluctuations of superfluid Bose gases in D dimensions. We then use this action to calculate the damping of phase fluctuations at zero temperature as a function of D. For D >1 and wavevectors | k | << 2 mc (where m is the mass of the bosons and c is the sound velocity) we find that the damping in units of the phonon energy E_k = c | k | is to leading order gamma_k / E_k = A_D (k_0^D / 2 pi rho) (| k | / k_0)^{2 D -2}, where rho is the boson density and k_0 =2 mc is the inverse healing length. For D -> 1 the numerical coefficient A_D vanishes and the damping is proportional to an additional power of |k | /k_0; a self-consistent calculation yields in this case gamma_k / E_k = 1.32 (k_0 / 2 pi rho)^{1/2} |k | / k_0. In one dimension, we also calculate the entire spectral function of phase fluctuations.
2. arXiv:1207.2911 [pdf, ps, other]
Mean-Field Analysis of Spinor Bosons in Optical Superlattices
Andreas Wagner, Andreas Nunnenkamp, Christoph Bruder
We study the ground-state phase diagram of spinless and spin-1 bosons in optical superlattices using a Bose-Hubbard Hamiltonian that includes spin-dependent interactions. We decouple the unit cells of the superlattice via a mean-field approach and take into account the dynamics within the unit cell exactly. The system supports Mott-insulating as well as superfluid phases. The transitions between these phases are second-order for spinless bosons and second- or first-order for spin-1 bosons. Anti-ferromagnetic interactions energetically penalize high-spin configurations and elongate all Mott lobes, especially the ones corresponding to an even atom number on each lattice site. We find that the quadratic Zeeman effect lifts the degeneracy between different polar superfluid phases leading to additional metastable phases and first-order phase transitions. Finally, we show that an energy offset between the two sites of the unit cell induces a staircase of single-atom tunneling resonances which surprisingly survives well into the superfluid regime.
3. arXiv:1207.2856 [pdf, ps, other]
Medium effects close to s- and p-wave Feshbach resonances in atomic Fermi gases
Renyuan Liao, Khandker F. Quader
Many-body effects may influence properties, such as scattering parameters, nature of pairing, etc., close to a Feshbach resonance in the fermion BEC-BCS crossover problem. We study effects such as these using a tractable crossing-symmetric approach. This method allow us to include quantum fluctuations, such as, density, current, spin, spin-current and the higher-order fluctuations in a self-consistent fashion. The underlying fermion interaction is reflected in the "driving" term. We perform calculations here on both Bose-Einstein condensate (BEC) and BCS sides, and taking the driving term to be finite range, and of arbitrary strength. These are related to two-body singlet and triplet scattering parameters, and can be connected with experimental s- and p-wave Feshbach resonances. We include the $\ell=0$ density and spin fluctuations, as well as $\ell=1$ current and spin-current fluctuations. We calculate renormalized scattering amplitudes, pairing amplitudes, nature of pairing, etc., on both the BEC and BCS sides. We then compare our results qualitatively with experiments.
4. arXiv:1207.2821 [pdf, other]
Non-equilibrium dynamics of a driven Bose-Einstein condensate at finite temperature
T. P. Billam, P. Mason, S. A. Gardiner
While the Gross-Pitaevskii equation is well-established as the canonical dynamical description of atomic Bose-Einstein condensates at zero-temperature, describing the dynamics of Bose-Einstein condensates at finite temperatures remains a difficult theoretical problem, particularly when considering low-temperature, non-equilibrium systems in which depletion of the condensate occurs dynamically as a result of external driving. The BEC analog of the quantum delta-kicked rotor is the prototypical example of such a system. In this paper, we describe a fully time-dependent numerical method to propagate the equations of motion of a second-order, number-conserving description; these equations describe the coupled dynamics of the condensate and non-condensate fractions in a self-consistent manner, and correctly capture the phonon-like nature of excitations at low temperature, making them ideal for the study of low-temperature, non-equilibrium, driven systems. We use this numerical method to systematically explore the finite-temperature dynamics of the delta-kicked-rotor-BEC. We demonstrate that several qualitative features of this system at zero temperature are well-preserved at finite temperatures, and predict a finite-temperature shift of resonance frequencies which could be verified by future experiments.
July 12
1. arXiv:1207.2645 [pdf, other]
Dynamically Slowed Collapse of a Bose-Einstein Condensate with Negative Scattering Length
R. L. Compton, Y.-J. Lin, K. Jimenez-Garcia, J. V. Porto, I. B. Spielman
We rapidly change the scattering length a_s of a 87Rb Bose-Einstein condensate by means of a Feshbach resonance, simultaneously releasing the condensate from its harmonic trapping potential. When a_s is changed from positive to negative, the subsequent collapse of the condensate is stabilized by the kinetic energy imparted during the release, resulting in a deceleration of the loss rate near the resonance. We also observe an increase in the Thomas-Fermi radius, near the resonance, that cannot be understood in terms of a simple scaling model. Instead, we describe this behavior using the Gross-Pitaevskii equation, including three-body recombination, and hypothesize that the increase in cloud radius is due to the formation of concentric shells.
2.arXiv:1207.2659 (cross-list from physics.atom-ph) [pdf, other]
Driven-dissipative dynamics of a strongly interacting Rydberg gas
A. W. Glaetzle, R. Nath, B. Zhao, G. Pupillo, P. Zoller
We study the non-equilibrium many-body dynamics of a cold gas of ground state alkali atoms weakly admixed by Rydberg states with laser light. On a timescale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the long-time dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing.
July 11
1. arXiv:1207.2391 [pdf, other]
Large-N approximation for single- and two-component dilute Bose gases
Chih-Chun Chien, Fred Cooper, Eddy Timmermans
We discuss the mean-field theories obtained from the leading order in a large-$N$ approximation for one- and two- component dilute Bose gases. For a one-component Bose gas this approximation has the following properties: the Bose-Einstein condensation (BEC) phase transition is second order but the critical temperature $T_c$ is not shifted from the non-interacting gas value $T_0$. The spectrum of excitations in the BEC phase resembles the Bogoliubov dispersion with the usual coupling constant replaced by the running coupling constant which depends on both temperature and momentum. We then study two-component Bose gases with both inter- and intra- species interactions and focus on the stability of the mixture state above $T_c$. Our mean-field approximation predicts an instability from the mixture state to a phase-separated state when the ratio of the inter-species interaction strength to the intra-species interaction strength (assuming equal strength for both species) exceeds a critical value. At high temperature this is a structural transition and the global translational symmetry is broken. Our work complements previous studies on the instability of the mixture phase in the presence of BEC.
2. arXiv:1207.2187 (cross-list from physics.atom-ph) [pdf, ps, other]
Dynamics of Feshbach Molecules in an Ultracold Three-Component Mixture
A. Y. Khramov, A. H. Hansen, A. O. Jamison, W. H. Dowd, S. Gupta
We present investigations of the formation rate and collisional stability of lithium Feshbach molecules in an ultracold three-component mixture composed of two resonantly interacting fermionic 6-Li spin states and bosonic 174-Yb. We observe long molecule lifetimes (> 100 ms) even in the presence of a large ytterbium bath and extract reaction rate coefficients of the system. We find good collisional stability of the mixture in the unitary regime, opening new possibilities for studies and probes of strongly interacting quantum gases in contact with a bath species.
July 10
1.arXiv:1207.1929 [pdf, other]
The finite temperature trapped dipolar Bose gas
R. N. Bisset, D. Baillie, P. B. Blakie
We develop a finite temperature Hartree theory for the trapped dipolar Bose gas. We use this theory to study thermal effects on the mechanical stability of the system and density oscillating condensate states. We present results for the stability phase diagram as a function of temperature and aspect ratio. In oblate traps above the critical temperature for condensation we find that the Hartree theory predicts significant stability enhancement over the semiclassical result. Below the critical temperature we find that thermal effects are well described by accounting for the thermal depletion of the condensate. Our results also show that density oscillating condensate states occur over a range of interaction strengths that broadens with increasing temperature.
2.arXiv:1207.1763 [pdf, ps, other]
Superfluid properties of one-component Fermi gas with an anisotropic p-wave interaction
Daisuke Inotani, Manfred Sigrist, Yoji Ohashi
We investigate superfluid properties and strong-coupling effects in a one-component Fermi gas with an anisotropic p-wave interaction. Within the framework of the Gaussian fluctuation theory, we determine the superfluid transition temperature $T_{\rm c}$, as well as the temperature $T_0$ at which the phase transition from the $p_x$-wave pairing state to the $p_x+ip_y$-wave state occurs below $T_{\rm c}$. We also show that while the anisotropy of the p-wave interaction enhances $T_{\rm c}$ in the strong-coupling regime, it suppresses $T_0$.
3. arXiv:1207.1728 [pdf, other]
SU(3) Spin-Orbit Coupling in Ultracold Atoms
Ryan Barnett, G. R. Boyd, Victor Galitski
Motivated by the recent experimental success in realizing synthetic spin-orbit coupling in ultracold atomic systems, we consider N-component atoms coupled to a non-Abelian SU(N) gauge field. More specifically, we focus on the case, referred to here as "SU(3) spin-orbit-coupling," where the internal states of three-component atoms are coupled to their momenta via a matrix structure that involves the Gell-Mann matrices (in contrast to the Pauli matrices in conventional SU(2) spin-orbit-coupled systems). It is shown that the SU(3) spin-orbit-coupling gives rise to qualitatively different phenomena and in particular we find that even a homogeneous SU(3) field on a simple square lattice enables a topologically non-trivial state to exist, while such SU(2) systems always have trivial topology. In deriving this result, we first establish an exact equivalence between the Hofstadter model with a 1/N Abelian flux per plaquette and a homogeneous SU(N) non-Abelian model. The former is known to have a topological spectrum for N>2, which is thus inherited by the latter. It is explicitly verified by an exact calculation for N=3, where we develop and use a new algebraic method to calculate topological indices in the SU(3) case. Finally, we consider a strip geometry and establish the existence of three gapless edge states -- the hallmark feature of such an SU(3) topological insulator.
4. arXiv:1207.2006 (cross-list from cond-mat.str-el) [pdf, ps, other]
Dynamics after Interaction Quenches in One-Dimensional Fermionic Systems
Simone A. Hamerla, Götz S. Uhrig
We show that the dynamics of quenches in one dimension far off equilibrium can be described by power laws, but with exponents differing from the fully renormalized ones at lowest energies. Instead they depend on the initial state and its excitation energy. Furthermore, we found that for quenches to strong interactions unexpected similarities between systems in one and in infinite dimensions occur, indicating the dominance of local processes.
July 9
1.arXiv:1207.1483 [pdf, ps, other]
Fast generation of spin-squeezed states in bosonic Josephson junctions
B. Juliá-Díaz, E. Torrontegui, J. Martorell, J. G. Muga, A. Polls
We describe methods for fast production of highly coherent-spin-squeezed many-body states in bosonic Josephson junctions (BJJs). We start from the known mapping of the two-site Bose-Hubbard (BH) Hamiltonian to that of a single effective particle evolving according to a Schr\"odinger-like equation in Fock space. Since, for repulsive interactions, the effective potential in Fock space is nearly parabolic, we extend recently derived protocols for shortcuts to adiabatic evolution in harmonic potentials to the many-body BH Hamiltonian. The best scaling of the squeezing parameter for large number of atoms N is \xi^2_S ~ 1/N.
2. arXiv:1207.1466 [pdf, other]
Vortices on demand in multicomponent Bose-Einstein condensates
Roberto Zamora-Zamora, Marcelo Lozada-Hidalgo, Santiago F. Caballero-Benitez, Victor Romero-Rochin
We present a novel and simple mechanism to produce vortices at any desired spatial locations in harmonically trapped Bose-Einstein condensates (BEC) with multicomponent spin states coupled to external transverse magnetic fields. The vortices appear at the spatial points where the spin-field interaction vanishes and, depending on the multipolar magnetic field order, the vortices acquire a corresponding charge. We explicitly demonstrate our findings, both numerically and analitically, by analyzing 2D and 3D BEC via the Gross-Pitaevskii equation for atomic systems with either two or three internal states. We further show that, by an spontaneous symmetry breaking mechanism, the vortices appear in only some spin components. We propose an experimental realization of this scenario using a gas of 87 Rb occupying all three F = 1 states in an optical trap. We suggest that a further time varying of the external magnetic field may lead to a route for quantum turbulence.
3.arXiv:1207.1441 [pdf, ps, other]
Thermally isolated Luttinger liquids with noisy Hamiltonians
Luca D'Alessio, Armin Rahmani
We study the dynamics of a quantum-coherent thermally isolated Luttinger liquid with noisy Luttinger parameter. We calculate the out-of-equilibrium energy fluctuations as well as the correlation functions of this noise-driven system, and find universal features. By identifying two types of energy moments, which can help tease apart the effects of classical and quantum sources of fluctuations, we argue that the classical source is dominant in the thermodynamic limit.
July 2 - July 6, Xiaopeng Li
July 6
1. arXiv:1207.1320 [pdf, other]
Entropy dependence of correlations in one-dimensional SU(N) antiferromagnets
Laura Messio, Frédéric Mila
Motivated by the possibility to load multi-color fermionic atoms in optical lattices, we study the entropy dependence of the properties of the one-dimensional antiferromagnetic SU(N) Heisenberg model, the effective model of the SU(N) Hubbard model with one particle per site (filling 1/N). Using continuous-time world line Monte Carlo simulations for N=2 to 5, we show that characteristic short-range correlations develop at low temperature as a precursor of the ground state algebraic correlations. We also calculate the entropy as a function of temperature, and we show that the first sign of short-range order appears at an entropy per particle that increases with N and already reaches 0.8k_B at N=4, in the range of experimentally accessible values.
2.arXiv:1207.1289 [pdf, ps, other]
Onsager relations in a two-dimensional electron gas with spin-orbit coupling
C. Gorini, R. Raimondi, P. Schwab
Theory predicts for the two-dimensional electrons gas with only Rashba spin-orbit interaction a vanishing spin Hall conductivity and at the same time a finite inverse spin Hall effect. We show how these seemingly contradictory results are compatible with the Onsager relations: the latter do hold for spin and particle (charge) currents in the two-dimensional electron gas, although (i) their form depends on the experimental setup and (ii) a vanishing bulk spin Hall conductivity does not necessarily imply a vanishing spin Hall effect. We also discuss the situation in which extrinsic spin orbit from impurities is present and the bulk spin Hall conductivity can be different from zero.
3. arXiv:1207.1104 [pdf, other]
$\mathbb Z_2$~Green's function topology of Majorana wires
Jan Carl Budich, Björn Trauzettel
We calculate the $\mathbb Z_2$ topological invariant characterizing a one dimensional topological superconductor using a Wess-Zumino-Witten dimensional extension. The invariant is formulated in terms of the single particle Green's function which allows to classify interacting systems. Employing a recently proposed generalized Berry curvature method, the topological invariant is represented independent of the extra dimension requiring only the single particle Green's function at zero frequency of the interacting system. Furthermore, a modified twisted boundary conditions approach is used to rigorously formulate the topological invariant for disordered systems.
July 5
1. arXiv:1207.1045 [pdf, ps, other]
Nematic and meta-nematic transitions in the iron pnictides
S. Kasahara, H.J. Shi, K. Hashimoto, S. Tonegawa, Y. Mizukami, T. Shibauchi, K. Sugimoto, T. Fukuda, T. Terashima, Andriy H. Nevidomskyy, Y. Matsuda
Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice. The electron nematicity has been recently observed in the iron-pnictide and cuprate high-temperature superconductors. Whether such a tendency of electrons to self-organise unidirectionally has a common feature in these superconductors is, however, a highly controversial issue. In the cuprates, the nematicity has been suggested as a possible source of the pseudogap phase, whilst in the iron-pnictides, it has been commonly associated with the tetragonal-to-orthorhombic structural phase transition at $T_s$. Here, we provide the first thermodynamic evidence in BaFe2(As1-xPx)2 that the nematicity develops well above the structural transition and persists to the nonmagnetic superconducting regime, resulting in a new phase diagram strikingly similar to the pseudogap phase diagram in the cuprates. Our highly sensitive magnetic anisotropy measurements using microcantilever torque-magnetometry under in-plane field rotation reveal pronounced two-fold oscillations, which break the tetragonal symmetry. Combined with complementary high-resolution synchrotron X-ray and resistivity measurements, our results consistently identify two distinct temperatures - one at $T^{\ast}$, signifying a true nematic transition, and the other at $T_s (< T^{\ast})$, which we show to be not a true phase transition, but rather what we refer to as a "meta-nematic transition", in analogy to the well-known metamagnetic transition in the theory of magnetism. Our observation of the extended nematic phase above the superconducting dome establishes that the nematicity has primarily an electronic origin, inherent in the normal state of high-temperature superconductors.
July 4
1.arXiv:1207.0691 [pdf, other]
Variational Matrix Product Ansatz for Nonuniform Dynamics in the Thermodynamic Limit
Ashley Milsted, Jutho Haegeman, Tobias J. Osborne,
We describe how to implement the time-dependent variational principle for matrix product states in the thermodynamic limit for nonuniform lattice systems. This is achieved by confining the nonuniformity to a (dynamically growable) finite region with fixed boundary conditions. The suppression of unphysical quasiparticle reflections from the boundary of the nonuniform region is also discussed. Using this algorithm we study the dynamics of localized excitations in infinite systems, which we illustrate in the case of the spin-1 anti-ferromagnetic Heisenberg model and the phi^4 model.
2.arXiv:1207.0641 [pdf, ps, other]
Ultracold lattice gases with periodically modulated interactions
Akos Rapp, Xiaolong Deng,
We show that a time-dependent magnetic field inducing a periodically modulated scattering length may lead to interesting novel scenarios for cold gases in optical lattices, characterized by a non-linear hopping depending on the number difference at neighboring sites. We discuss the rich physics introduced by this hopping, including pair superfluidity, exactly defect-free Mott-insulator states for finite hopping, and pure holon- and doublon superfluids. We also address experimental detection, showing that the introduced non-linear hopping may lead in harmonically trapped gases to abrupt drops in the density profile marking the interface between different superfluid regions.
3. arXiv:1207.0542 [pdf, other]
Topological blockade and measurement of topological charge
B. van Heck, M. Burrello, A. Yacoby, A. R. Akhmerov
The fractionally charged quasiparticles appearing in the 5/2 fractional quantum Hall plateau are predicted to have an extra non-local degree of freedom, known as topological charge. We show how this topological charge can block the tunnelling of these particles, and how such 'topological blockade' can be used to readout their topological charge. We argue that the short time scale required for this measurement is favorable for the detection of the non-Abelian anyonic statistics of the quasiparticles. We also show how topological blockade can be used to measure braiding statistics, and to couple a topological qubit with a conventional one.
4.arXiv:1207.0498 [pdf, other]
Featureless and Non-Fractionalized Bose Insulator on the Honeycomb Lattice at 1/2 site-filling
Itamar Kimchi, S. A. Parameswaran, Ari M. Turner, Ashvin Vishwanath
It is well known that band insulators require an integer number of electrons (of each spin) per unit cell. Similarly, in bosonic insulators where the number of particles per unit cell (f) is fractional, the ground state either enlarges the unit cell, or realizes an exotic topologically ordered (fractionalized) state. Symmetric non-fractionalized Mott insulators only appear at integer f. However, the converse problem is relatively unexplored - is such a symmetric non-fractionalized insulator always allowed at integer f, or can it be prohibited by other lattice symmetries? An especially relevant example is the honeycomb lattice - where free spinless fermions at f=1 (or, 1/2 site filling) are always metallic, due to point group symmetries. Here we argue that bosons at the same filling however can realize a Mott phase. We propose a wave function for this state and by a mapping to a classical partition function we compute its properties and demonstrate that the state is insulating, fully symmetric and has no topological order. Thus the absence of symmetry breaking in this case does not imply topological order. Our construction suggests that featureless insulators are generically allowed for bosons at unit filling on any symmorphic lattice in any dimension.
July 3
1. arXiv:1207.0478 [pdf, ps, other]
Weyl points and line nodes in gapless gyroid photonic crystals
Ling Lu, Liang Fu, John D. Joannopoulos,
Weyl points and line nodes are three-dimensional linear point- and line-degeneracies between two bands. In contrast to Dirac points, which are their two-dimensional analogues, Weyl points are stable in the momentum space and the associated surface states are predicted to be topologically non-trivial. However, Weyl points are yet to be discovered in nature. Here, we report photonic crystals, based on the double-gyroid structures, exhibiting frequency-isolated Weyl points with intricate phase diagrams. The surface states associated with the non-zero Chern numbers are demonstrated. Line nodes are also found in similar geometries; the associated surface states are shown to be flat bands. Our results are readily experimentally realizable at both microwave and optical frequencies.
2.arXiv:1207.0307 [pdf, other]
Experimental observation of the optical spin-orbit torque
N. Tesarova, P. Nemec, E. Rozkotova, J. Zemen, F. Trojanek, K. Olejnik, V. Novak, P. Maly, T. Jungwirth
Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) on the magnetization. The phe- nomenon belongs to the area of spintronics research focusing on manipulating magnetic moments by electric fields and is the basis of the emerging technologies for scalable magnetoresistive random access memories. In our previous work we have reported experimental observation of the optical counterpart of STT in which a circularly polarized pump laser pulse acts as the external polarizer, allowing to study and utilize the phenomenon on several orders of magnitude shorter timescales than in the electric current induced STT. Recently it has been theoretically proposed and experimentally demonstrated that in the absence of an external polarizer, carriers in a magnet under applied electric field can develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) acting on the magnetization. In this paper we report the observation of the optical counterpart of SOT. At picosecond time-scales, we detect excitations of magnetization of a ferromagnetic semiconductor (Ga,Mn)As which are independent of the polarization of the pump laser pulses and are induced by non-equilibrium spin-orbit coupled photo-holes.
3.arXiv:1207.0457 (cross-list from hep-ph) [pdf, ps, other]
Remarks on nonrelativistic Goldstone bosons
We discuss excitations in nonrelativistic field theories with spontaneous breaking of a continuous global symmetry. It is known that in such systems there are two types of Goldstone bosons (Type A and Type B) whose dispersion law is generically linear or quadratic, respectively. We show that Type B Goldstone bosons may have gapped partners which we call almost-Goldstone bosons. With some nondegeneracy assumption about the low-energy effective action, the total number of Goldstone and almost-Goldstone bosons adds up to the number of broken symmetry generators. We propose that deviations of the dispersion law of Goldstone bosons from linearity at small momenta may serve as a signature of small breaking of time-reversal symmetry.
4.arXiv:1207.0020 (cross-list from q-bio.PE) [pdf, ps, other]
Spontaneously Broken Neutral Symmetry in an Ecological System
Claudio Borile, Miguel A. Muñoz, Sandro Azaele, Jayanth R. Banavar, Amos Maritan**
Spontaneous symmetry breaking plays a fundamental role in many areas of condensed matter and particle physics. A fundamental problem in ecology is the elucidation of the mechanisms responsible for biodiversity and stability. Neutral theory, which makes the simplifying assumption that all individuals (such as trees in a tropical forest) --regardless of the species they belong to-- have the same prospect of reproduction, death, etc., yields gross patterns that are in accord with empirical data. We explore the possibility of birth and death rates that depend on the population density of species while treating the dynamics in a species-symmetric manner. We demonstrate that the dynamical evolution can lead to a stationary state characterized simultaneously by both biodiversity and spontaneously broken neutral symmetry.
July 2
1.arXiv:1206.7109 [pdf, other]
Manipulating Majorana Fermions in Quantum Nanowires with Broken Inversion Symmetry
Xiong-Jun Liu, Alejandro M. Lobos
We predict that a Majorana quantum wire, driven into a trivial phase by tilting the external magnetic field as observed in experiment [V. Mourik et al., Science 336, 1003 (2012)], can be restored into the topological phase by applying a supercurrent in the proximate superconductor (SC). A tilted external magnetic field breaks spatial inversion symmetry and closes the superconducting gap in the wire when the tilt angle exceeds a critical value, leading to a topological phase transition. Interestingly, we show that a supercurrent applied in the SC is able to restore the Majorana end-states. Using Abelian bosonization, we further confirm this result in the presence of electron-electron interactions and show the analogy of this phenomenon to the commensurate-incommensurate transition driven by doping a one-dimensional Mott-insulator system. The present results have important applications in e.g., realizing the supercurrent assisted braiding (SAB) of Majorana fermions, which proves highly useful in topological quantum computation with realistic Majorana networks.
2.arXiv:1206.7010 [pdf, ps, other]
Quantum simulation of small-polaron formation with trapped ions
Vladimir M. Stojanovic, Tao Shi, C. Bruder,
We propose a method for simulating polaron physics using a one-dimensional system of trapped ions acted upon by off-resonant standing waves. This system, envisioned as an array of ion microtraps, in the single-excitation case provides a realization of the anti-adiabatic regime of the Holstein model. We show that the strong excitation-phonon coupling regime, characterized by the formation of small polarons, can be reached using realistic values of the relevant system parameters. Finally, we propose measurements of the quasiparticle residue and the average number of phonons in the ground state, experimental probes validating the polaronic character of the phonon-dressed excitation.
3.arXiv:1206.6959 [pdf, other]
Magnetic and Superconducting Ordering at LaAlO3/SrTiO3 Interfaces
Lukasz Fidkowski, Hong-Chen Jiang, Roman M. Lutchyn, Chetan Nayak
We formulate a model for magnetic and superconducting ordering at LaAlO3/SrTiO3 interfaces containing both localized magnetic moments and itinerant electrons. Though these both originate in Ti 3d orbitals, the former may be due to electrons more tightly-bound to the interface while the latter are extended over several layers. Only the latter contribute significantly to metallic conduction and superconductivity. In our model, the interplay between the two types of electrons, which is argued to be ferromagnetic, combined with strong spin-orbit coupling of the itinerant electrons, leads to magnetic ordering. Furthermore, we propose a model for interfacial superconductivity, consisting of random superconducting grains in the bulk STO driven, via coupling to the interface conduction band, towards long-ranged or quasi-long-ranged order. Most interestingly, the magnetic order and strong spin orbit coupling can lead in this manner to unconventional interfacial superconductivity, yielding a possible realization of Majorana physics.
