Nov 26 - Nov 30, Xiaopeng Li
Nov 30
1. arXiv:1211.7010 [pdf, ps, other]
Exact matrix product solution for the boundary-driven Lindblad $XXZ$-chain
D. Karevski, V. Popkov, G.M. Schütz
We demonstrate that the exact non-equilibrium steady state of the one-dimensional Heisenberg XXZ spin chain driven by boundary Lindblad operators can be constructed explicitly with a matrix product ansatz for the non-equilibrium density matrix where the matrices satisfy a {\it quadratic algebra}. This algebra turns out to be related to the quantum algebra $U_q[SU(2)]$. Coherent state techniques are introduced for the exact solution of the isotropic Heisenberg chain with and without quantum boundary fields and Lindblad terms that correspond to two different completely polarized boundary states. We show that this boundary twist leads to non-vanishing stationary currents of all spin components. Our results suggest that the matrix product ansatz can be extended to more general quantum systems kept far from equilibrium by Lindblad boundary terms.
2. arXiv:1211.6959 [pdf, other]
Few-particle quantum dynamics---comparing Nonequilibrium Green's functions with the generalized Kadanoff-Baym ansatz to density operator theory
S. Hermanns, K. Balzer, M. Bonitz
The nonequilibrium description of quantum systems requires, for more than two or three particles, the use of a reduced description to be numerically tractable. Two possible approaches are based on either reduced density matrices or nonequilibrium Green's functions (NEGF). Both concepts are formulated in terms of hierarchies of coupled equations---the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy for the reduced density operators and the Martin-Schwinger-hierarchy (MS) for the Green's functions, respectively. In both cases, similar approximations are introduced to decouple the hierarchy, yet still many questions regarding the correspondence of both approaches remain open.
3. arXiv:1211.6924 [pdf, other]
Topological transport in a spin-orbit coupled bosonic Mott insulator
C. H. Wong, R.A. Duine
We investigate topological transport in a spin-orbit coupled bosonic Mott insulator. We show that interactions can lead to anomalous quasi-particle dynamics even when the spin-orbit coupling is abelian. To illustrate the latter, we consider the spin-orbit coupling realized in the experiment of Lin \textit{et al}. [Nature (London) \textbf{471}, 83 (2011)]. For this spin-orbit coupling, we compute the quasiparticle dispersions and spectral weights, the interaction-induced momentum space Berry curvature, and the momentum space distribution of spin density, and propose experimental signatures. Furthermore, we find that in our approximation for the single-particle propagator, the ground state can in principle support an integer Hall conductivity if the sum of the Chern numbers of the hole bands is nonzero.
Nov 29
1. arXiv:1211.6469 (cross-list from quant-ph) [pdf, other]
Dynamical correlation functions and the quantum Rabi model
F. Alexander Wolf, Fabio Vallone, Guillermo Romero, Marcus Kollar, Enrique Solano, Daniel Braak
We study the quantum Rabi model within the framework of the analytical solution developed in Phys. Rev. Lett. 107,100401 (2011). In particular, through time-dependent correlation functions, we give a quantitative criterion for classifying two regions of the quantum Rabi model, involving the Jaynes-Cummings, the ultrastrong, and deep strong coupling regimes. In addition, we find a stationary qubit-field entangled basis that governs the whole dynamics as the coupling strength overcomes the mode frequency.
Nov 28
1. arXiv:1211.6224 [pdf, ps, other]
Critical parameters from trap-size scaling in trapped particle systems
Giacomo Ceccarelli, Christian Torrero, Ettore Vicari
We investigate the critical behavior of trapped particle systems at the low-temperature superfluid transition. In particular, we consider the three-dimensional Bose-Hubbard model in the presence of a trapping harmonic potential coupled with the particle density, which is a realistic model of cold bosonic atoms in optical lattices. We present a numerical study based on quantum Monte Carlo simulations, analyzed in the framework of the trap-size scaling (TSS).
We show how the critical parameters can be derived from the trap-size dependences of appropriate observables, matching them with TSS. This provides a systematic scheme which is supposed to exactly converge to the critical parameters of the transition in the large trap-size limit. Our numerical analysis may provide a guide for experimental investigations of trapped systems at finite-temperature and quantum transitions, showing how critical parameters may be determined by looking at the scaling of the critical modes with respect to the trap size, i.e. by matching the trap-size dependence of the experimental data with the expected TSS Ansatz.
Nov 27
1. arXiv:1211.6084 [pdf, other]
Photon Counting as a Probe of Superfluidity in a Two-Band Bose Hubbard System Coupled to a Cavity Field
Sara Rajaram, Nandini Trived
We show that photon number measurement can be used to detect superfluidity for a two-band Bose-Hubbard model coupled to a cavity field. The atom-photon coupling induces transitions between the two internal atomic levels and results in entangled polaritonic states. In the presence of a cavity field, we find different photon numbers in the Mott-insulating versus superfluid phases, providing a method of distinguishing the atomic phases by photon counting. Furthermore, we examine the dynamics of the photon field after a rapid quench to zero atomic hopping by increasing the well depth. We find a robust correlation between the field's quench dynamics and the initial superfluid order parameter, thereby providing a novel and accurate method of determining the order parameter.
2. arXiv:1211.5848 [pdf, ps, other]
Stability of p-orbital Bose-Einstein condensates in optical checkerboard and square lattices
Yong Xu, Zhu Chen, Hongwei Xiong, W. Vincent Liu, Biao Wu
We investigate $p$-orbital Bose-Einstein condensates in both the square and checkerboard lattice by numerically solving the Gross-Pitaevskii equation. The periodic potential for the latter lattice is taken exactly from the recent Hamburg experiment. It is confirmed that the staggered orbital-current state is the lowest energy state in the $p$ band. Our numerical calculation further reveals that for both lattices the staggered $p$-orbital state suffers Landau instability but the situation is remarkably different for dynamical instability. A dynamically stable parameter region is found for the checkerboard lattice, but not for the square.
3. arXiv:1211.5649 [pdf, other]
Direct observation of a Berezinskii-Kosterlitz-Thouless superfluid in an atomic gas
Jae-yoon Choi, Sang Won Seo, Yong-il Shin
Understanding the emerging mechanisms of superfluidity has been a central theme in many-body physics. In particular, the superfluid state in a two-dimensional (2D) system is intriguing because large thermal fluctuations prohibit the formation of long-range order and consequently the picture of Bose-Einstein condensation is not applicable to the phase transition. The Berezinskii-Kosterlitz-Thouless (BKT) theory provides a microscopic mechanism for the superfluid phase transition, where vortices with opposite circulation are paired below the critical temperature, establishing quasi-long-range phase coherence. While this mechanism has been tested experimentally, there has been no direct observation of thermal vortex pairs in a 2D superfluid. Here we report the observation of a BKT superfluid in a trapped quasi-2D atomic gas by measuring the spatial distribution of thermally activated vortices and revealing their pair correlations. The vortex population concentrates in the low-density superfluid region of the trapped sample and decreases gradually as the temperature is lowered, showing a crossover from a BKT phase to a Bose-Einstein condensate with no thermal vortices. These observations clarify the nature of the superfluid state of a trapped 2D Bose gas.
Nov 26
1. arXiv:1211.5440 [pdf, other]
Topological Solitons versus Non-Solitonic Phase Defects in Quasi-One-Dimensional Charge Density Wave
Tae-Hwan Kim, Han Woong Yeom
We investigated phase defects in a quasi-one-dimensional commensurate charge density wave (CDW) system, an In atomic wire array on Si(111), using low temperature scanning tunneling microscopy. The unique four-fold degeneracy of the CDW state leads to various phase defects, among which intrinsic solitons are clearly distinguished. The solitons exhibit a characteristic variation of the CDW amplitude with a coherence length of about 4 nm, as expected from the electronic structure, and a localized electronic state within the CDW gap. While most of the observed solitons are trapped by extrinsic defects, moving solitons are also identified and their novel interaction with extrinsic defects is disclosed.
2. arXiv:1211.5381 [pdf, ps, other]
Topological Rényi entropy after a quantum quench
Gábor B. Halász, Alioscia Hamma
We present an analytical study on the resilience of topological order after a quantum quench. The system is initially prepared in the ground state of the Toric Code Model, and then quenched by switching on an external magnetic field. During the subsequent time evolution, the variation in topological order is detected via the topological Renyi entropy of order 2. We consider two different quenches: the first one has an exact solution, while the second one requires perturbation theory. In both cases, we find that the long-term time average of the topological Renyi entropy in the thermodynamic limit is the same as its initial value. Based on our results, we argue that topological order is resilient against a wide range of quenches.
3. arXiv:1211.5145 [pdf, other]
Universal "rephasing" dynamics
Emanuele G. Dalla Torre, Eugene Demler, Anatoli Polkovnikov
We consider a quantum quench in which two initially independent condensates are suddenly coupled, and study the subsequent "rephasing" dynamics. For weak couplings the time-evolution of physical observables is predicted to follow universal scaling laws, connecting the short-time dynamics to the long-time non-perturbative regime. We first describe a two-modes model valid in two and three dimensions and then move to one dimension, where the problem is described by a massive Sine-Gordon theory. Combining analytical and numerical methods we compute universal time-dependent expectation values and distribution functions, allowing a qualitative comparison with future experiments.
Nov 19 - Nov 23, Saubhik Sarkar
Nov 22
1. arXiv:1211.5133 [pdf, ps, other]
Quantum Degenerate Fermi Gas with Spin-orbit Coupling and Crossed Zeeman Fields
Kangjun Seo, Li Han, C. A. R. Sá de Melo
We study quantum degenerate ultra-cold Fermi gases in the presence of artificial spin-orbit coupling and crossed Zeeman fields. We emphasize the case where parity is violated in the excitation spectrum and compare it with the simpler situation where parity is preserved. We investigate in detail spectroscopic properties such as the excitation spectrum, the spectral function, momentum distribution and density of states for the cases where parity is preserved or violated. Similarly, we show that thermodynamic properties such as pressure, chemical potential, entropy, specific heat, isothermal compressibility and induced spin polarization become anisotropic as a function of Zeeman field components, when parity is violated. Lastly, we discuss the effects of interactions and present results for the pairing temperature as the precursor for the transition to a superfluid state. In particular, we find that the pairing temperature is dramatically reduced in the weak interaction regime as parity violation gets stronger, and that the momentum dependence of the order parameter for superfluidity violates parity when crossed Zeeman fields are present for finite spin-orbit coupling.
2.arXiv:1211.5014 [pdf, ps, other]
Quantum interferometry at zero and finite temperature with two-mode bosonic Josephson junctions
G. Mazzarella
We analyze phase interferometry realized with a bosonic Josephson junction made of trapped dilute and ultracold atoms. By using a suitable phase sensitivity indicator we study the zero temperature junction states useful to achieve sub shot-noise precisions. Sub shot-noise phase shift sensitivities can be reached even at finite temperature under a suitable choice of the junction state. We infer a scaling law in terms of the size system (that is, the number of particles) for the temperature at which the shot-noise limit is not overcome anymore
3. arXiv:1211.4987 [pdf, ps, other]
Simulation of frustrated classical XY models with ultra-cold atoms in 3D triangular optical lattices
Arkadiusz Kosior, Krzysztof Sacha
Miscellaneous magnetic systems are being recently intensively investigated because of their potential applications in modern technologies. Nonetheless, a many body dynamical description of complex magnetic systems may be cumbersome, especially when the system exhibits a geometrical frustration. This paper deals with simulations of the classical XY model on a three dimensional triangular lattice with anisotropic couplings, including an analysis of the phase diagram and a Bogoliubov description of the dynamical stability of mean-field stationary solutions. We also discuss the possibilities of the realization of Bose-Hubbard models with complex tunneling amplitudes in shaken optical lattices without breaking the time-reversal symmetry and the opposite, i.e. real tunneling amplitudes in systems with the time-reversal symmetry broken.
Nov 21
1. arXiv:1211.4863 [pdf, other]
Absence of the Twisted Superfluid Phase in a mean field model of bosons on a Honeycomb Lattice
Sayan Choudhury, Erich J MuellerMotivated by recent observations (P. Soltan-Panahi {\it et al.}, Nature Physics {\bf 8}, 71-75 (2012)), we study the stability of a Bose-Einstein Condensate within a spin-dependent honeycomb lattice towards forming a "Twisted Superfluid phase". Our exhaustive numerical search fails to find this phase, pointing to possible non-mean field physics.
2. arXiv:1211.4772 [pdf, ps, other]
Slow Thermalization Between a Lattice and Free Bose Gas
David C. McKay, Brian DeMarco
Using a 3D spin-dependent optical lattice, we study thermalization and energy exchange between two ultracold Bose gases, one of which is bound to the lattice and another that is free from the lattice potential. Disruption of inter-species thermalization is revealed through measurements of condensate fraction after the lattice is superimposed on the parabolic confining potential. By selectively heating the lattice-bound species and measuring the rate of heat transfer to the free state, suppression of energy exchange is observed. Comparison with a Fermi's golden rule prediction confirms that this effect is caused by a dispersion mismatch that reduces the phase space available for elastic collisions. This result has critical implications for methods proposed to cool strongly correlated lattice gases.
Nov 20
1. arXiv:1211.4556 [pdf, ps, other]
Optical Control of the Scattering Length and Effective Range for Magnetically Tunable Feshbach Resonances in Ultracold Gases
Haibin Wu, J. E. Thomas
We describe two-field optical techniques to control interactions in Feshbach resonances for two-body scattering in ultra-cold gases. These techniques create a molecular dark state in the closed channel of a magnetically tunable Feshbach resonance, greatly suppressing optical scattering compared to single optical field methods. The dark-state method enables control of the effective range, by creating narrow features that modify the energy dependence of the scattering phase shift, as well as control of the elastic and inelastic parts of the zero-energy s-wave scattering amplitude. We determine the scattering length and the effective range from an effective range expansion, by calculating the momentum-dependent scattering phase shift from the two-body scattering state.
2.arXiv:1211.4350 [pdf, ps, other]
Mean-field dynamics to negative absolute temperatures in the Bose-Hubbard model
Akos Rapp
We apply time-dependent Gutzwiller mean-field theory to provide a qualitative understanding for bosons in optical lattices that approach states corresponding to negative absolute temperatures. We perform the dynamical simulations to relate to the recent experiments by Braun et al., Ref.[1]. Time-of-flight images calculated from the two-dimensional numerical simulations reproduce characteristics of the experimental observations, in particular, the emergence of the four peaks at the corners of the Brillouin zone.
3. arXiv:1211.4537 (cross-list from physics.atom-ph) [pdf, other]
Long-range interacting many-body systems with alkaline-earth-metal atoms
B. Olmos, D. Yu, Y. Singh, F. Schreck, K. Bongs, I. Lesanovsky
Alkaline-earth-metal atoms exhibit long-range dipolar interactions, which are generated via the coherent exchange of photons on the 3P_0-3D_1-transition of the triplet manifold. In case of bosonic strontium, which we discuss here, this transition has a wavelength of 2.7 \mu m and a dipole moment of 2.46 Debye, and there exists a magic wavelength permitting the creation of optical lattices that are identical for the states 3P_0 and 3D_1. This interaction enables the realization and study of mixtures of hard-core lattice bosons featuring long-range hopping, with tuneable disorder and anisotropy. We derive the many-body Master equation, investigate the dynamics of excitation transport and analyze spectroscopic signatures stemming from coherent long-range interactions and collective dissipation. Our results show that lattice gases of alkaline-earth-metal atoms permit the creation of long-lived collective atomic states and constitute a simple and versatile platform for the exploration of many-body systems with long-range interactions. As such, they represent an alternative to current related efforts employing Rydberg gases, atoms with large magnetic moment, or polar molecules.
4.arXiv:1211.4087 (cross-list from cond-mat.str-el) [pdf, other]
Emergence of long distance pair coherence through incoherent local environmental coupling
Jean-Sebastien Bernier, Peter Barmettler, Dario Poletti, Corinna Kollath
We demonstrate that quantum coherence can be generated by the interplay of coupling to an incoherent environment and kinetic processes. This joint effect even occurs in a repulsively interacting fermionic system initially prepared in an incoherent Mott insulating state. In this case, coupling a dissipative noise field to the local spin density produces coherent pairs of fermions. The generated pair coherence extends over long distances as typically seen in Bose-Einstein condensates. This conceptually surprising approach provides a novel path towards a better control of quantum many-body correlations.
Nov 19
1. arXiv:1211.4020 [pdf, other]
Topological superfluid phases of an atomic Fermi gas with equal Rashba-Dresselhaus spin-orbit coupling and in- and out-of-plane Zeeman fields
M. Iskin, A. L. SubasiWe analyze the effects of in- and out-of-plane Zeeman fields on the BCS-BEC evolution of a spin-orbit coupled Fermi gas with an equal contribution from Rashba and Dresselhaus terms, and show that the ground-state of the system involves novel gapless superfluid phases that can be distinguished with respect to the topological structure of the momentum-space regions with zero excitation energy. Depending on the combination of Zeeman and spin-orbit fields, these topological structures correspond to one or two doubly-degenerate round spheres, two or four round spheres, two or four concave spheres, or one or two doubly-degenerate circles. Such changes in the topology signal a quantum phase transition between distinct superfluid phases, and leave their signatures on some of the thermodynamic quantities.
Nov 12 - Nov 16, Johannes Schachenmayer
Nov 16
1. arXiv:1211.3598 [pdf, other]
Trion and Dimer Formation of Three-Color Fermions
J. Pohlmann, A. Privitera, I. Titvinidze, W. HofstetterWe study the problem of three ultracold fermions in different hyperfine states loaded into a lattice with spatial dimension D=1,2. We consider SU(3)-symmetric attractive interactions and also eventually include a three-body constraint, which mimics the effect of three-body losses in the strong-loss regime. We combine exact diagonalization with the Lanczos algorithm, and evaluate both the eigenvalues and the eigenstates of the problem. In D=1, we find that the ground state is always a three-body bound state (trion) for arbitrarily small interaction, while in D=2, due to the stronger influence of finite-size effects, we are not able to provide conclusive evidence of the existence of a finite threshold for trion formation. Our data are however compatible with a threshold value which vanishes logarithmically with the size of the system. Moreover we are able to identify the presence of a fine structure inside the spectrum, which is associated with off-site trionic states. The characterization of these states shows that only the long-distance behavior of the eigenstate wavefunctions provides clear-cut signatures about the nature of bound states and that onsite observables are not enough to discriminate between them. The inclusion of a three-body constraint due to losses promotes these off-site trions to the role of lowest energy states, at least in the strong-coupling regime.
2. arXiv:1211.3456 [pdf, ps, other]
Strongly interacting two-dimensional Bose gases
Li-Chung Ha, Chen-Lung Hung, Xibo Zhang, Ulrich Eismann, Shih-Kuang Tung, Cheng ChinWe prepare and study strongly interacting two-dimensional Bose gases in the superfluid, the classical Berezinskii-Kosterlitz-Thouless (BKT) transition, and the vacuum-to-superfluid quantum critical regimes. A wide range of the two-body interaction strength 0.05 < g < 3 is covered by tuning the scattering length and by loading the sample into an optical lattice. Based on the equations of state measurements, we extract the coupling constants as well as critical thermodynamic quantities in different regimes. In the superfluid and the BKT transition regimes, the extracted coupling constants show significant downshifts from the mean-field and perturbation calculations when g approaches or exceeds 1. In the BKT and the quantum critical regimes, all measured thermodynamic quantities show logarithmic dependence on the interaction strength, a tendency confirmed by the extended classical-field and renormalization calculations.
Nov 15
1. arXiv:1211.3317 [pdf, ps, other]
Quantum phases of quadrupolar Fermi gases in optical lattices
Satyan G. Bhongale, Ludwig Mathey, Erhai Zhao, Susanne F. Yelin, Mikhail LemeshkoWe introduce a new platform for quantum simulation of many-body systems based on nonspherical particles with zero dipole moment but possessing a significant value of the electric quadrupole moment. Considering a quadrupolar quantum gas trapped on a 2D square lattice, we show that the peculiar symmetry and broad tunability of the quadrupole-quadrupole interactions results in a rich phase diagram encompassing unconventional BCS and charge density wave phases, and paves the way to create topological superfluid ground states of $p_x+ip_y$ symmetry. Quadrupolar species, such as metastable alkaline-earth atoms and homonuclear molecules, are stable against chemical reactions and collapse and are readily available in experiment at high densities.
2. arXiv:1211.3127 [pdf, other]
Quenched binary Bose-Einstein condensates: spin domain formation and coarsening
S. De, D. L. Campbell, R. M. Price, A. Putra, B. M. Anderson, I. B. SpielmanWe explore the time evolution of quasi-1D two component Bose-Einstein condensates (BEC's) following a quench from one component BEC's with a ${\rm U}(1)$ order parameter into two component condensates with a ${\rm U}(1)\shorttimes{\rm Z}_2$ order parameter. In our case, these two spin components have a propensity to phase separate, i.e., they are immiscible. Remarkably, these spin degrees of freedom can equivalently be described as a single component attractive BEC. A spatially uniform mixture of these spins is dynamically unstable, rapidly amplifing any quantum or pre-existing classical spin fluctuations. This coherent growth process drives the formation of numerous spin polarized domains, which are far from the system's ground state. At much longer times these domains grow in size, coarsening, as the system approaches equilibrium. The experimentally observed time evolution is fully consistent with our stochastic-projected Gross-Pitaevskii calculation.
3. arXiv:1211.3267 [pdf, ps, other]
Universal local pair correlations of Lieb-Liniger bosons at quantum criticality
M.-S. Wang, J.-H. Huang, C.-H. Lee, X.-G. Yin, X.-W. Guan, M. T. BatchelorThe one-dimensional Lieb-Liniger Bose gas is a prototypical many-body system featuring universal Tomonaga-Luttinger liquid (TLL) physics and free fermion quantum criticality. We analytically calculate finite temperature local pair correlations for this model at quantum criticality using the polylog function in the framework of the Yang-Yang thermodynamic equations. We show that the local pair correlation $g^{(2)}(0)$ has the universal value $g^{(2)}(0)\approx 2 p/(n\varepsilon)$ at quantum criticality, where $n$ is the linear density and $p$ is the pressure per unit length rescaled by the interaction energy $\varepsilon=\frac{\hbar^2}{2m} c^2$ with interaction strength $c$. This suggests the possibility to test finite temperature local pair correlations for the TLL over the relativistic dispersion regime and to probe quantum criticality with the local correlations beyond the TLL phase. Furthermore, thermodynamic properties at high temperatures are obtained by both high temperature and virial expansion of the Yang-Yang thermodynamic equation. The virial expansion is more suitable for the weak interaction regime.
Nov 14
1. arXiv:1211.2842 [pdf, other]
Phonon mediated quantum spin simulator employing a planar ionic crystal in a Penning trap
C.-C. Joseph Wang, Adam C. Keith, J. K. FreericksWe derive the normal modes for a rotating Coulomb ion crystal in a Penning trap, quantize the motional degrees of freedom, and illustrate how they can by driven by a spin-dependent optical dipole force to create a quantum spin simulator on a triangular lattice with hundreds of spins. The analysis for the axial modes (oscillations perpendicular to the two-dimensional crystal plane) follow a standard normal-mode analysis, while the remaining planar modes are more complicated to analyze because they have velocity-dependent forces in the rotating frame. After quantizing the normal modes into phonons, we illustrate some of the different spin-spin interactions that can be generated by entangling the motional degrees of freedom with the spin degrees of freedom via a spin-dependent optical dipole force. In addition to the well-known power-law dependence of the spin-spin interactions when driving the axial modes blue of phonon band, we notice certain parameter regimes in which the level of frustration between the spins can be engineered by driving the axial or planar phonon modes at different energies. These systems may allow for the analog simulation of quantum spin glasses with large numbers of spins.
2. arXiv:1211.2901 [pdf, ps, other]
Supersolid in Bose-Bose-Fermi Mixtures subjected to a Square Lattice
Zhongbo Yan, Xiaosen Yang, Shaolong WanTwo-component Bose condensates with repulsive interaction are stable when $g_{\rm \scriptscriptstyle 1} g_{\rm \scriptscriptstyle 2}<g_{\rm \scriptscriptstyle 12}^{2}$ is satisfied. By tuning the interactions, we show that the instability corresponding to bose-bose phase separation always happens at a higher temperature than corresponding to bose-fermi phase separation happens. Moreover, we find both the transition temperature $T_{\rm \scriptscriptstyle DW}$ of supersolid and the coherence peak at $k_{\rm \scriptscriptstyle DW}$ are enhanced in the mixtures studied. These will make the observation of supersolid in experiments more reachable.
3. arXiv:1211.2888 [pdf, other]
Ultracold mixtures of atomic Li-6 and Cs-133 with tunable interactions
Shih-Kuang Tung, Colin Parker, Jacob Johansen, Cheng Chin, Yujun Wang, Paul S. JulienneWe report the experimental and theoretical study of two-body interactions in a $^{6}$Li-$^{133}$Cs Fermi- Bose mixture. Using a translatable dipole trap setup, we have successfully trapped the two species in the same trap with temperatures of a few micro Kelvins. By monitoring atom number loss and inter-species thermalization, we identify five s-wave interspecies Feshbach resonances in the lowest two scattering channels. We construct a coupled channels model using molecular potentials to fit and characterize these resonances. Two of the resonances are as wide as 60 G and thus should be suitable for creating Feshbach molecules and searching for universal few-body scaling.
Nov 13
1. arXiv:1211.2704 [pdf, other]
Simulations of non-Abelian gauge theories with optical lattices
L. Tagliacozzo, A. Celi, P. Orland, M. LewensteinMany phenomena occurring in strongly correlated quantum systems still await conclusive explanations. The absence of isolated free quarks in nature is an example. It is attributed to quark confinement, whose origin is not yet understood. The phase diagram for nuclear matter at general temperatures and densities, studied in heavy-ion collisions, is largely conjectural. Finally, we have no definitive theory of high-temperature superconductivity. Though we have theories that could underlie such physics, we lack the tools to determine the experimental consequences of these theories. Quantum simulators may provide such tools. Here we show how to engineer quantum simulators of non-Abelian lattice gauge theories. The systems we consider have several applications: they can be used to mimic quark confinement or to study dimer and valence-bond states (which may be relevant for high-temperature superconductors).
Comments: 5+3 pages, 3 figures, any comment is welcomeSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Lattice (hep-lat); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
2. arXiv:1211.2659 [pdf, ps, other]
Collective Modes in a Unitary Fermi Gas across the Superfluid Phase Transition
Meng Khoon Tey, Leonid A. Sidorenkov, Edmundo R. Sánchez Guajardo, Rudolf Grimm, Mark J. H. Ku, Martin W. Zwierlein, Yan-Hua Hou, Lev Pitaevskii, Sandro StringariWe provide a joint theoretical and experimental investigation of the temperature dependence of the collective oscillations of first sound nature exhibited by a highly elongated harmonically trapped Fermi gas at unitarity, including the region below the critical temperature for superfluidity. Differently from the lowest axial breathing mode, the hydrodynamic frequencies of the higher nodal excitations show a temperature dependence, which is calculated starting from Landau two-fluid theory and using the available experimental knowledge of the equation of state. The experimental results agree with high accuracy with the predictions of theory and provide the first evidence for the temperature dependence of the collective frequencies near the superfluid phase transition.
4. arXiv:1211.2509 [pdf, ps, other]
Nontrivial Haldane phase of an atomic two-component Fermi gas trapped in a 1d optical lattice
Keita Kobayashi, Masahiko Okumura, Yukihiro Ota, Susumu Yamada, Masahiko MachidaWe propose how to create a non-trivial Haldane phase in atomic two-component Fermi-gas loaded on one-dimensional (1-D) optical lattice with trap potential. The Haldane phase is naturally formed on $p$-band Mott core in a wide range of the strong on-site repulsive interaction. The present proposal is composed of two steps, one of which is theoretical derivation of an effective 1-D S=1 interacting-chain model from the original tight-binding Hamiltonian handling the two $p$-orbitals, and the other of which is numerical demonstration employing the density-matrix renormalization-group for the formation of the Haldane phase on $p$-band Mott core and its associated features in the original tight-binding model with the harmonic trap potential.
5. arXiv:1211.2242 [pdf, other]
Atomic Quantum Simulation of U(N) and SU(N) Non-Abelian Lattice Gauge Theories
D. Banerjee, M.Bögli, M. Dalmonte, E. Rico, P. Stebler, U.-J. Wiese, P. Zoller
Using ultracold alkaline-earth atoms in optical lattices, we construct a quantum simulator for U(N) and SU(N) lattice gauge theories with fermionic matter based on quantum link models. These systems share qualitative features with QCD, including chiral symmetry breaking and restoration at non-zero temperature or baryon density. Unlike classical simulations, a quantum simulator does not suffer from sign problems and can address the corresponding chiral dynamics in real time.
6. arXiv:1211.2683 [pdf, ps, other]
AC-driven Quantum Phase Transition in the Lipkin-Meshkov-Glick Model
G. Engelhardt, V. M. Bastidas, C. Emary, T. BrandesWe establish a set of nonequilibrium quantum phase transitions in the Lipkin-Meshkov-Glick model driven under monochromatic nonadiabatic modulation of the transverse field. We show that the external driving induces a rich phase diagram that characterizes the multistability in the system. Interestingly, the number of stable configurations can be tuned by increasing the amplitude of the driving field. Furthermore, by studying the quantum evolution, we demonstrate that the system exhibits a set of quantum phases that correspond to dynamically stabilized states.
7. arXiv:1211.2572 [pdf, ps, other]
Quantum quenches, dynamical transitions and off-equilibrium quantum criticality
Bruno Sciolla, Giulio BiroliSeveral mean-field computations have revealed the existence of an out of equilibrium dynamical transition induced by quantum quenching an isolated system starting from its symmetry broken phase. In this work we focus on the quantum phi^4 N-component field theory. By taking into account dynamical fluctuations at the Hartree-Fock level, corresponding to the leading order of the 1/N expansion, we derive the critical properties of the dynamical transition beyond mean-field theory (including at finite temperature). We find diverging time and length-scales, dynamic scaling and aging. Finally, we unveil a relationship with coarsening, an off-equilibrium dynamical regime that can be induced by quenching from the symmetric toward the symmetry broken phase.
8. arXiv:1211.2241 [pdf, other]
A cold-atom quantum simulator for SU(2) Yang-Mills lattice gauge theory
Erez Zohar, J. Ignacio Cirac, Benni ReznikNon-abelian gauge theories play an important role in the standard model of particle physics, and unfold a partially unexplored world of exciting physical phenomena. In this letter, we suggest a realization of a non-abelian lattice gauge theory - SU(2) Yang-Mills in 1+1 dimensions, using ultracold atoms. Remarkably, and in contrast to previous proposals, in our model gauge invariance is a direct consequence of angular momentum conservation and thus is fundamental and robust. Our proposal may serve as well as a starting point for higher dimensional realizations.
Nov 12
1. arXiv:1211.2097 [pdf, ps, other]
Spin-orbit Coupled Bose-Einstein Condensates in Spin-dependent Optical Lattices
Wei Han, Suying Zhang, Wu-Ming LiuWe investigate ground-state structures of spin-orbit coupled Bose-Einstein condensates in spin-dependent optical lattices. Our results show that the competition between the spin-orbit coupling and the optical lattice leads to three distinct phases---density stripe, triangular vortex lattice and rectangular vortex lattice. In the density stripe phase, the condensates act as alternately arranged spin domains, and the domain wall responses to spin-orbit coupling with its structure changing from N\'{e}el wall to Bloch wall. In the vortex lattice phases, both the triangular and rectangular lattices are composed of alternately arranged vortex and antivortex chains, with alternating-direction plane waves propagating on two sides of each chain. We demonstrate that the exotic vortex phase is essentially equivalent to a lattice composed of meron pairs and antimeron pairs in the pseudospin representation. Furthermore, we discuss how to observe these phases in future experiments.
2. arXiv:1211.2123 [pdf, ps, other]
Estimation of classical parameters via continuous probing of complementary quantum observables
Antonio Negretti, Klaus Molmer
We discuss how continuous probing of a quantum system allows estimation of unknown classical parameters embodied in the Hamiltonian of the system. We generalize the stochastic master equation associated with continuous observation processes to a Bayesian filter equation for the probability distribution of the desired parameters, and we illustrate its application by estimating the direction of a magnetic field. In our example, the field causes a ground state spin precession in a two-level atom which is detected by the polarization rotation of off-resonant optical probes, interacting with the atomic spin components.
Nov 5 - Nov 9, Stephan Langer
Nov 9
1. arXiv:1211.1907 [pdf, ps, other]
Energy dependence of the entanglement entropy of composite boson (quasiboson) systems
A.M. Gavrilik, Yu.A. Mishchenko
Bipartite composite boson (quasiboson) systems, which admit realization in terms of deformed oscillators, were considered in our previous paper from the viewpoint of entanglement characteristics. These characteristics, including entanglement entropy, were expressed through the relevant deformation parameter for different quasibosonic states. On the other hand, it is of interest to present the entanglement entropy as function of energy for those states. In this work, the corresponding dependencies are found for different states of composite bosons realized by deformed oscillators and, for comparison, also for the hydrogen atom viewed as composite boson. The obtained results are analyzed and expressed graphically.
Nov 8
1. arXiv:1211.1504 [pdf, ps, other]
An efficient Monte Carlo algorithm for the evaluation of Renyi entanglement entropy of a general quantum dimer model at the R-K point
Jiquan Pei, Qiang Han, Haijun Liao, Tao Li
A highly efficient and simple to implement Monte Carlo algorithm is proposed for the evaluation of the Renyi entanglement entropy(REE) of quantum dimer model(QDM) at the Rokhsar-Kivelson(R-K) point. It makes possible the evaluation of REE at the R-K point to the thermodynamic limit for a general QDM. We apply the algorithm to QDM on both triangular and square lattice as demonstrations and find the REE on both lattices follow perfect linear scaling in the thermodynamic limit, apart from an even-odd oscillation in the latter case. We also evaluate the topological entanglement entropy(TEE) on both lattices with a subtraction procedure. While the expected TEE of $\ln2$ is clearly demonstrated for QDM on triangular lattice, a strong oscillation of the result is found for QDM on square lattice, which implies the relevance of boundary perturbation in such a critical system.
2. arXiv:1211.1522 [pdf, ps, other]
Nonequilibrium phase transitions in systems with long-range interactions
Tarcśio N. Teles, Fernanda Benetti, Renato Pakter, Yan Levin
We introduce a generalized Hamiltonian Mean Field Model (gHMF)-XY model with both linear and quadratic coupling between spins and explicit Hamiltonian dynamics. In addition to the usual paramagnetic and ferromagnetic phases, this model also possesses a nematic phase. The gHMF can be solved explicitly using Boltzmann-Gibbs (BG) statistical mechanics, in both canonical and microcanonical ensembles. However, when the resulting microcanonical phase diagram is compared with the one obtained using molecular dynamics simulations, it is found that the two are very different. We will present a dynamical theory which allows us to explicitly calculate the phase diagram obtained using molecular dynamics simulations without any adjustable parameters. The model illustrates the fundamental role played by dynamics as well the inadequacy of BG statistics for systems with long-range forces in the thermodynamic limit.
Nov 7
1. arXiv:1211.1195 [pdf, ps, other]
Linear quantum quench in the Heisenberg XXZ chain: Out of equilibrium Luttinger liquid description of a lattice system
Frank Pollmann, Masudul Haque, Balázs Dóra
We study variable-rate linear quenches in the anisotropic Heisenberg (XXZ) chain, starting at the XX point. This is equivalent to swithcing on a nearest neighbour interaction for hard-core bosons or an interaction quench for free fermions. The physical observables we investigate are: the energy pumped into the system during the quench, the spin-flip correlation function, and the bipartite fluctuations of the z component of the spin in a box. We find excellent agreement between exact numerics (infinite system time-evolving block decimation, iTEBD) and analytical results from bosonization, as a function of the quench time, spatial coordinate and interaction strength. This provides a stringent and much-needed test of Luttinger liquid theory in a non-equilibrium situation.
Nov 6
1. arXiv:1211.0545 [pdf, other]
Negative Absolute Temperature for Motional Degrees of Freedom
Simon Braun, Jens Philipp Ronzheimer, Michael Schreiber, Sean S. Hodgman, Tim Rom, Immanuel Bloch, Ulrich SchneiderAbsolute temperature, the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special conditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. So far, such states have been demonstrated in localized systems with finite, discrete spectra. Here, we were able to prepare a negative temperature state for motional degrees of freedom. By tailoring the Bose-Hubbard Hamiltonian we created an attractively interacting ensemble of ultracold bosons at negative temperature that is stable against collapse for arbitrary atom numbers. The quasi-momentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states and counterintuitive effects such as Carnot engines above unity efficiency.
2. arXiv:1211.0581 [pdf, ps, other]
Entanglement and area laws in weakly correlated gaussian states
J.M. Matera, R. Rossignoli, N. Canosa
We examine the evaluation of entanglement measures in weakly correlated gaussian states. It is shown that they can be expressed in terms of the singular values of a particular block of the generalized contraction matrix. This result enables to obtain in a simple way asymptotic expressions and related area laws for the entanglement entropy of bipartitions in pure states, as well as for the logarithmic negativity associated with bipartitions and also pairs of arbitrary subsystems. As illustration, we consider different types of contiguous and noncontiguous blocks in two dimensional lattices. Exact asymptotic expressions are provided for both first neighbor and full range couplings, which lead in the first case to area laws depending on the orientation and separation of the blocks.
3. arXiv:1211.1201 [pdf, other]
Non-perturbative stochastic method for driven quantum impurity systems
Peter P. Orth, Adilet Imambekov, Karyn Le Hur
We introduce and apply a numerically exact method for investigating the real-time dissipative dynamics of quantum impurities embedded in a macroscopic environment beyond the weak-coupling limit. We focus on the spin-boson Hamiltonian that describes a two-level system interacting with a bosonic bath of harmonic oscillators. This model is archetypal for investigating dissipation in quantum systems and tunable experimental realizations exist in mesoscopic and cold-atom systems. It finds abundant applications in physics ranging from the study of decoherence in quantum computing and quantum optics to extended dynamical mean-field theory. Starting from the real-time Feynman-Vernon path integral, we derive an exact stochastic Schr\"odinger equation that allows to compute the full spin density matrix and spin-spin correlation functions beyond weak coupling. We greatly extend our earlier work (P. P. Orth, A. Imambekov, K. Le Hur, Phys. Rev. A {\bf 82}, 032118 (2010)) by fleshing out the core concepts of the method and by presenting a number of interesting applications. Methodologically, we present an analogy between the dissipative dynamics of a quantum spin and that of a classical spin in a random magnetic field. This analogy is used to recover the well-known Non-Interacting-Blip-Approximation (NIBA) in the weak-coupling limit. We explain in detail how to compute spin-spin autocorrelation functions. As interesting applications of our method, we explore the non-Markovian effects of the initial spin-bath preparation on the dynamics of the coherence $\sigma^x(t)$ and of $\sigma^z(t)$ under a Landau-Zener sweep of the bias field. We also compute to a high precision the asymptotic long-time dynamics of $\sigma^z(t)$ without bias and demonstrate the wide applicability of our approach by calculating the spin dynamics at non-zero bias and different temperatures.
Nov 5
1. arXiv:1211.0293 [pdf, ps, other]
Nonequilibrium dynamics of the ohmic spin-boson model
D. M. Kennes, O. Kashuba, M. Pletyukhov, H. Schoeller, V. Meden
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We study the nonequilibrium dynamics of the unbiased, ohmic spin-boson model close to the coherent-to-incoherent transition. In a first setup the system is prepared in a product state. Using complementary renormalization group methods we obtain numerical results for the spin expectation value on all time scales. They are supplemented by an analytical study for intermediate times. In the coherent regime the time evolution for those is characterized by the subtle interplay of damped oscillatory and purely exponential terms. Secondly, we investigate the dynamics when abruptly switching the coupling from a value in the incoherent regime to one in the coherent one. The incoherent dynamics before the quench heavily affects the one after it up to the extent that a critical coupling strength is required to observe nonmonotonic behavior. This exemplifies the importance of non-markovian memory.
2. arXiv:1211.0321 [pdf, other]
Entanglement Entropy at Generalized RK Points of Quantum Dimer Models
Alexander Selem, C. M. Herdman, K. Birgitta Whaley
We study the $n=2$ R\'enyi entanglement entropy of the triangular quantum dimer model via Monte Carlo sampling of Rokhsar-Kivelson(RK)-like ground state wavefunctions. Using the construction proposed by Kitaev and Preskill [Phys. Rev. Lett. 96, 110404 (2006)] and an adaptation of the Monte Carlo algorithm described in [Phys. Rev. Lett. 104, 157201 (2010)], we compute the topological entanglement entropy (TEE) at the RK point $\gamma = (1.001 \pm .003) \ln 2$ confirming earlier results. Additionally, we compute the TEE of the ground state of a generalized RK-like Hamiltonian and demonstrate the universality of TEE over a wide range of parameter values within a topologically ordered phase approaching a quantum phase transition. For systems sizes that are accessible numerically, we find that the quantization of TEE depends sensitively on correlations. We characterize corner contributions to the entanglement entropy and show that these are well described by shifts proportional to the number and types of corners in the bipartition.
3. arXiv:1211.0539 [pdf, other]
A Majorana smoking gun for the superconductor-semiconductor hybrid topological system
S. Das Sarma, Jay D. Sau, Tudor D. Stanescu
Recent observations of a zero bias conductance peak in tunneling transport measurements in superconductor--semiconductor nanowire devices provide evidence for the predicted zero--energy Majorana modes, but not the conclusive proof for their existence. We establish that direct observation of a splitting of the zero bias conductance peak can serve as the smoking gun evidence for the existence of the Majorana mode. We show that the splitting has an oscillatory dependence on the Zeeman field (chemical potential) at fixed chemical potential (Zeeman field). By contrast, when the density is constant rather than the chemical potential -- the likely situation in the current experimental set-ups -- the splitting oscillations are generically suppressed. Our theory predicts the conditions under which the splitting oscillations can serve as the smoking gun for the experimental confirmation of the elusive Majorana mode.
Oct 29 - Nov 2, Xiaopeng Li
Nov 2
1.arXiv:1211.0006 [pdf, other]
Oscillating terms in the Renyi entropy of Fermi liquids
Brian Swingle, Jeremy McMinis, **Norm M. Tubman**
In this work we compute subleading oscillating terms in the Renyi entropy of Fermi gases and Fermi liquids corresponding to $2k_F$-like oscillations. Our theoretical tools are the one dimensional formulation of Fermi liquid entanglement familiar from discussions of the logarithmic violation of the area law and quantum Monte Carlo calculations. The main result is a formula for the oscillating term for any region geometry and a spherical Fermi surface. We compare this term to numerical calculations of entanglement using the correlation function method and find excellent agreement. We also compare with quantum Monte Carlo data on interacting Fermi liquids where we also find excellent agreement up to moderate interaction strengths.
2. arXiv:1211.0199 [pdf, ps, other]
Matter-wave bright solitons in spin-orbit coupled Bose-Einstein condensates
V. Achilleos, D. J. Frantzeskakis, P. G. Kevrekidis, **D. E. Pelinovsky**
We study matter-wave bright solitons in spin-orbit (SO) coupled Bose-Einstein condensates (BECs) with attractive interatomic interactions. We use a multiscale expansion method to identify solution families for chemical potentials in the semi-infinite gap of the linear energy spectrum. Depending on the linear and spin-orbit coupling strengths, the solitons may resemble either standard bright nonlinear Schr\"{o}dinger solitons or exhibit a modulated density profile, reminiscent of the stripe phase of SO-coupled repulsive BECs. Our numerical results are in excellent agreement with our analytical findings, and demonstrate the potential robustness of such solitons for experimentally relevant conditions through stability analysis and direct numerical simulations.
3. arXiv:1211.0202 [pdf, ps, other]
Superconducting proximity effect in quantum wires without time-reversal symmetry
M. A. Skvortsov, P. M. Ostrovsky, D. A. Ivanov, **Ya. V. Fominov**
We study the superconducting proximity effect in a quantum wire with broken time-reversal (TR) symmetry connected to a conventional superconductor. We consider the situation of a strong TR-symmetry breaking, so that Cooper pairs entering the wire from the superconductor are immediately destroyed. Nevertheless, some traces of the proximity effect survive: for example, the local electronic density of states (LDOS) is influenced by the proximity to the superconductor, provided that localization effects are taken into account. With the help of the supersymmetric sigma model, we calculate the average LDOS in such a system. The LDOS in the wire is strongly modified close to the interface with the superconductor at energies near the Fermi level. The relevant distances from the interface are of the order of the localization length, and the size of the energy window around the Fermi level is of the order of the mean level spacing at the localization length. Remarkably, the sign of the effect is sensitive to the way the TR symmetry is broken: In the spin-symmetric case (orbital magnetic field), the LDOS is depleted near the Fermi energy, whereas for the broken spin symmetry (magnetic impurities), the LDOS at the Fermi energy is enhanced.
Nov 1
1. arXiv:1210.8426 [pdf, other]
Nonlocal correlations in a proximity-coupled normal metal
Taewan Noh, Sam Davis, Venkat Chandrasekhar
We report evidence of large, nonlocal correlations between two spatially separated normal metals in superconductor/normal-metal (SN) heterostructures, which manifest themselves a nonlocal voltage generated in response to a driving current. Unlike prior experiments in SN heterostructures, the nonlocal correlations are mediated not by a superconductor, but by a proximity-coupled normal metal. The nonlocal correlations extend over relatively long length scales in comparison to the superconduncting case. At very low temperatures, we find a reduction in the nonlocal voltage for small applied currents that cannot be explained by the quasiclassical theory of superconductivity. We believe is a signature of new long-range quantum correlations in the system.
2. arXiv:1210.8403 [pdf, other]
Scaling of entanglement entropy across Lifshitz transitions
Marlon Rodney, H. Francis Song, Sung-Sik Lee, Karyn Le Hur, **Erik Sorensen**
We investigate the scaling of the bipartite entanglement entropy across Lifshitz quantum phase transitions, where the topology of the Fermi surface changes without any changes in symmetry. We present both numerical and analytical results which show that Lifshitz transitions are characterized by a well-defined set of critical exponents for the entanglement entropy near the phase transition. In one dimension, we show that the entanglement entropy exhibits a length scale that diverges as the system approaches a Lifshitz critical point. In two dimensions, the leading and sub-leading coefficients of the scaling of entanglement entropy show distinct power-law singularities at critical points. The effect of weak interactions is investigated using the density matrix renormalization group algorithm.
3. arXiv:1210.8282 [pdf, ps, other]
Magnetic edge states in spin triplet superconductors
Alfonso Romano, Paola Gentile, Canio Noce, Ilya Vekhter, Mario Cuoco
We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel. To unveil the microscopic mechanism behind such effect we combine numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a potential barrier modulating the electronic density near the edge of the system leads to a non-unitary superconducting state close to the boundary where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and is manifested via lifting of the spin-degeneracy of the Andreev bound-states.
4. arXiv:1210.8349 (cross-list from quant-ph) [pdf, ps, other]
Topological Flat Bands and Fractional Quantum Hall Effects in Trapped Ion Systems
T. Shi, J. I. Cirac
We propose and analyze a scheme to observe topological phenomena with ions in microtraps. We consider a set of trapped ions forming a regular structure in two spatial dimensions and interacting with lasers. We find phonon bands with non-trivial topological properties, which are caused by the breaking of time reversal symmetry induced by the lasers. We investigate the appearance of edge modes, as well as their robustness against perturbations. Long-range hopping of phonons caused by the Coulomb interaction gives rise to flat bands which, together with induced phonon-phonon interactions, can be used to produce and explore strongly correlated states. Furthermore, some of these ideas can also be implemented with cold atoms in optical lattices.
5. arXiv:1210.8158 (cross-list from hep-th) [pdf, ps, other]
Kinetic theory with Berry curvature from quantum field theories
Dam Thanh Son, Naoki Yamamoto
It has been recently shown that a kinetic theory can be modified to incorporate triangle anomalies and the chiral magnetic effect by taking into account the Berry curvature flux through the Fermi surface. We show how such a kinetic theory can be derived from underlying quantum field theories. Using the new kinetic theory, we also compute the parity-odd correlation function that is found to be identical to the result in the perturbation theory in the next-to-leading-order hard dense loop approximation.
Oct 31
1. arXiv:1210.8134 [pdf, other]
Classical Topological Order in Abelian and Non-Abelian Generalized Height Models
R. Zach Lamberty, Stefanos Papanikolaou, Christopher L. Henley
We present Monte Carlo simulations on a new class of lattice models in which the degrees of freedom are elements of an abelian or non-abelian finite symmetry group G, placed on directed edges of a two-dimensional lattice. The plaquette group product is constrained to be the group identity. In contrast to discrete gauge models (but similar to past work on height models) only elements of symmetry-related subsets S of G are allowed on edges. These models have topological sectors labeled by group products along topologically non-trivial loops. Measurement of relative sector probabilities and the distribution of distance between defect pairs are done to characterize the types of order (topological or quasi-LRO) exhibited by these models. We present particular models in which fully local non-abelian constraints lead to global topological liquid properties.
2. arXiv:1210.8035 [pdf, ps, other]
Fractional quantum Hall states of a Bose gas with spin-orbit coupling
T. Grass, B. Juliá-Díaz, **M. Lewenstein**
We study the fractional quantum Hall phases of a pseudospin-1/2 Bose gas in an artificial gauge field. In addition to an external magnetic field, the gauge field also mimics an intrinsic spin-orbit coupling of the Rashba type. While the spin degeneracy of the Landau levels is lifted by the spin-orbit coupling, the crossing of two Landau levels at certain coupling strengths gives rise to a new degeneracy. We therefore take into account two Landau levels, and perform exact diagonalization of the many-body Hamiltonian. We study and characterize the quantum Hall phases which occur in the vicinity of the degeneracy point. Notably, we describe the different states appearing at the Laughlin filling, \nu=1/2. While for this filling incompressible phases disappear at the degeneracy point, denser systems at \nu=3/2 and \nu=2 are found to be clearly gapped. For filling factors \nu=2/3 and \nu=4/3, we discuss the connection of the exact ground state to the non-Abelian spin singlet states, obtained as the ground state of k+1 body contact interactions.
Oct 30
1. arXiv:1210.7370 [pdf, ps, other]
Model for magnetic flux pattern induced by in-plane magnetic fields on spatially inhomogeneous superconducting interfaces of LaAlO_3-SrTiO_3
Kazushi Aoyama, Manfred Sigrist
The effect of spatial inhomogeneity on the properties of a two-dimensional non-centrosymmetric superconductor in an in-plane magnetic field is investigated, as it can be realized in LaAlO_3-SrTiO_3 interfaces. We demonstrate that the spatial variation of Rashba spin-orbit coupling (RSOC) yields a local magnetic flux pattern due to the field-induced inhomogeneous helical phase. For sufficiently strong fields, vortices can nucleate at inhomogeneities of the RSOC.
Oct 29
1. arXiv:1210.7113 [pdf, other]
Direct band structure measurement of a buried two-dimensional electron gas
Jill A. Miwa, Philip Hofmann, Michelle Y. Simmons, Justin W. Wells
Buried two dimensional electron gasses (2DEGs) have recently attracted considerable attention as a testing ground for both fundamental physics and quantum computation applications. Such 2DEGs can be created by phosphorus delta (\delta) doping of silicon, a technique in which a dense and narrow dopant profile is buried beneath the Si surface. Phosphorous \delta-doping is a particularly attractive platform for fabricating scalable spin quantum bit architectures, compatible with current semiconductor technology. The band structure of the \delta-layers that underpin these devices has been studied intensely using different theoretical methods, but it has hitherto not been possible to directly compare these predictions with experimental data. Here we report the first measurement of the electronic band structure of a \delta-doped layer below the Si(001) surface by angle resolved photoemission spectroscopy (ARPES). Our measurements confirm the layer to be metallic and give direct access to the Fermi level position. Surprisingly, the direct observation of the states is possible despite them being buried far below the surface. Using this experimental approach, buried states in a wide range of other material systems, including metallic oxide interfaces, could become accessible to direct spectroscopic investigations.
