Oct 22 - Oct 26, Saubhik Sarkar
Oct 26
1.[3] arXiv:1210.6767 [pdf, ps, other]
Bound states in the one-dimensional two-particle Hubbard model with an impurity
J. M. Zhang, Daniel Braak, Marcus Kollar
We investigate bound states in the one-dimensional two-particle Bose-Hubbard model with an attractive ($V> 0$) impurity potential. This is a one-dimensional, discrete analogy of the hydrogen negative ion H$^-$ problem. There are several different types of bound states in this system, each of which appears in a specific region. For given $V$, there exists a (positive) critical value $U_{c1}$ of $U$, below which the ground state is a bound state. Interestingly, close to the critical value ($U\lesssim U_{c1}$), the ground state can be described by the Chandrasekhar-type variational wave function, which was initially proposed for H$^-$. For $U>U_{c1}$, the ground state is no longer a bound state. However, there exists a second (larger) critical value $U_{c2}$ of $U$, above which a molecule-type bound state is established and stabilized by the repulsion. We have also tried to solve for the eigenstates of the model using the Bethe ansatz. The model possesses a global $\Zz_2$-symmetry (parity) which allows classification of all eigenstates into even and odd ones. It is found that all states with odd-parity have the Bethe form, but none of the states in the even-parity sector. This allows us to identify analytically two odd-parity bound states, which appear in the parameter regions $-2V<U<-V$ and $-V<U<0$, respectively. Remarkably, the latter one can be \textit{embedded} in the continuum spectrum with appropriate parameters. Moreover, in part of these regions, there exists an even-parity bound state accompanying the corresponding odd-parity bound state with almost the same energy.
2. arXiv:1210.6701 [pdf, ps, other]
Two trapped particles interacting by a finite-ranged two-body potential in two spatial dimensions
Rostislav A. Doganov, Shachar Klaiman, Ofir E. Alon, Alexej I. Streltsov, Lorenz S. Cederbaum
We examine the problem of two particles confined in an isotropic harmonic trap, which interact via a finite-ranged Gaussian-shaped potential in two spatial dimensions. We derive an approximative transcendental equation for the energy and study the resulting spectrum as a function of the interparticle interaction strength. Both the attractive and repulsive systems are analyzed. We study the impact of the potential's range on the ground-state energy. Complementary, we also explicitly verify by a variational treatment that in the zero-range limit the positive delta potential in two dimensions only reproduces the non-interacting results, if the Hilbert space in not truncated. Finally, we establish and discuss the connection between our finite-range treatment and regularized zero-range results from the literature.
Oct 25
1.arXiv:1210.6504 [pdf, other]
Collisions of anisotropic two-dimensional bright solitons in dipolar Bose-Einstein condensates
Rüdiger Eichler, Damir Zajec, Patrick Köberle, Jörg Main, Günter Wunner
We investigate the coherent collision of anisotropic quasi-two-dimensional bright solitons in dipolar Bose-Einstein condensates. Our analysis is based on the extended Gross-Pitaevskii equation, and we use the split-operator method for the grid calculations and the time-dependent variational principle with an ansatz of coupled Gaussian functions to calculate the time evolution of the ground state. We compare the results of both approaches for collisions where initially the solitons are in the repelling side-by-side configuration and move towards each other with a specific momentum. We change the relative phases of the condensates, and introduce a total angular momentum by shifting the solitons in opposite direction along the polarization axis. Our calculations show that collisions result in breathing-mode-like excitations of the solitons.
Oct 24
1.arXiv:1210.6025 (cross-list from quant-ph) [pdf, ps, other]
Controlling the Momentum Current of an Off-resonant Ratchet
R. K. Shrestha, J. Ni, W. K. Lam, S. Wimberger, G. S. Summy
We experimentally investigate the phenomenon of a quantum ratchet created by exposing a Bose-Einstein Condensate to short pulses of a potential which is periodic in both space and time. Such a ratchet is manifested by a directed current of particles, even though there is an absence of a net bias force. We confirm a recent theoretical prediction [M. Sadgrove and S. Wimberger, New J. Phys. \textbf{11}, 083027 (2009)] that the current direction can be controlled by experimental parameters which leave the underlying symmetries of the system unchanged. We demonstrate that this behavior can be understood using a single variable containing many of the experimental parameters and thus the ratchet current is describable using a single universal scaling law.
Oct 23
1.arXiv:1210.5745 [pdf, ps, other]
Phase diagram of the Bose-Hubbard model on a ring-shaped lattice with tunable weak links
Kalani Hettiarachchilage, Valéry G. Rousseau, Ka-Ming Tam, Mark Jarrell, Juana Moreno
Motivated by recent experiments on toroidal Bose-Einstein condensates in all-optical traps with tunable weak links, we study the one-dimensional Bose-Hubbard model on a ring-shaped lattice with a small region of weak hopping integrals using quantum Monte Carlo simulations. Besides the usual Mott insulating and superfluid phases, we find a novel phase which is compressible but non superfluid with a local Mott region. This `local Mott' phase extends in a large region of the phase diagram. These results suggest that the insulating and conducting phases can be tuned by a local parameter which may provide a new insight to the design of atomtronic devices.
2. arXiv:1210.5518 [pdf, other]
Topological phases in polar-molecule quantum magnets
Salvatore R. Manmana, E. M. Stoudenmire, Kaden R. A. Hazzard, Ana Maria Rey, Alexey V. Gorshkov
We show how to use polar molecules in an optical lattice to engineer quantum spin models with arbitrary spin S >= 1/2 and with interactions featuring a direction-dependent spin anisotropy. This is achieved by encoding the effective spin degrees of freedom in microwave-dressed rotational states of the molecules and by coupling the spins through dipolar interactions. We demonstrate how one of the experimentally most accessible anisotropies stabilizes symmetry protected topological phases in spin ladders. Using the numerically exact density matrix renormalization group method, we find that these phases - previously studied only in the nearest-neighbor case - survive in the presence of long-range dipolar interactions. We also show how to use our approach to realize the bilinear-biquadratic spin-1 and the Kitaev honeycomb models. Experimental detection schemes and imperfections are discussed.
Oct 22
1.arXiv:1205.1806 (replaced) [pdf, ps, other]
Quantum phases of Bose-Bose mixtures on a triangular lattice
Liang He, Yongqiang Li, Ehud Altman, Walter Hofstetter
We investigate the zero temperature quantum phases of a Bose-Bose mixture on a triangular lattice using Bosonic Dynamical Mean Field Theory (BDMFT). We consider the case of total filling one where geometric frustration arises for asymmetric hopping. We map out a rich ground state phase diagram including xy-ferromagnetic, spin-density wave, superfluid, and supersolid phases. In particular, we identify a stripe spin-density wave phase for highly asymmetric hopping. On top of the spin-density wave, we find that the system generically shows weak charge (particle) density wave order.
2.arXiv:1111.5371 (replaced) [pdf, ps, other]
Transport criticality in triangular lattice Hubbard model
Toshihiro Sato, Kazumasa Hattori, Hirokazu Tsunetsugu
We study electric transport near the Mott metal-insulator transition. Optical conductivity of the half-filled Hubbard model on a triangular lattice is calculated based on a cellular dynamical mean field theory including vertex corrections inside the cluster. By investigating the spectrum at low frequencies, we find that a Drude peak on the metallic side smoothly connects to an "ingap" peak on the insulating side. The optical weight of these peaks exhibits a critical behavior with power-law near the Mott critical end point, $|D-D^*|\propto|U-U^*|^{1/\delta}$. We find that the critical exponent $1/\delta$ differs from the exponents in the thermodynamics.
Oct 15 - Oct 19, Johannes Schachenmayer
Oct 19
1. arXiv:1210.5030 [pdf, ps, other]
Zitterbewegung effect in spin-1 ultracold atoms
Yi-Cai Zhang, Song-Wei Song, Chao-Fei Liu, W. M. Liu
The Zitterbewegung-like effect in spin-1 cold atoms is investigated in the presence of the Zeeman field and external parabolic trap. It is shown that the Zeeman field and parabolic trap have significant effect on the Zitterbewegung oscillatory behaviors. Multi-frequency Zitterbewegung oscillation can be induced by the applied Zeeman field in comparison to that in two components atoms. In addition, a much slowly damping Zitterbewegung oscillation can be achieved by adjusting the linear and quadratic zeeman parameters properly, which is favorable to the observation of Zitterbewegung oscillation in the experiments. In the presence of the harmonic trap, the subpackets corresponding to different eigenenergies would always keep coherent, resulting in persistent Zitterbewegung oscillations. If the initial condition of Gaussian packet is prepared, the Zitterbewegung oscillation would display very complicated and irregular oscillation characteristics due to coexistence of different frequencies of the Zitterbewegung oscillation.
2. arXiv:1210.5139 [pdf, ps, other]
Measuring nontrivial fusion rule of Majorana fermions in inhomogeneous transverse Ising chain
Yin-Chen He, Yan Chen
We describe various dynamical processes aimed to create and fuse Majorana fermions in inhomogeneous transverse field Ising chain. The number of excitations involved in the process can be greatly suppressed when a part of the inhomogeneous system crosses the quantum phase transition point so that one can reach the limit when only few Majorana fermions are involved. It is shown that fusion of Majorana fermions may result in universal excitations, which may serve as a direct verification of the Majorana fermions fusion rule. Moreover, oscillatory tunneling between two unpaired Majorana fermions may show up. Our theoretical proposal provides a promising way to test nontrivial properties of Majorana fermions experimentally.
3. arXiv:1210.4807 [pdf, ps, other]
Disordered bosons in one dimension: from weak to strong randomness criticality
Fawaz Hrahsheh, Thomas Vojta
We investigate the superfluid-insulator quantum phase transition of one-dimensional bosons with off-diagonal disorder by means of large-scale Monte-Carlo simulations. For weak disorder, we find the transition to be in the same universality class as the superfluid-Mott insulator transition of the clean system. The nature of the transition changes for stronger disorder. Beyond a critical disorder strength, we find nonuniversal, disorder-dependent critical behavior. We compare our results to recent perturbative and strong-disorder renormalization group predictions. We also discuss experimental implication as well as extensions of our results to other systems.
4. arXiv:1210.4774 [pdf, ps, other]
Nagaoka states in the SU($n$) Hubbard model
Hosho Katsura, Akinori Tanaka
We present an extension of Nagaoka's theorem in the SU($n$) generalization of the infinite-$U$ Hubbard model. It is shown that, when there is exactly one hole, the fully polarized states analogous to the ferromagnetic states in the SU(2) Hubbard model are ground states. For a restricted class of models satisfying the connectivity condition, these fully polarized states are the unique ground states up to the trivial degeneracy due to the SU($n$) symmetry. We also give examples of lattices in which the connectivity condition can be verified explicitly. The examples include the triangular, kagome, and the hypercubic lattices in $d (\ge 2)$ dimensions, among which $d=2$ and 3 cases are experimentally realizable in ultracold atomic gases loaded into optical lattices.
Oct 18
1. arXiv:1210.4834 [pdf, other]
Persistent currents in spinor condensates
Scott Beattie, Stuart Moulder, Richard J. Fletcher, Zoran Hadzibabic
We create and study persistent currents in a toroidal two-component Bose gas, consisting of $^{87}$Rb atoms in two different spin states. For a large spin-population imbalance we observe supercurrents persisting for over two minutes. However we find that the supercurrent is unstable for spin polarisation below a well defined critical value. We also investigate the role of phase coherence between the two spin components and show that only the magnitude of the spin-polarisation vector, rather than its orientation in spin space, is relevant for supercurrent stability.
2. arXiv:1210.4554 [pdf, other]
Sudden quantum quenches in the presence of frustration or background synthetic gauge fields: Probing equilibrium currents of ultracold atoms in an optical lattice
Matthew Killi, Stefan Trotzky, Arun Paramekanti
Bosons and fermions, in the presence of frustration or background gauge fields, can form manybody ground states which support equilibrium 'charge' or 'spin' currents. Motivated by the experimental creation of frustration or artificial gauge fields in ultracold atomic systems, we propose a general scheme by which making a sudden anisotropic quench of the atom tunneling across the lattice and tracking the ensuing density modulations provides a powerful and gauge invariant route to visualizing diverse equilibrium current patterns. Using illustrative examples of trapped superfluid Bose and normal Fermi systems in the presence of artificial magnetic fluxes on square lattices, and frustrated bosons in a triangular lattice, we show that this scheme to probe equilibrium bulk current order works independent of particle statistics. We also show that such quenches can detect chiral edge currents in gapped topological states, such as quantum Hall or quantum spin Hall insulators.
3. arXiv:1210.4772 [pdf, ps, other]
A Multi-path Interferometer with Ultracold Atoms Trapped in an Optical Lattice
J. Chwedenczuk, F. Piazza, A. Smerzi
We study an ultra-cold gas of $N$ bosons trapped in a one dimensional $M$-site optical lattice perturbed by a spatially dependent potential $g\cdot x^j$, where the unknown coupling strength $g$ is to be estimated. We find that the measurement uncertainty is bounded by $\Delta g\propto\frac1{N (M^j-1)}$. For a typical case of a linear potential, the sensitivity improves as $M^{-1}$, which is a result of multiple interferences between the sites -- an advantage of multi-path interferometers over the two-mode setups. Next, we calculate the estimation sensitivity for a specific measurement where, after the action of the potential, the particles are released from the lattice and form an interference pattern. If the parameter is estimated by a least-square fit of the average density to the interference pattern, the sensitivity still scales like $M^{-1}$ for linear potentials and can be further improved by preparing a properly correlated initial state in the lattice.
Oct 17
1. arXiv:1210.4270 [pdf, ps, other]
Dynamics of an impurity in a one-dimensional lattice
Francesco Massel, Adrian Kantian, Andrew J. Daley, Thierry Giamarchi, Päivi Törmä
We study the non-equilibrium dynamics of an impurity in an harmonic trap that is kicked with a well-defined quasi-momentum, and interacts with a bath of free fermions or interacting bosons in a 1D lattice configuration. Using numerical and analytical techniques we investigate the full dynamics beyond linear response, which allows us to quantitatively characterise states of the impurity in the bath for different parameter regimes. These vary from a tightly bound molecular state in a strongly interacting limit to a polaron (dressed impurity) and a free particle for weak interactions, with composite behaviour in the intermediate regime. These dynamics and different parameter regimes should be readily realizable in systems of cold atoms in optical lattices.
2. arXiv:1210.4338 [pdf, other]
Spontaneous time-reversal symmetry breaking for spinless fermions on a triangular lattice
Olivier Tieleman, Omjyoti Dutta, Maciej Lewenstein, André Eckardt
As a minimal fermionic model with kinetic frustration, we study a system of spinless fermions in the lowest band of a triangular lattice with long-range repulsion. We find that the combination of interactions and kinetic frustration leads to spontaneous symmetry breaking in various ways. Time-reversal symmetry can be broken by two types of loop current patterns, a chiral one and one that breaks the translational lattice symmetry. Moreover, the translational symmetry can also be broken by a density wave forming a kagome pattern or by a Peierls-type trimerization characterized by enhanced correlations among the sites of certain triangular plaquettes (giving a plaquette-centered density wave). We map out the phase diagram as it results from leading order Ginzburg-Landau mean-field theory. Several experimental realizations of the type of system under study are possible with ultracold atoms in optical lattices.
3. arXiv:1210.4183 [pdf, ps, other]
Effective potential renormalization and polaronic mass shift in a trapped dynamical impurity-luttinger liquid system
Julius Bonart, Leticia F. Cugliandolo
Recent experiments with cold atoms on the impurity motion in one-dimensional liquids of interacting bosons have revealed an interesting interplay between the polaronic impurity mass shift and the renormalization of the optical potential. We show that the influence of the external trap on the Bose gas leads to a steeper effective potential for the impurity. We propose a framework in which this potential renormalization and the mass shift can be quantitatively understood by combining a semi-classical theory of density wave excitations in the Luttinger liquid with the non equilibrium formalism of a quantum Brownian particle. The obtained theoretical results reproduce well recent experimental data.
Oct 16
1. arXiv:1210.4131 [pdf, ps, other]
Pair condensation in a Finite Trapped Fermi Gas
C. N. Gilbreth, Y. Alhassid
We study signatures of the superfluid phase transition and the pseudogap phase in a trapped finite-size system of spin-1/2 fermions with infinite scattering length. Applying the auxiliary-field quantum Monte Carlo (AFMC) method in the canonical ensemble, we compute the energy-staggering pairing gap, heat capacity, and condensate fraction as a function of temperature. Our calculations reveal clear signatures of the superfluid phase transition in all three quantities, including a signature of the lambda peak in the heat capacity. Comparing the temperature dependence of these quantities shows that the pairing gap does not exhibit an obvious pseudogap effect in this system.
2. arXiv:1210.3994 [pdf, other]
The Snake Instability of Ring Dark Solitons in Toroidally Trapped Bose-Einstein Condensates
Lauri Toikka, Kalle-Antti Suominen
We show that the onset of the snake instability of ring dark solitons requires a broken symmetry. We also elucidate explicitly the connection between imaginary Bogoliubov modes and the snake instability, predicting the number of vortex-anti-vortex pairs produced. In addition, we propose a simple model to give a physical motivation as to why the snake instability takes place. Finally, we show that tight confinement in a toroidal potential actually enhances soliton decay due to inhibition of soliton motion.
3. arXiv:1210.3969 [pdf, ps, other]
Ground state properties and excitation spectrum of a two dimensional gas of bosonic dipoles
A. Macia, F. Mazzanti, J. Boronat
We present a quantum Monte Carlo study of two-dimensional dipolar Bose gases in the limit of zero temperature. The analysis is mainly focused on the anisotropy effects induced in the homogeneous gas when the polarization angle with respect to the plane is changed. We restrict our study to the regime where the dipolar interaction is strictly repulsive, although the strength of the pair repulsion depends on the vector interparticle distance. Our results show that the effect of the anisotropy in the energy per particle scales with the gas parameter at low densities as expected, and that this scaling is preserved for all polarization angles even at the largest densities considered here. We also evaluate the excitation spectrum of the dipolar Bose gas in the context of the Feynman approximation and compare the results obtained with the Bogoliubov ones. As expected, we find that these two approximations agree at very low densities, while they start to deviate from each other as the density increases. For the largest densities studied, we observe a significant influence of the anisotropy of the dipole-dipole interaction in the excitation spectrum.
4. arXiv:1210.3703 [pdf, other]
Inter-band tunneling near the merging transition of Dirac cones
J.-N. Fuchs, L.-K. Lim, G. Montambaux
Motivated by a recent experiment in a tunable graphene analog [L. Tarruell et al., Nature 483, 302 (2012)], we consider a generalization of the Landau-Zener problem to the case of a quadratic crossing between two bands in the vicinity of the merging transition of Dirac cones. The latter is described by the so-called universal hamiltonian. In this framework, the inter-band tunneling problem depends on two dimensionless parameters: one measures the proximity to the merging transition and the other the adiabaticity of the motion. Under the influence of a constant force, the probability for a particle to tunnel from the lower to the upper band is computed numerically in the whole range of these two parameters and analytically in different limits using (i) the Stueckelberg theory for two successive linear band crossings, (ii) diabatic perturbation theory, (iii) adiabatic perturbation theory and (iv) a modified Stueckelberg formula. We obtain a complete phase diagram and explain the presence of unexpected probability oscillations in terms of interferences between two poles in the complex time plane. We also compare our results to the above mentioned experiment.
5. arXiv:1210.3623 [pdf, other]
Keldysh approach for non-equilibrium phase transitions in quantum optics: Dicke model in optical cavities
Emanuele G. Dalla Torre, Sebastian Diehl, Mikhail Lukin, Philipp Strack
We investigate non-equilibrium phase transitions for driven atomic ensembles interacting with a cavity mode that is subject to dissipation. We first discuss that the relevant modes in the photon spectrum of the driven Dicke model are effectively thermal at low frequencies. The effective temperature of the photons is set by the atom-photon interaction strength and characterizes the static and dynamic critical exponents of the associated superradiance transition. Motivated by these considerations, we develop a general Keldysh path integral approach to describe phase transitions in open quantum-optical systems. This approach is used to compute both the low-frequency and the high frequency dynamics and of the driven Dicke model with Markovian dissipation for the cavity photons and for the atoms. We re-derive the thermal properties of the low-temperature dynamics, while at higher frequencies, we show that the photon and atom distribution functions exhibit quantum and non-equilibrium regimes.
6. arXiv:1210.4040 [pdf, ps, other]
Quenched dynamics of two-dimensional solitons andvortices in the Gross-Pitaevskii equation
Qian-Yong Chen, P. G. Kevrekidis, Boris A. Malomed
We consider a two-dimensional (2D) counterpart of the experiment that led to the creation of quasi-1D bright solitons in Bose-Einstein condensates (BECs) [Nature 417, 150--153 (2002)]. We start by identifying the ground state of the 2D Gross-Pitaevskii equation for repulsive interactions, with a harmonic-oscillator (HO) trap, and with or without an optical lattice (OL). Subsequently, we switch the sign of the interaction to induce interatomic attraction and monitor the ensuing dynamics. Regions of the stable self-trapping and catastrophic collapse of 2D fundamental solitons are identified in the parameter plane of the OL strength and BEC norm. The increase of the OL strength expands the persistence domain for the solitons to larger norms. For single-charged solitary vortices, in addition to the survival and collapse regimes, an intermediate one is identified, where the vortex resists the collapse but loses its structure, transforming into a fundamental soliton. The same setting may also be implemented in the context of optical solitons and vortices, using photonic-crystal fibers.
Oct 15
Oct 8 - Oct 12, Xiaopeng Li
Oct 12
1. arXiv:1210.3341 [pdf, other]
Large deviations and universality in quantum quenches
Andrea Gambassi, Alessandro Silva
We study the large deviations statistics of the intensive work done by changing globally a control parameter in a thermally isolated quantum many-body system. We show that, upon approaching a critical point, large deviations well below the mean work display universal features related to the critical Casimir effect in the corresponding classical system. Large deviations well above the mean are, instead, of quantum nature and not captured by the quantum-to-classical correspondence. For a bosonic system we show that in this latter regime a transition from exponential to power-law statistics, analogous to the equilibrium Bose-Einstein condensation, may occur depending on the parameters of the quench and on the spatial dimensionality.
2. arXiv:1210.3237 [pdf, other]
Signatures of topological phase transitions in mesoscopic superconducting rings
Falko Pientka, Alessandro Romito, Mathias Duckheim, Yuval Oreg, Felix von Oppen
We investigate Josephson currents in mesoscopic rings with a weak link which are in or near a topological superconducting phase. As a paradigmatic example, we consider the Kitaev model of a spinless p-wave superconductor in one dimension, emphasizing how this model emerges from more realistic settings based on semiconductor nanowires. We show that the flux periodicity of the Josephson current provides signatures of the topological phase transition and the emergence of Majorana fermions situated on both sides of the weak link even when fermion parity is not a good quantum number. In large rings, the Majorana fermions hybridize only across the weak link. In this case, the Josephson current is h/e periodic in the flux threading the loop when fermion parity is a good quantum number but reverts to the more conventional h/2e periodicity in the presence of fermion-parity changing relaxation processes. In mesoscopic rings, the Majorana fermions also hybridize through their overlap in the interior of the superconducting ring. We find that in the topological superconducting phase, this gives rise to an h/e-periodic contribution even when fermion parity is not conserved and that this contribution exhibits a peak near the topological phase transition. This signature of the topological phase transition is robust to the effects of disorder. As a byproduct, we find that close to the topological phase transition, disorder drives the system deeper into the topological phase. This is in stark contrast to the known behavior far from the phase transition, where disorder tends to suppress the topological phase.
3.arXiv:1210.3070 [pdf, ps, other]Anyonic braiding phases in a rotating strongly correlated photon gas
R. O. Umucalilar, I. Carusott
We present a theoretical study of a rotating trapped photon gas where a Laguerre-Gauss laser pump with a non-zero orbital angular momentum is used to inject rotating photons into a cavity with strong optical nonlinearity. The Laughlin-like few-photon eigenstates appear as sharp resonances in the transmission spectra. Using additional localized repulsive potentials, quasi-holes can be created in the quantum Hall liquid of photons and then braided around in space: an unambiguous signature of the many-body Berry phase under exchange of two quasi-holes is observed as a spectral shift of the corresponding transmission resonance.
Oct 11
1. arXiv:1210.2898 [pdf, ps, other]
Quasihole dynamics as a detection tool for quantum Hall phases
Tobias Graß, Bruno Juliá-Díaz, Maciej Lewenstein
Existing techniques for synthesizing gauge fields are able to bring a two-dimensional cloud of harmonically trapped bosonic atoms into a regime where the occupied single-particle states are restricted to the lowest Landau level (LLL). Repulsive short-range interactions drive various transitions from fully condensed into strongly correlated states. In these different phases we study the response of the system to quasihole excitations induced by a laser beam. We find that in the Laughlin state the quasihole performs a coherent constant rotation around the center, ensuring conservation of angular momentum. This is distinct to any other regime with higher density, where the quasihole is found to decay. At a characteristic time, the decay process is reversed, and revivals of the quasihole can be observed in the density. Measuring the period and position of the revival can be used as a spectroscopic tool to identify the strongly correlated phases in systems with a finite number of atoms.
2. arXiv:1210.2846 [pdf, other]
The electronic structure of NaIrO$_3$, Mott insulator or band insulator?
Liang Du, Xianlei Sheng, Hongming Weng, Xi Dai
Motivated by the unveiled complexity of nonmagnetic insulating behavior in pentavalent post-perovskite NaIrO$_3$, we have studied its electronic structure and phase diagram in the plane of Coulomb repulsive interaction and spin-orbit coupling (SOC) by using the newly developed local density approximation plus Gutzwiller method. Our theoretical study proposes the metal-insulator transition can be generated by two different physical pictures: renormalized band insulator or Mott insulator regime. For the realistic material parameters in NaIrO$_3$, Coulomb interaction $U=2.0 (J=U/4)$ eV and SOC strength $\eta=0.33$ eV, it tends to favor the renormalized band insulator picture as revealed by our study.
3. arXiv:1210.2791 [pdf, ps, other]
Many-body energy localization transition in periodically driven systems
Luca D'Alessio, Anatoli Polkovnikov
According to the second law of thermodynamics the total entropy of a system is increased during almost any dynamical process. The positivity of the specific heat implies that the entropy increase is associated with heating. This is generally true both at the single particle level, like in the Fermi acceleration mechanism of charged particles reflected by magnetic mirrors, and for complex systems in everyday devices. Notable exceptions are known in noninteracting systems of particles moving in periodic potentials. Here the phenomenon of dynamical localization can prevent heating beyond certain threshold. The dynamical localization is known to occur both at classical (Fermi-Ulam model) and at quantum levels (kicked rotor). However, it was believed that driven ergodic systems will always heat without bound. Here, on the contrary, we report strong evidence of dynamical localization transition in periodically driven ergodic systems in the thermodynamic limit. This phenomenon is reminiscent of many-body localization in energy space.
Oct 10
1. arXiv:1210.2616 [pdf, other]
A microscopic model for ultrafast remagnetization dynamics
Raghuveer Chimata, Anders Bergman, Lars Bergqvist, Biplab Sanyal, Olle Eriksson
In this letter, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above Stoner-Curie temperature by a strong laser fluence. Combining first principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation and a three temperature model, we show the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macro spin moment that evolves non-monotonically.
Oct 9
1. arXiv:1210.2114 [pdf, ps, other]
The nonlinear Dirac equation: Relativistic vortices and experimental realization in Bose-Einstein condensates
L. H. Haddad, K. M. O'Hara, Lincoln D. Carr
We present a detailed experimental procedure for preparing relativistic vortices, governed by the nonlinear Dirac equation, in a two-dimensional Bose-Einstein condensate (BEC) in a honeycomb optical lattice. Our setup contains Dirac points, in direct analogy to graphene. We determine a range of practical values for all relevant physical parameters needed to realize relativistic vortices in BEC of $^{87}\mathrm{Rb}$ atoms. Seven distinct vortex types, including Anderson-Toulouse and Mermin-Ho skyrmion textures and half-quantum vortices, are obtained, and their discrete spectra and stability properties are calculated in a weak harmonic trap. We find that most vortices are stable with a lifetime between 1 and 10 seconds.
2.arXiv:1210.2072 [pdf, other]Symmetry-protected topological phases of alkaline-earth cold fermionic atoms in one dimension
H. Nonne, M. Moliner, S. Capponi, P. Lecheminant, K. Totsuka
We investigate the existence of symmetry-protected topological phases in one-dimensional alkaline-earth cold fermionic atoms with general half-integer nuclear spin I at half filling. Using complementary techniques, we show that SU(2) topological phases are stabilized where the SU(2) symmetry stems from the existence of a metastable excited state in alkaline-earth atoms. On top of these phases, we find the emergence of topological phases with enlarged SU(2I+1) symmetry which depend only on the nuclear spins degrees of freedom. The main physical properties of the latter phases are further studied using a matrix-product state approach. We find that these phases are symmetry-protected topological phases, with respect to inversion symmetry, when I=1/2,5/2,9/2..., which is directly relevant to ytterbium and strontium cold fermions.
3.arXiv:1210.1859 [pdf, ps, other]Orbital phases of fermions in an asymmetric optical ladder
Xiaopeng Li, W. Vincent Liu
We study a quantum ladder of interacting fermions with coupled s and p orbitals. Such a model describes dipolar molecules or atoms loaded into a double-well optical lattice, dipole moments being aligned by an external field. The two orbital components have distinct hoppings. The tunneling between them is equivalent to a partial Rashba spin-orbital coupling when the orbital space (s, p) is identified as spanned by pseudo-spin 1/2 states. A rich phase diagram, including incommensurate orbital density wave, pair density wave and other exotic superconducting phases, is proposed with bosonization analysis. In particular, superconductivity is found in the repulsive regime.
Oct 8
1. arXiv:1210.1826 [pdf]
Topological Flat Band in a Two-Dimensional Organometallic Framework
Zheng Liu, Zheng-Fei Wang, Jia-Wei Mei, Yong-Shi Wu, Feng Liu
Discoveries of unconventional band structures in solid-state materials, such as graphene and topological insulators, have inspired a wide spectrum of theoretical advances and technological innovations. By combining exotic band dispersion with nontrivial band topology, an interesting type of band, namely the topological flat band (TFB), has recently been proposed, in which carriers experience strong Coulomb interaction as well as topological frustration that in together spawn unprecedented topological many-body electronic states, i.e. high-temperature fractional quantum hall state. Despite the proposal of several theoretical lattice models, however, it remains a doubt whether such a "romance of flatland" could exist in a real material, which requires a delicate balance of (1) lattice geometry, (2) spin-orbit coupling (SOC) and (3) ferromagnetism. Here, we present a first-principles design to fulfill all three requirements in a two-dimensional (2D) Indium-Phenylene Organometallic Framework (IPOF) that realizes a nearly flat topological band right around the Fermi level. Our design in addition provides a general strategy to synthesize topologically nontrivial materials in virtue of organic chemistry and nanotechnology.
2. arXiv:1210.1587 [pdf, ps, other]
Unconventional superconducting states of interlayer pairing in bilayer and trilayer graphene
Mir Vahid Hosseini, Malek Zareyan
We develop a theory for interlayer pairing of chiral electrons in graphene materials which results in an unconventional superconducting (S) state with s-wave spin-triplet order parameter. In a pure bilayer graphene, this superconductivity exhibits a gapless property with an exotic effect of temperature-induced condensation causing an increase of the pairing amplitude (PA) with increasing temperature. We find that a finite doping opens a gap in the excitation spectrum and weakens this anomalous temperature-dependence. We further explore the possibility of realizing variety of pairing patterns with different topologies of the Fermi surface, by tuning the difference in the doping of the two layers. In trillayer graphene, the interlayer superconductivity is characterized by a two components order parameter which can be used to define two distinct phases in which only one of the components is non vanishing. For ABA stacking the stable state is determined by a competition between these two phases. By varying the relative amplitude of the corresponding coupling strenghes, a first order phase transition can occur between these two phases. For ABC stacking, we find that the two phases coexist with a possibility of a similar phase transition which turns out to be second order.
Oct 1 - Oct 5, Saubhik Sarkar
Oct 5
1.arXiv:1210.1426 [pdf, other]
Observing the Drop of Resistance in the Flow of a Superfluid Fermi Gas
David Stadler, Sebastian Krinner, Jakob Meineke, Jean-Philippe Brantut, Tilman EsslingerIn this work, we investigate the conduction properties of strongly interacting fermions flowing through a quasi two-dimensional, multimode channel, which connects two atomic reservoirs. The atomic current in the channel is controlled using a repulsive potential created by an off-resonant laser beam. In analogy with an electronic field-effect transistor, this gate potential controls the chemical potential in the channel while keeping the temperature imposed by the reservoirs unchanged. With the gate potential as a control parameter, we measure the current through the channel over a large dynamic range and determine the density distribution in the channel region. This allows us to observe the onset of superfluid flow of strongly interacting fermions. These measurements are compared to the case of a weakly interacting Fermi gas.
2.arXiv:1210.1400 [pdf, other]
Fractional Excitations in Cold Atomic Gases
Jens Honer, Jad C. Halimeh, Ian McCulloch, Ulrich Schollwöck, Hans Peter Büchler
We study the behavior of excitations in the tilted one-dimensional Bose-Hubbard model. In the phase with broken symmetry, fundamental excitations are domain-walls which show fractional statistics. Using perturbation theory, we derive an analytic model for the time evolution of these fractional excitations, and demonstrate the existence of a repulsively bound state above a critical center of mass momentum. The validity of the perturbative analysis is confirmed by the use of t- DMRG simulations. These findings open the path for experimental detection of fractional particles in cold atomic gases.
Oct 4
1. arXiv:1210.1079 [pdf, other]
Propagation and amplification dynamics of 1D polariton condensates
E. Wertz, A. Amo, D. D. Solnyshkov, L. Ferrier, T. C. H. Liew, D. Sanvitto, P. Senellart, I. Sagnes, A. Lemaître, A. V. Kavokin, G. Malpuech, J. BlochThe dynamics of propagating polariton condensates in one-dimensional microcavities is investigated through time resolved experiments. We find a strong increase in the condensate intensity when it travels through the non-resonantly excited area. This amplification is shown to come from bosonic stimulated relaxation of reservoir excitons into the polariton condensate, allowing for the repopulation of the condensate through non-resonant pumping. Thus, we experimentally demonstrate a polariton amplifier with a large band width, opening the way towards the transport of polaritons with high densities over macroscopic distances.
2. arXiv:1210.0904 [pdf, other]
Double transfer through Dirac points in a tunable honeycomb optical lattice
Thomas Uehlinger, Daniel Greif, Gregor Jotzu, Leticia Tarruell, Tilman Esslinger, Lei Wang, Matthias Troyer
We report on Bloch-Zener oscillations of an ultracold Fermi gas in a tunable honeycomb lattice. The quasi-momentum distribution of the atoms is measured after sequentially passing through two Dirac points. We observe a double-peak feature in the transferred fraction to the second band, both as a function of the band gap at the Dirac points and the quasi-momentum of the trajectory. Our results are in good agreement with a simple analytical model based on two successive Landau-Zener transitions. Owing to the variation of the potential gradient over the cloud size, coherent St\"uckelberg oscillations are not visible in our measurements. This effect of the harmonic confinement is confirmed by a numerical simulation of the dynamics of a trapped 2D system.
Oct 3
1. arXiv:1210.0799 [pdf, other]
Enhancing the thermal stability of entanglement between Majorana fermions with dipoles in optical lattices
Fei Lin, V.W. Scarola
Pairing between spinless fermions can generate Majorana fermion excitations. Such excitations may exhibit intriguing properties arising from non-local entanglement, including anyonic braid statistics, teleportation, and enough stability to encode quantum information. But simple models indicate that non-local entanglement between Majorana fermions becomes unstable at non-zero temperatures. We address this issue here by showing that anisotropic interactions between dipolar fermions in optical lattices can be used to form domains that significantly enhance thermal stability. We construct a model of oriented dipolar fermions in a square optical lattice. We explicitly compute the correlation functions defining entanglement. We find that domains established by strong interactions exhibit enhanced entanglement between Majorana fermions over large distances and long times even at finite temperatures. Our approach can be generalized to a variety of configurations and other systems, such as quantum wire arrays.
2. arXiv:1210.0657 [pdf, ps, other]
Localization of an inhomogeneous Bose-Einstein condensate in a moving random potential
Ardavan Alamir, Pablo Capuzzi, Patrizia Vignolo
We study the dynamics of a harmonically trapped quasi-one-dimensional Bose-Einstein condensate subjected to a moving disorder potential of finite extent. We show that, due to the inhomogeneity of the sample, only a percentage of the atoms is localized at supersonic velocities of a random potential. We find that this percentage can be sensitively increased by introducing suitable correlations in the disorder potential such as those provided by random dimers.
Oct 2
1. arXiv:1210.0110 [pdf, other]
Ground-state and dynamical properties of two-dimensional dipolar Fermi liquids
Saeed H. Abedinpour, Reza Asgari, B. Tanatar, Marco Polini
We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler-Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schr\"odinger equation for the "pair amplitude" $\sqrt{g(r)}$, where $g(r)$ is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow-Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree-Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition (QPT). Using a certain approximate model for the dynamical density-density response function, we furthermore demonstrate that: i) the liquid phase is stable towards the formation of density waves up to the liquid-to-crystal QPT and ii) an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings.
2. arXiv:1210.0030 [pdf, ps, other]
Spin modulation instabilities and phase separation dynamics in trapped two-component Bose condensates
Ivana Vidanovic, N. J. van Druten, Masudul Haque
In the study of trapped two-component Bose gases, a widely used dynamical protocol is to start from the ground state of a one-component condensate and then switch half the atoms into another hyperfine state. The slightly different intra-component and inter-component interactions can then lead to highly nontrivial dynamics. We study and classify the possible subsequent dynamics, over a wide variety of parameters spanned by the trap strength and by the inter- to intra-component interaction ratio. A stability analysis suited to the trapped situation provides us with a framework to explain the various types of dynamics in different regimes.
Oct 1
1. arXiv:1209.6468 [pdf, other]
Quantum dynamics of a single, mobile spin impurity
Takeshi Fukuhara, Adrian Kantian, Manuel Endres, Marc Cheneau, Peter Schauß, Sebastian Hild, David Bellem, Ulrich Schollwöck, Thierry Giamarchi, Christian Gross, Immanuel Bloch, Stefan Kuhr
Quantum magnetism describes the properties of many materials such as transition metal oxides and cuprate superconductors. One of its elementary processes is the propagation of spin excitations. Here we study the quantum dynamics of a deterministically created spin-impurity atom, as it propagates in a one-dimensional lattice system. We probe the full spatial probability distribution of the impurity at different times using single-site-resolved imaging of bosonic atoms in an optical lattice. In the Mott-insulating regime, a post-selection of the data allows to reduce the effect of temperature, giving access to a space- and time-resolved measurement of the quantum-coherent propagation of a magnetic excitation in the Heisenberg model. Extending the study to the bath's superfluid regime, we determine quantitatively how the bath strongly affects the motion of the impurity. The experimental data shows a remarkable agreement with theoretical predictions allowing us to determine the effect of temperature on the coherence and velocity of impurity motion. Our results pave the way for a new approach to study quantum magnetism, mobile impurities in quantum fluids, and polarons in lattice systems.
2. arXiv:1209.6411 (cross-list from cond-mat.str-el) [pdf, ps, other]
A novel topological antiferromagnetic spin-density-wave phase in an extended Kondo lattice model
Yin Zhong, Yu-Feng Wang, Yong-Qiang Wang, Hong-Gang Luo
By using an extended mean-field theory we study the phase diagram of the topological Kondo lattice model on the honeycomb lattice at half-filling, in which the conduction electrons are described by the Haldane model. Besides the well-defined Kondo insulator and normal antiferromagnetic spin-density-wave (N-SDW) state, it is found that a novel and nontrivial topological antiferromagnetic SDW state (T-SDW) with a quantized Hall conductance is possible if the quasiparticle gap is dominated by the next-nearest neighbor hopping rather than the antiferromagnetic order. By analyzing the low-energy effective Chern-Simon action and the corresponding chiral edge-state, the T-SDW could be considered as a quantum anomalous Hall insulator with antiferromagnetic long-range order. This novel state is apparently beyond Landau-Ginzburg paradigm, which can be attributed to the interplay of quantum anomalous Hall effect and the subtle antiferromagnetic order in the Kondo lattice-like model. While the transition between the SDW states and the Kondo insulator is found to be conventional (a first order transition), the transition between the N- and T-SDWs is, however, a topological quantum phase transition. It is expected that such a T-SDW state could be realized by experiments in ultra-cold atoms on the honeycomb optical lattice. The present work sheds light on the interplay between conduction electrons and the densely localized spins on the honeycomb lattice.
