Publications (103)484.25 Total impact
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ABSTRACT: We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the SuperfluidtoInsulator quantum critical point (QCP) in a system of twodimensional ultracold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schau{\ss}, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature 487, 454458 (2012)], and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement.  [Show abstract] [Hide abstract]
ABSTRACT: We study attractively interacting spin1/2 fermions on the square lattice subject to a spin population imbalance. Using unbiased diagrammatic Monte Carlo simulations we find an extended region in the parameter space where the Fermi liquid is unstable towards formation of Cooper pairs with nonzero centerofmass momentum, known as the FuldeFerrellLarkinOvchinnikov (FFLO) state. The highest temperature where the FFLO instability can be observed is about half of the superfluid transition temperature in the unpolarized system.  [Show abstract] [Hide abstract]
ABSTRACT: We study the instabilities towards (exotic) superconductivity of mixtures of spin$1/2$ fermions coupled to scalar bosons on a twodimensional square lattice with the DynamicalClusterApproximation (DCA) using a numerically exact continuoustime MonteCarlo solver. The Bogoliubov bosons provide an effective phononic bath for the fermions and induce a nonlocal retarded interaction between the fermions, which can lead to (exotic) superconductivity. Because of the sign problem the biggest clusters we can study are limited to $2 \times 2$ in size, but this nevertheless allows us to study the pairing instablilities, and their possible divergence, in the $s$, extended $s$, $p$ and $d$ wave channels as well as the competition with antiferromagnetic fluctuations. At fermionic halffilling we find that $d$wave is stable when the mediated interaction by the bosons is of the same order as the bare fermionic repulsion. Its critical temperature can be made as high as the maximum one for $s$wave, which opens perspectives for its detection in a cold atom experiment.  [Show abstract] [Hide abstract]
ABSTRACT: We derive the Bogoliubov+U formalism to study the thermodynamical properties of the BoseHubbard model. The framework can be viewed as the zerofrequency limit of bosonic dynamical meanfield theory (BDMFT), but equally well as an extension of the meanfield decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The selfenergy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just $3$ parameters that are determined selfconsistently reproduces the $T=0$ phase diagrams of the 3d and 2d BoseHubbard model with an accuracy of $1 \%$ or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in BDMFT.  [Show abstract] [Hide abstract]
ABSTRACT: We use quantum Monte Carlo simulations to study a finitetemperature dimensionalcrossoverdriven evolution of spin and charge dynamics in weakly coupled Hubbard chains with a halffilled band. The lowtemperature behavior of the charge gap indicates a crossover between two distinct energy scales: highenergy onedimensional (1D) Mott gap due to umklapp process and a lowenergy gap which stems from longrange antiferromagnetic (AF) fluctuations. Away from the 1D regime and at temperature scales above the charge gap, the emergence of zerofrequency Drudelike feature in the interchain optical conductivity $\sigma_{\perp}(\omega)$ implies the onset of a higherdimensional metal. In this metallic phase, enhanced quasiparticle scattering off finiterange AF fluctuations results in incoherent singleparticle dynamics. The coupling between spin and charge fluctuations is also seen in the spin dynamical structure factor $S({\pmb q},\omega)$ displaying damped spin excitations (paramagnons) close to the AF wavevector ${\pmb q}=(\pi,\pi)$ and particlehole continua near 1D momentum transfers spanning quasiparticles at the Fermi surface. We relate our results to the charge deconfinement in quasi1D organic BechgaardFabre salts. 
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ABSTRACT: We study attractively interacting fermions on a square lattice with dispersion relations exhibiting strong spindependent anisotropy. The resulting Fermi surface mismatch suppresses the swave BCStype instability, clearing the way for unconventional types of order. Unbiased sampling of the Feynman diagrammatic series using diagrammatic Monte Carlo methods reveals a rich phase diagram in the regime of intermediate coupling strength. Instead of a proposed Cooperpair Bose metal phase [A. E. Feiguin and M. P. A. Fisher, Phys. Rev. Lett. 103, 025303 (2009).  [Show abstract] [Hide abstract]
ABSTRACT: We apply the diagrammatic Monte Carlo approach to threedimensional Fermipolaron systems with massimbalance, where an impurity interacts resonantly with a noninteracting Fermi sea whose atoms have a different mass. This method allows to go beyond frequently used variational techniques by stochastically summing all relevant impurity Feynman diagrams up to a maximum expansion order limited by the sign problem. Polaron energy and quasiparticle residue can be accurately determined over a broad range of impurity masses. Furthermore, the spectral function of an imbalanced polaron demonstrates the stability of the quasiparticle and allows to locate in addition also the repulsive polaron as an excited state. The quantitative exactness of twoparticlehole wavefunctions is investigated, resulting in a relative lowering of polaronic energies in the massimbalance phase diagram. Tan's contact coefficient for the massbalanced polaron system is found in good agreement with variational methods. Massimbalanced systems can be studied experimentally by ultracold atom mixtures like $^6$Li$^{40}$K.  [Show abstract] [Hide abstract]
ABSTRACT: We argue that the three key phenomena recently observed in solid $^4$Hemass supertransport, anomalous isochoric compressibility (syringe effect), and giant plasticityare closely linked to each other through the physics of an interconnected network of tilted quantumrough dislocations. As immediate implications of this connection several predictions follow: In the absence of $^3$He impurities, the syringe effect and giant plasticity persist down to $T=0$; the dynamical lowfrequency syringe and giantplasticity responses are dispersionless; and similarly to giant plasticity but without direct relationship to the supertransport along the dislocation cores, $^3$He impurities should suppress the syringe effect partially or completely at appropriately low temperatures.  [Show abstract] [Hide abstract]
ABSTRACT: We study the ground state phase diagram of a onedimensional hardcore bosonic model with nearestneighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spinboson models to interacting spinboson systems. %The coupling with the bath is taken to be Ohmic ($s=1)$. We show that a bathinduced Bose metal phase can occur in the ground state phase diagram away from half filling. This phase is compressible, gapless, and conducting but not superfluid. At haffilling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of KosterlitzThouless type where the Luttinger parameter takes a nonuniversal value.The applied quantum Monte Carlo method can be used for all open bosonic and unfrustrated spin systems, regardless of their dimension, filling factor and spectrum of the dissipation as long as the quantum system couples to the bath via the density operators. 
Article: Thermalization of strongly interacting bosons after spontaneous emissions in optical lattices
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ABSTRACT: We study the outofequilibrium dynamics of bosonic atoms in a 1D optical lattice, after the groundstate is excited by a single spontaneous emission event, i.e. after an absorption and reemission of a lattice photon. This is an important fundamental source of decoherence for current experiments, and understanding the resulting dynamics and changes in the manybody state is important for controlling heating in quantum simulators. Previously it was found that in the superfluid regime, simple observables relax to values that can be described by a thermal distribution on experimental timescales, and that this breaks down for strong interactions (in the Mott insulator regime). Here we expand on this result, investigating the relaxation of the momentum distribution as a function of time, and discussing the relationship to eigenstate thermalization. For the strongly interacting limit, we provide an analytical analysis for the behavior of the system, based on an effective lowenergy Hamiltonian in which the dynamics can be understood based on correlated doublonholon pairs.  [Show abstract] [Hide abstract]
ABSTRACT: The ground state of ^{4}He confined in a system with the topology of cylinder can display properties of solid, superfluid, and liquid crystal. This phase, which we call a compactified supersolid (CSS), originates from wrapping the basal planes of the bulk hcp solid into concentric cylindrical shells, with several central shells exhibiting superfluidity along the axial direction. Its main feature is the presence of a topological defect which can be viewed as a disclination with Frank index n=1 observed in liquid crystals, and which, in addition, has a superfluid core. The CSS as well as its transition to an insulating compactified solid with a very wide hysteresis loop are found by ab initio Monte Carlo simulations. A simple analytical model captures qualitatively correctly the main property of the CSSa gradual decrease of the superfluid response with increasing pressure.  [Show abstract] [Hide abstract]
ABSTRACT: We apply diagrammatic Monte Carlo to the problem of an impurity interacting resonantly with a homogeneous Fermi bath for a quasitwodimensional setup. Notwithstanding the series divergence we can show numerically that the three particlehole diagrammatic contributions are not contributing significantly to the final answer, thus demonstrating a nearly perfect destructive interference of contributions in subspaces with higherorder particlehole lines. Consequently, for strong enough confinement in the third direction, the transition between the polaron and the molecule ground state is found to be in good agreement with the pure 2D case and agrees very well with the one found by the wavefunction approach in the two particlehole subspace.  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experiments, we study the relaxation dynamics and thermalization in the onedimensional BoseHubbard model induced by a global interaction quench. Specifically, we start from an initial state that has exactly one boson per site and is the ground state of a system with infinitely strong repulsive interactions at unit filling. Using exact diagonalization and the density matrix renormalization group method, we compute the time dependence of such observables as the multiple occupancy and the momentum distribution function. Typically, the relaxation to stationary values occurs over just a few tunneling times. The stationary values are identical to the socalled diagonal ensemble on the system sizes accessible to our numerical methods and we further observe that the microcanonical ensemble describes the steady state of many observables reasonably well for small and intermediate interaction strength. The expectation values of observables in the canonical ensemble agree quantitatively with the time averages obtained from the quench at small interaction strengths, and qualitatively provide a good description of steadystate values even in parameter regimes where the microcanonical ensemble is not applicable due to finitesize effects. We discuss our numerical results in the framework of the eigenstate thermalization hypothesis. Moreover, we also observe that the diagonal and the canonical ensemble are practically identical for our initial conditions already on the level of their respective energy distributions for small interaction strengths. Finally, we discuss implications of our results for the interpretation of a recent sudden expansion experiment [Phys. Rev. Lett. 110, 205301 (2013)], in which the same interaction quench was realized.  [Show abstract] [Hide abstract]
ABSTRACT: The ground state of solid $^4$He in a cylindrical nanopore hosts a topological linear defect which can be viewed as a nematictype Frank's disclination. The associated singular strain (or, rather, splay) may cause partial melting around the line to create a superfluid core of the disclination. The resulting phase, compactified supersolid (CSS), is studied by ab initio Monte Carlo simulations and by a simple model explaining its main feature  a gradual decrease of the superfluid response with pressure observed in vycor. The CSS is found to transform into insulating compactified solid (CS) by a first order transition with very wide hysteresis.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the groundstate properties of mixtures consisting of scalar bosons and spin1/2 fermions using a meanfield treatment of the local bosonfermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCStype equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mottinsulatortosuperfluid transition of the boson sector in the presence of a staggered densityinduced alternating potential provided by the fermions, and determine the meanfield transition line. In the twosuperfluids phase of the mixture, we restrict to nearestneighborinduced interactions between the fermions and consider the extended Hubbard model. We perform a meanfield analysis of the critical temperature for the formation of bosonassisted s, extended s, d, and pwave pairs at fermionic halffilling. We compare our results with a recent dynamical meanfield study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012), 10.1103/PhysRevLett.109.206401]. 
Article: Groundstate phase diagram of the twodimensional BoseHubbard model with anisotropic hopping
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ABSTRACT: We compute the groundstate phase diagram of the twodimensional (2D) BoseHubbard model with anisotropic hopping using quantum Monte Carlo simulations, connecting the onedimensional (1D) to the 2D system. We find that the tip of the lobe lies on a curve controlled by the 1D limit over the full anisotropy range, while the universality class is always the same as in the isotropic 2D system. This behavior can be derived analytically from the lowest renormalizationgroup equations and has a shape typical for the underlying KosterlitzThouless transition in one dimension. We also compute the phase boundary of the Mott lobe at unit density for strong anisotropy and compare it to the 1D system. Our calculations shed light on recent cold gas experiments monitoring the dynamics of an expanding cloud. 
Article: Universal Conductivity in a TwoDimensional SuperfluidtoInsulator Quantum Critical System
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ABSTRACT: We compute the universal conductivity of the (2+1)dimensional XY universality class, which is realized for a superfluidtoMott insulator quantum phase transition at constant density. Based on largescale Monte Carlo simulations of the classical (2+1)dimensional Jcurrent model and the twodimensional BoseHubbard model, we can precisely determine the conductivity on the quantum critical plateau, σ(∞)=0.359(4)σ_{Q} with σ_{Q} the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottomsup AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the σ(iω_{n})σ(∞) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gaugegravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation. 
Article: Asymptotically Exact Scenario of StrongDisorder Criticality in OneDimensional Superfluids
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ABSTRACT: We present a controlled rareweaklink theory of the superfluidtoBose/Mott glass transition in onedimensional disordered systems. The transition has KosterlitzThouless critical properties but may occur at an arbitrary large value of the Luttinger parameter $K$. In contrast to the scenario by Altman {\it et al.} [Phys. Rev. B {\bf 81}, 174528 (2010)], the hydrodynamic description is valid under the correlation radius and defines criticality via the renormalization of microscopically weak links, along the lines of Kane and Fisher [Phys. Rev. Lett. {\bf 68}, 1220 (1992)]. The hallmark of the theory is the relation $K^{(c)}=1/\zeta$ between the critical value of the Luttinger parameter at macroscopic scales and the microscopic (irrenormalizable) exponent $\zeta$ describing the scaling $\propto 1/N^{1\zeta}$ for the strength of the weakest link among the $N/L \gg 1$ disorder realizations in a system of fixed mesoscopic size $L$. 
Dataset: Observation of Correlated ParticleHole Pairs and String Order in LowDimensional Mott Insulators
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ABSTRACT: Quantum phases of matter are characterized by the underlying correlations of the manybody system. Although this is typically captured by a local order parameter, it has been shown that a broad class of manybody systems possesses a hidden nonlocal order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particlehole pairs that lead to the emergence of a nonlocal string order in one dimension. By using highresolution imaging of lowdimensional quantum gases in an optical lattice, we directly detect these pairs with singlesite and singleparticle sensitivity and observe string order in the onedimensional case.
Publication Stats
3k  Citations  
484.25  Total Impact Points  
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Institutions

20122015

Technische Universität München
München, Bavaria, Germany 
LudwigMaximiliansUniversity of Munich
 Department of Physics
München, Bavaria, Germany


2013

Kurchatov Institute
Moskva, Moscow, Russia


20082013

University of Massachusetts Amherst
 Department of Physics
Amherst Center, Massachusetts, United States


20092011

Harvard University
 Department of Physics
Cambridge, Massachusetts, United States 
Columbia University
 Department of Physics
New York, New York, United States


20052011

ETH Zurich
Zürich, Zurich, Switzerland


20032011

Ghent University
 Center for Molecular Modeling
Gand, Flemish, Belgium


2007

Hochschule für Technik Zürich
Zürich, Zurich, Switzerland 
University of NiceSophia Antipolis
Nice, ProvenceAlpesCôte d'Azur, France
