Lode Pollet

Ludwig-Maximilian-University of Munich, München, Bavaria, Germany

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Publications (97)435.88 Total impact

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    ABSTRACT: We study attractively interacting fermions on a square lattice with dispersion relations exhibiting strong spin-dependent anisotropy. The resulting Fermi surface mismatch suppresses the s-wave BCS-type 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 Cooper-pair Bose metal phase [A. E. Feiguin and M. P. A. Fisher, Phys. Rev. Lett. 103, 025303 (2009).
    Physical Review Letters 11/2014; 113(19):195301. · 7.73 Impact Factor
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    ABSTRACT: We argue that the three key phenomena recently observed in solid $^4$He---mass supertransport, anomalous isochoric compressibility (syringe effect), and giant plasticity---are closely linked to each other through the physics of an interconnected network of tilted quantum-rough 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 low-frequency syringe and giant-plasticity 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.
    09/2014;
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    Zi Cai, Ulrich Schollwock, Lode Pollet
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    ABSTRACT: We study the ground state phase diagram of a one-dimensional hard-core bosonic model with nearest-neighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spin-boson models to interacting spin-boson systems. %The coupling with the bath is taken to be Ohmic ($s=1)$. We show that a bath-induced 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 haf-filling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of Kosterlitz-Thouless type where the Luttinger parameter takes a non-universal 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.
    08/2014;
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    ABSTRACT: We study attractively interacting fermions on a square lattice with dispersion relations exhibiting strong spin-dependent anisotropy. The resulting Fermi surface mismatch suppresses the s-wave BCS-type 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 Cooper-pair Bose metal phase [A. E. Feiguin and M. P. A. Fisher, Phys. Rev. Lett. 103, 025303 (2009)] we find an incommensurate density wave at strong anisotropy and two different p-wave superfluid states with unconventional symmetry at intermediate anisotropy.
    08/2014;
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    ABSTRACT: We study the out-of-equilibrium dynamics of bosonic atoms in a 1D optical lattice, after the ground-state is excited by a single spontaneous emission event, i.e. after an absorption and re-emission of a lattice photon. This is an important fundamental source of decoherence for current experiments, and understanding the resulting dynamics and changes in the many-body 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 time-scales, 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 low-energy Hamiltonian in which the dynamics can be understood based on correlated doublon-holon pairs.
    08/2014;
  • Lode Pollet, Anatoly B Kuklov
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    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 CSS-a gradual decrease of the superfluid response with increasing pressure.
    Physical Review Letters 07/2014; 113(4):045301. · 7.73 Impact Factor
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    Lode Pollet, Anatoly Kuklov
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    ABSTRACT: The ground state of solid $^4$He in a cylindrical nanopore hosts a topological linear defect which can be viewed as a nematic-type 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.
    03/2014;
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    Marin Bukov, Lode Pollet
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    ABSTRACT: We analyze the ground-state properties of mixtures consisting of scalar bosons and spin-1/2 fermions using a mean-field treatment of the local boson-fermion 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 BCS-type 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 Mott-insulator-to-superfluid transition of the boson sector in the presence of a staggered density-induced alternating potential provided by the fermions, and determine the mean-field transition line. In the two-superfluids phase of the mixture, we restrict to nearest-neighbor-induced interactions between the fermions and consider the extended Hubbard model. We perform a mean-field analysis of the critical temperature for the formation of boson-assisted s-, extended s--, d-, and p-wave pairs at fermionic half-filling. We compare our results with a recent dynamical mean-field study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012), 10.1103/PhysRevLett.109.206401].
    02/2014; 89(9).
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    ABSTRACT: We compute the ground-state phase diagram of the two-dimensional (2D) Bose-Hubbard model with anisotropic hopping using quantum Monte Carlo simulations, connecting the one-dimensional (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 renormalization-group equations and has a shape typical for the underlying Kosterlitz-Thouless 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.
    01/2014; 89(2).
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    ABSTRACT: We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional J-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, 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 bottoms-up 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 gauge-gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.
    Physical Review Letters 01/2014; 112(3):030402. · 7.73 Impact Factor
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    ABSTRACT: We present a controlled rare-weak-link theory of the superfluid-to-Bose/Mott glass transition in one-dimensional disordered systems. The transition has Kosterlitz-Thouless 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 $L$.
    11/2013; 89(5).
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    ABSTRACT: Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body 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 particle-hole pairs that lead to the emergence of a nonlocal string order in one dimension. By using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case.
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    ABSTRACT: We compute the ground state phase diagram of the 2d Bose-Hubbard model with anisotropic hopping using quantum Monte Carlo simulations, connecting the 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 RG equations and has a form typical for the underlying Kosterlitz-Thouless transition in 1d. We also compute the phase boundary of the Mott lobe 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.
    08/2013;
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    ABSTRACT: Correlation functions play an important role for the theoretical and experimental characterization of many-body systems. In solid-state systems, they are usually determined through scattering experiments, whereas in cold gases systems, time-of-flight, and in situ absorption imaging are the standard observation techniques. However, none of these methods allow the in situ detection of spatially resolved correlation functions at the single-particle level. Here, we give a more detailed account of recent advances in the detection of correlation functions using in situ fluorescence imaging of ultracold bosonic atoms in an optical lattice. This method yields single-site- and single-atom-resolved images of the lattice gas in a single experimental run, thus gaining direct access to fluctuations in the many-body system. As a consequence, the detection of correlation functions between an arbitrary set of lattice sites is possible. This enables not only the detection of two-site correlation functions but also the evaluation of non-local correlations, which originate from an extended region of the system and are used for the characterization of quantum phases that do not possess (quasi-)long-range order in the traditional sense.
    Applied Physics B 08/2013; · 1.78 Impact Factor
  • Lode Pollet
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    ABSTRACT: We review the physics of the Bose-Hubbard model with disorder in the chemical potential focusing on recently published analytical arguments in combination with quantum Monte Carlo simulations. Apart from the superfluid and Mott insulator phases that can occur in this system without disorder, disorder allows for an additional phase, called the Bose glass phase. The topology of the phase diagram is subject to strong theorems proving that the Bose Glass phase must intervene between the superfluid and the Mott insulator and implying a Griffiths transition between the Mott insulator and the Bose glass. The full phase diagrams in 3d and 2d are discussed, and we zoom in on the insensitivity of the transition line between the superfluid and the Bose glass in the close vicinity of the tip of the Mott insulator lobe. We briefly comment on the established and remaining questions in the 1d case, and give a short overview of numerical work on related models.
    Comptes Rendus Physique 07/2013; 14(8). · 1.82 Impact Factor
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    ABSTRACT: The droplet crystal phase of a symmetric binary mixture of Rydberg-blockaded dipolar Bose gases is studied by computer simulation. At high temperature each droplet comprises on average equal numbers of particles of either component, but the two components demix below the supersolid transition temperature, i.e., droplets mostly consist of particles of one component. Droplets consisting of the same component will also favor clustering. Demixing is driven by quantum tunnelling of particles across droplets over the system, and does not take place in a non-superfluid crystal. This effect should be easily detectable in a cold gas experiment.
    Physical Review A 06/2013; 88(3). · 3.04 Impact Factor
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    ABSTRACT: We study the thermalization of excitations generated by spontaneous emission events for cold bosons in an optical lattice. Computing the dynamics described by the many-body master equa- tion, we characterize equilibration timescales in different parameter regimes. For simple observables, we find regimes in which the system relaxes rapidly to values in agreement with a thermal distribu- tion, and others where thermalization does not occur on typical experimental timescales. Because spontaneous emissions lead effectively to a local quantum quench, this behavior is strongly depen- dent on the low-energy spectrum of the Hamiltonian, and undergoes a qualitative change at the Mott Insulator-superfluid transition point. These results have important implications for the un- derstanding of thermalization after localized quenches in isolated quantum gases, as well as the characterization of heating in experiments.
    05/2013;
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    ABSTRACT: We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the two-dimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particle-hole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a low-frequency resonance which is relatively sharp and is followed by a damped oscillation (in the first two phases only) before saturating to the quantum critical plateau. The counterintuitive resonance feature in the insulating and normal phases calls for deeper understanding of collective modes in the strongly coupled (2+1)-dimensional relativistic field theory. Our results are derived from analytically continued correlation functions obtained from path-integral Monte Carlo simulations of the Bose-Hubbard model.
    Physical Review Letters 04/2013; 110(17):170403. · 7.73 Impact Factor
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    ABSTRACT: The low-temperature phase diagram of 4He adsorbed on a single graphene sheet is studied by computer simulations of a system consisting of nearly a thousand helium atoms. In the first layer, two commensurate solid phases are observed with fillings 1/3 and 7/16, respectively, separated by a domain wall phase, as well as an incommensurate crystal at a higher coverage. No evidence of a thermodynamically stable superfluid phase is found for the first adlayer. Second-layer promotion occurs at a coverage of 0.111(4) Å−2. In the second layer, two phases are observed, namely a superfluid and an incommensurate solid, with no commensurate solid intervening between these two phases. The computed phase diagram closely resembles that predicted for helium on graphite.
    Physical review. B, Condensed matter 03/2013; 87(9). · 3.77 Impact Factor

Publication Stats

2k Citations
435.88 Total Impact Points

Institutions

  • 2013–2014
    • Ludwig-Maximilian-University of Munich
      • Department of Physics
      München, Bavaria, Germany
  • 2008–2013
    • University of Massachusetts Amherst
      • Department of Physics
      Amherst Center, Massachusetts, United States
    • UPMC
      Pittsburgh, Pennsylvania, United States
  • 2006–2012
    • ETH Zurich
      • • Institute for Theoretical Physics
      • • Department of Physics
      Zürich, ZH, Switzerland
  • 2011
    • Max Planck Institute of Quantum Optics
      Arching, Bavaria, Germany
    • Georg-August-Universität Göttingen
      • Institute for Theoretical Physics
      Göttingen, Lower Saxony, Germany
    • Adam Mickiewicz University
      • Faculty of Physics
      Posen, Greater Poland Voivodeship, Poland
  • 2003–2011
    • Ghent University
      • Center for Molecular Modeling
      Gand, Flanders, Belgium
  • 2009–2010
    • Harvard University
      • Department of Physics
      Cambridge, MA, United States
    • Università degli Studi di Trento
      • Department of Physics
      Trient, Trentino-Alto Adige, Italy
    • University of California, Berkeley
      • Pitzer Center for Theoretical Chemistry
      Berkeley, CA, United States
  • 2007
    • University of Nice-Sophia Antipolis
      Nice, Provence-Alpes-Côte d'Azur, France
    • Hochschule für Technik Zürich
      Zürich, Zurich, Switzerland
  • 2006–2007
    • University of Alberta
      • Department of Physics
      Edmonton, Alberta, Canada