Publications (91)426.91 Total impact
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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 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.03/2014;  [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].02/2014; 89(9). 
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.01/2014; 89(2). 
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.Physical Review Letters 01/2014; 112(3):030402. · 7.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the ground state properties of BoseFermi mixtures using a meanfield treatment of the bosonfermion interaction on a simple cubic lattice. In the deep superfluid limit of the bosonic 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 Mott insulator to superfluid transition of the bosonic sector in the presence of a staggered densityinduced alternating potential provided by the fermions, and determine the meanfield transition line. In the twosuperfluid phase of the mixture we restrict to nearestneighbor induced 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 $p$wave pairs at fermionic half filling. We compare our results with a recent dynamical meanfield study [Anders \emph{et al.} Phys. Rev. Lett. {\bf 109}, 206401].11/2013; 
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 $L$.11/2013; 89(5). 
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.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the ground state phase diagram of the 2d BoseHubbard 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 KosterlitzThouless 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;  [Show abstract] [Hide abstract]
ABSTRACT: Correlation functions play an important role for the theoretical and experimental characterization of manybody systems. In solidstate systems, they are usually determined through scattering experiments, whereas in cold gases systems, timeofflight, 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 singleparticle 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 singlesite and singleatomresolved images of the lattice gas in a single experimental run, thus gaining direct access to fluctuations in the manybody 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 twosite correlation functions but also the evaluation of nonlocal correlations, which originate from an extended region of the system and are used for the characterization of quantum phases that do not possess (quasi)longrange order in the traditional sense.Applied Physics B 08/2013; · 1.78 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We review the physics of the BoseHubbard 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  [Show abstract] [Hide abstract]
ABSTRACT: The droplet crystal phase of a symmetric binary mixture of Rydbergblockaded 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 nonsuperfluid crystal. This effect should be easily detectable in a cold gas experiment.Physical Review A 06/2013; 88(3). · 3.04 Impact Factor  [Show abstract] [Hide abstract]
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 manybody 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 lowenergy spectrum of the Hamiltonian, and undergoes a qualitative change at the Mott Insulatorsuperfluid 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;  [Show abstract] [Hide abstract]
ABSTRACT: We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the twodimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particlehole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a lowfrequency 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 pathintegral Monte Carlo simulations of the BoseHubbard model.Physical Review Letters 04/2013; 110(17):170403. · 7.94 Impact Factor 
Article: Phase diagram of 4He on graphene
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ABSTRACT: The lowtemperature 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. Secondlayer 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  [Show abstract] [Hide abstract]
ABSTRACT: We find that despite strong decay into Goldstone modes the Higgs boson survives as a welldefined resonance in the twodimensional relativistic field theory realized in the cold atomic system near the quantum critical point between the superfluid (SF) and Mott insulator(MI) states. Using scaling analysis of analytically continued results from quantum Monte Carlo simulations we construct universal spectral functions for scalar response both for SF and MI phases and reveal that they share similar properties: a resonant peak followed by a broader secondary peak before saturating to a near plateau behavior at higher frequencies, i.e. the Higgs amplitude mode is present in the MI phase under the correlation length scale. Our simulations of a trapped system of ultracold ^87Rb atoms explain recent experimental data and how the signal is modified by tight confinement.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We study attractively interacting fermions on a square lattice whose Fermi surfaces exhibit a spindependent anisotropy. Such a system was proposed to harbor several exotic phases, most notably a Cooperpair Bosemetal featuring a gap for fermionic excitations but gapless, uncondensed pair excitations along a Bose surface in momentum space. We present unbiased numeric results obtained with Diagrammatic Monte Carlo, a new technique for correlated fermionic systems based on sampling Feynman diagrammatic series directly in the thermodynamic limit. For the relevant regime of intermediate coupling strength our data show that the Fermi surface mismatch indeed suppresses the BCS transition to superfluidity. At strong anisotropy we find no sign of an ordering transition down to very low temperature suggesting existence of a quantumphase transition from the conventional superconductor to an uncondensed state driven by the Fermi surface anisotropy.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We show that in the regime when strong disorder is more relevant than field quantization the superfluidtoBoseglass criticality of onedimensional bosons is preceded by the prolonged logarithmically slow classicalfield renormalization flow of the superfluid stiffness at mesoscopic scales. With the system compressibility remaining constant, the quantum nature of the system manifests itself only in the renormalization of dilute weak links. On the insulating side, the flow ultimately reaches a value of the Luttinger parameter at which the instantonantiinstanton pairs start to proliferate, in accordance with the universal quantum scenario. This happens first at astronomic system sizes because of the suppressed instanton fugacity. We illustrate our result by firstprinciples simulations.Physical review. B, Condensed matter 02/2013; 87(14). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the twodimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particlehole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a lowfrequency 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 pathintegral Monte Carlo simulations of the BoseHubbard model.Physical Review Letters 01/2013; 110(17):170403. · 7.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The low temperature phase diagram of $^4$He adsorbed on a single graphene sheet is studied by computer simulation of a system comprising nearly 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 higher coverage. No evidence of a thermodynamically stable superfliuid phase is found for the first adlayer. Second layer promotion occurs at a coverage of 0.111(4) $\AA^{2}$. 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.12/2012;  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the phase diagram of the BoseFermi Hubbard model on the 3d cubic lattice at fermionic half filling and bosonic unit filling by means of singlesite dynamical meanfield theory. For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce swave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the interspecies coupling strength. In contrast, slow bosons favor fermionic charge density wave structures for attractive fermionic interactions. These competing instabilities lead to a rich phase diagram within reach of cold gas experiments.Physical Review Letters 11/2012; 109(20):206401. · 7.94 Impact Factor
Publication Stats
2k  Citations  
426.91  Total Impact Points  
Top Journals
Institutions

2013–2014

LudwigMaximilianUniversity of Munich
 Department of Physics
München, Bavaria, Germany


2008–2013

University of Massachusetts Amherst
 Department of Physics
Amherst Center, Massachusetts, United States 
French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France


2006–2012

ETH Zurich
 • Institute for Theoretical Physics
 • Department of Physics
Zürich, ZH, Switzerland


2011

Max Planck Institute of Quantum Optics
Arching, Bavaria, Germany 
GeorgAugustUniversität 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, TrentinoAlto Adige, Italy 
University of California, Berkeley
 Pitzer Center for Theoretical Chemistry
Berkeley, CA, United States


2007

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


2006–2007

University of Alberta
 Department of Physics
Edmonton, Alberta, Canada
