Publications (43)82.19 Total impact
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ABSTRACT: We calculate the zerotemperature equation of state of massimbalanced resonant Fermi gases in an ab initio fashion, by implementing the recent proposal of imaginaryvalued mass difference to bypass the sign problem in lattice Monte Carlo calculations. The fully nonperturbative results thus obtained are analytically continued to real mass imbalance to yield the physical equation of state, providing predictions for upcoming experiments with massimbalanced atomic Fermi gases. In addition, we present an exact relation for the rate of change of the equation of state at small mass imbalances, showing that it is fully determined by the energy of the massbalanced system.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the manyflavor phase diagram of quantum electrodynamics (QED) in 2+1 (Euclidean) spacetime dimensions. We compute the critical flavor number above which the theory is in the quasiconformal massless phase. For this, we study the renormalization group fixedpoint structure in the space of gauge interactions and pointlike fermionic selfinteractions, the latter of which are induced dynamically by fermionphoton interactions. We find that a reliable estimate of the critical flavor number crucially relies on a careful treatment of the Fierz ambiguity in the fermionic sector. Using a Fierzcomplete basis, our results indicate that the phase transition towards a chirallybroken phase occurring at small flavor numbers could be separated from the quasiconformal phase at larger flavor numbers, allowing for an intermediate phase which is dominated by fluctuations in a vector channel. If these interactions approach criticality, the intermediate phase could be characterized by a Lorentzbreaking vector condensate.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the phase structure of mass and spinimbalanced unitary Fermi gases in harmonic traps. To this end, we employ Density Functional Theory in the local density approximation. Depending on the values of the control parameters measuring mass and spin imbalance, we observe that three regions exist in the trap, namely: a superfluid region at the center, surrounded by a mixed region of resonantly interacting spinup and spindown fermions, and finally a fully polarized phase surrounding the previous two regions. We also find regimes in the phase diagram where the existence of a superfluid region at the center of the trap is not energetically favored. We point out the limitations of our approach at the present stage, and call for more detailed (ab initio) studies of the equation of state of uniform, massimbalanced unitary Fermi gases.02/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We compute the phase diagram of strongly interacting fermions in one dimension at finite temperature, with mass and spin imbalance. By including the possibility of the existence of a spatially inhomogeneous ground state, we find regions where spatially varying superfluid phases are favored over homogeneous phases. We obtain estimates for critical values of the temperature, mass and spin imbalance, above which these phases disappear. Finally, we show that an intriguing relation exists between the general structure of the phase diagram and the binding energies of the underlying twobody boundstate problem.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the effect of a finite volume on the critical behavior of the theory of the strong interaction (QCD) by means of a quarkmeson model for two quark flavors. In particular, we analyze the effect of a finite volume on the location of the critical point in the phase diagram existing in our model. In our analysis, we take into account the effect of longrange fluctuations with the aid of renormalization group techniques. We find that these quantum and thermal fluctuations, absent in meanfield studies, play an import role for the dynamics in a finite volume. We show that the critical point is shifted towards smaller temperatures and larger values of the quark chemical potential if the volume size is decreased. This behavior persists for antiperiodic as well as periodic boundary conditions for the quark fields as used in many lattice QCD simulations.08/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Fermi gases in strongly coupled regimes, such as the unitary limit, are inherently challenging for manybody methods. Although much progress has been made with purely analytic methods, quantitative results require ab initio numerical approaches, such as Monte Carlo (MC) calculations. However, massimbalanced and spinimbalanced gases are not accessible to MC calculations due to the infamous sign problem. It was recently pointed out that the sign problem, for finite spin imbalance, can be circumvented by resorting to imaginary polarizations and analytic continuation. Large parts of the phase diagram spanned by temperature and polarization then become accessible to MC calculations. We propose to apply a similar strategy to the massimbalanced case, which opens up the possibility to study the associated phase diagram with MC calculations. In particular, our analysis suggests that a detection of a (tri)critical point in this phase diagram is possible. We also discuss calculations in the zerotemperature limit with our approach.06/2013;  [Show abstract] [Hide abstract]
ABSTRACT: From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is wellknown, however, that in the relativistic case imaginary chemical potentials solve this problem, assuming the data can be analytically continued to the real axis. Is this feasible for nonrelativistic systems? Are the interesting features of the phase diagram accessible in this manner? By introducing complex chemical potentials, for real total particle number and imaginary polarization, the sign problem is avoided in the nonrelativistic case. To give a first answer to the above questions, we perform a meanfield study of the finitetemperature phase diagram of spin1/2 fermions with imaginary polarization.Physical Review Letters 03/2013; 110(13):130404. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analyse the role of the quark backreaction on the gaugefield dynamics and its impact on the Polyakovloop potential. Based on our analysis we construct an improved Polyakovloop potential that can be used in future model studies. In the present work, we employe this improved potential in a study of a 2+1 flavour Polyakovquarkmeson model and show that the temperature dependence of the order parameters and thermodynamics is closer to full QCD. We discuss the results for QCD thermodynamics and outline briefly the dependence of our results on the critical temperature and the parametrisation of the Polyakovloop potential as well as the mass of the sigmameson.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the quark backreaction on the Polyakov loop and its impact on the thermodynamics of quantum chromodynamics. The dynamics of the gluons generating the Polyakovloop potential is altered by the presence of dynamical quarks. However, this backreaction of the quarks has not yet been taken into account in Polyakovloop extended model studies. In the present work, we show within a 2+1 flavour Polyakovquarkmeson model that a quarkimproved Polyakovloop potential leads to a smoother transition between the lowtemperature hadronic phase and the hightemperature quarkgluon plasma phase. In particular, we discuss the dependence of our results on the remaining uncertainties that are the critical temperature and the parametrisation of the Polyakovloop potential as well as the mass of the sigmameson.Physical review D: Particles and fields 02/2013; 87(7).  [Show abstract] [Hide abstract]
ABSTRACT: We study the phase diagram of the GrossNeveu model in d=2+1 spacetime dimensions in the plane spanned by temperature and the number of massless fermion flavors. We use a functional renormalization group approach based on a nonperturbative derivative expansion that accounts for fermionic as well as composite bosonic fluctuations. We map out the phase boundary separating the ordered massive lowtemperature phase from the disordered hightemperature phase. The phases are separated by a secondorder phase transition in the 2d Ising universality class. We determine the size of the Ginzburg region and show that it scales to zero for large $\Nf$ following a powerlaw, in agreement with large$\Nf$ and lattice studies. We also study the regimes of local order above as well as the classical regime below the critical temperature.Journal of Physics A Mathematical and Theoretical 12/2012; 46(28). · 1.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is well known, however, that in the relativistic case imaginary chemical potentials solve this problem, assuming the data can be analytically continued to the real axis. Is this feasible for nonrelativistic systems? Are the interesting features of the phase diagram accessible in this manner? Introducing complex chemical potentials, for real total particle number and imaginary polarization, the sign problem is avoided in the nonrelativistic case. To give a first answer to the above questions, we perform a meanfield study of the finitetemperature phase diagram of spin1/2 fermions with imaginary polarization.09/2012;  [Show abstract] [Hide abstract]
ABSTRACT: The question of the exact nature of the phase transition in twoflavor QCD is still under discussion. Recent results for small quark masses in simulations with 2+1 flavors show scaling behavior consistent with the O(4) or O(2) universality class. For a precise determination, an assessment of deviations from the ideal scaling behavior due to finite quark masses and finite simulation volumes is necessary. We study the scaling behavior at the chiral phase transition with an effective quarkmeson model. In our Renormalization Group approach, the quark masses in the model can be varied from the chiral limit over a wide range of values, which allows us to estimate scaling deviations due to large quark masses and the extent of the scaling region. We conclude that scaling deviations are already large at pion masses of 75 MeV, but that the effect is difficult to see in the absence of results for even smaller masses. Comparing results only in a narrow window of pion masses leads to the observation of apparent scaling behavior. While the scaling deviations are not necessarily universal, we expect that this may affect current lattice simulation results. By placing the system in a finite box, we investigate the transition between infinitevolume scaling behavior and finitesize scaling. We estimate that finitesize scaling behavior can be tested in regions where pion mass times box size is approximately 2  3, which is smaller than in most current lattice simulations. We expect that finitevolume effects are small for pion masses of 75 MeV and lattice aspect ratios with TL > 8, but that they will become significant when pion masses in lattice simulations become smaller.11/2011;  [Show abstract] [Hide abstract]
ABSTRACT: The chiral phase transition in QCD at finite chemical potential and temperature can be characterized for small chemical potential by its curvature and the transition temperature. The curvature is accessible to QCD lattice simulations, which are always performed at finite pion masses and in finite simulation volumes. We investigate the effect of a finite volume on the curvature of the chiral phase transition line. We use functional renormalization group methods with a two flavor quarkmeson model to obtain the effective action in a finite volume, including both quark and meson fluctuation effects. Depending on the chosen boundary conditions and the pion mass, we find pronounced finitevolume effects. For periodic quark boundary conditions in spatial directions, we observe a decrease in the curvature in intermediate volume sizes, which we interpret in terms of finitevolume quark effects. Our results have implications for the phase structure of QCD in a finite volume, where the location of a possible critical endpoint might be shifted compared to the infinitevolume case.Physics Letters B 10/2011; · 4.57 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The theory of the strong interaction, Quantum Chromodynamics (QCD), describes the generation of hadronic masses and the state of hadronic matter during the early stages of the evolution of the universe. As a complement, experiments with ultracold fermionic atoms provide a clean environment to benchmark our understanding of dynamical formation of condensates and the generation of bound states in strongly interacting manybody systems. Renormalization group (RG) techniques offer great potential for theoretical advances in both hot and dense QCD as well as manybody physics, but their connections have not yet been investigated in great detail. We aim to take a further step to bridge this gap. A crossfertilization is indeed promising since it may eventually provide us with an abinitio description of hadronization, condensation, and boundstate formation in strongly interacting theories. After giving a thorough introduction to the derivation and analysis of fermionic RG flows, we give an introductory review of our present understanding of universal longrange behavior in various different theories, ranging from nonrelativistic manybody problems to relativistic gauge theories, with an emphasis on scaling behavior of physical observables close to quantum phase transitions (i. e. phase transitions at zero temperature) as well as thermal phase transitions.Journal of Physics G Nuclear and Particle Physics 08/2011; · 5.33 Impact Factor 
Article: Algorithmic derivation of functional renormalization group equations and DysonSchwinger equations
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ABSTRACT: We present the Mathematica application DoFun which allows to derive DysonSchwinger equations and renormalization group flow equations for npoint functions in a simple manner. DoFun offers several tools which considerably simplify the derivation of these equations from a given physical action. We discuss the application of DoFun by means of two different types of quantum field theories, namely a bosonic O(N) theory and the GrossNeveu model.Computer Physics Communications 02/2011; · 2.41 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the fixedpoint structure of fourfermion interactions in twoflavor QCD with Nc colors close to the finitetemperature phase boundary. In particular, we analyze how the fixedpoint structure of fourfermion interactions is related to the confining dynamics in the gauge sector. We show that there exists indeed a mechanism which dynamically locks the chiral phase transition to the deconfinement phase transition. This mechanism allows us to determine a window for the values of physical observables in which the two phase transitions lie close to each other.Physical review D: Particles and fields 02/2011;  [Show abstract] [Hide abstract]
ABSTRACT: We study the phase diagram of twoflavor QCD at imaginary chemical potentials in the chiral limit. To this end we compute order parameters for chiral symmetry breaking and quark confinement. The interrelation of quark confinement and chiral symmetry breaking is analyzed with a new order parameter for the confinement phase transition. We show that it is directly related to both the quark density as well as the Polyakov loop expectation value. Our analytical and numerical results suggest a close relation between the chiral and the confinement phase transition.Physical Review Letters 01/2011; 106(2):022002. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present results for the phase diagram of QCD with two massless quark flavours as obtained from a firstprinciples functional renormalisation group approach. In particular we compute order parameters for chiral symmetry breaking and quark confinement at vanishing and nonzero imaginary chemical potential. Our analytical and numerical results suggest a close relation between the chiral and the deconfinement phase transition. We discuss the properties of dual order parameters at imaginary and real chemical potential. Comment: 3 pages, 5 figures; Contribution to the proceedings 'Quark Confinement and the Hadron Spectrum IX', 2010, Madrid12/2010; 
Article: Beyond Miransky Scaling
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ABSTRACT: We study the scaling behavior of physical observables in stronglyflavored asymptotically free gauge theories, such as manyflavor QCD. Such theories approach a quantum critical point when the number of fermion flavors is increased. It is wellknown that physical observables at this quantum critical point exhibit an exponential scaling behavior (Miransky scaling), provided the gauge coupling is considered as a constant external parameter. This scaling behavior is modified when the scale dependence of the gauge coupling is taken into account. Provided that the gauge coupling approaches an IR fixed point, we derive the resulting universal powerlaw corrections to the exponential scaling behavior and show that they are uniquely determined by the IR critical exponent of the gauge coupling. To illustrate our findings, we compute the universal corrections in manyflavor QCD with the aid of nonperturbative functional renormalization group methods. In this case, we expect the powerlaw scaling to be quantitatively more relevant if the theories are probed, for instance, at integer Nf as done in lattice simulations.Physical review D: Particles and fields 12/2010; 
Article: Asymptotic safety: a simple example
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ABSTRACT: We use the GrossNeveu model in 2<d<4 as a simple fermionic example for Weinberg's asymptotic safety scenario: despite being perturbatively nonrenormalizable, the model defines an interacting quantum field theory being valid to arbitrarily high momentum scales owing to the existence of a nonGaussian fixed point. Using the functional renormalization group, we study the UV behavior of the model in both the purely fermionic as well as a partially bosonized language. We show that asymptotic safety is realized at nonGaussian fixed points in both formulations, the universal critical exponents of which we determine quantitatively. The partially bosonized formulation allows to make contact to the largeNf expansion where the model is known to be renormalizable to allorders. In this limit, the fixedpoint action as well as all universal critical exponents can be computed analytically. As asymptotic safety has become an important scenario for quantizing gravity, our description of a wellunderstood model is meant to provide for an easily accessible and controllable example of modern nonperturbative quantum field theory.Physical review D: Particles and fields 11/2010; 83(8).
Publication Stats
804  Citations  
82.19  Total Impact Points  
Top Journals
Institutions

2013

Technical University Darmstadt
Darmstadt, Hesse, Germany


2009–2012

FriedrichSchillerUniversity Jena
 Department of Theoretical Physics
Jena, Thuringia, Germany


2008–2010

TRIUMF
Vancouver, British Columbia, Canada


2003–2006

Heidelberg University
 Institute of Theoretical Physics
Heidelberg, BadenWuerttemberg, Germany
