J. Frenkel

University of São Paulo, San Paulo, São Paulo, Brazil

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Publications (136)269.93 Total impact

  • Source
    Ashok K. Das, J. Frenkel
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    ABSTRACT: We study the behavior of out of equilibrium retarded, advanced and correlated Green's functions within the context of an exactly soluble (quenched) model. We show, to the lowest order, that even though the pinch singularities cancel, there is a residual linear dependence on the time interval (after the quench) in the correlated Green's function which may invalidate perturbation theory. We sum the perturbation series to all orders in this simple model and show explicitly that the complete Green's functions are well behaved even for large time intervals. The exact form of the correlated Green's function allows us to extract a manifestly positive distribution function, for large times after the quench, which has a memory of the frequency of the initial system before the quench.
    04/2014; 89(8).
  • 02/2014; 105(5).
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    ABSTRACT: We obtain the effective Lagrangian of static gravitational fields interacting with a QED plasma at high temperature. Using the equivalence between the static hard thermal loops and those with zero external energy-momentum, we compute the effective Lagrangian up to two-loop order. We also obtain a non-perturbative contribution which arises from the sum of all infrared divergent ring-diagrams. From the gauge and Weyl symmetries of the theory, we deduce to all orders that this effective Lagrangian is equivalent to the pressure of a QED plasma in Minkowski space-time, with the global temperature replaced by the Tolman local temperature.
    11/2013; 89(4).
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    ABSTRACT: We discuss the high temperature behavior of retarded thermal loops in static external fields. We employ an analytic continuation of the imaginary time formalism and use a spectral representation of the thermal amplitudes. We show that, to all orders, the leading contributions of static hard thermal loops can be directly obtained by evaluating them at zero external energies and momenta.
    European Physical Journal C 10/2013; 73(10). · 5.25 Impact Factor
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    ABSTRACT: We study the question of complete gauge independence of the fermion pole mass by choosing a general class of gauge fixing which interpolates between the covariant, the axial and the Coulomb gauges for different values of the gauge fixing parameters. We derive the Nielsen identity describing the gauge parameter variation of the fermion two point function in this general class of gauges. Furthermore, we relate the denominator of the fermion propagator to the two point function. This then allows us to study directly the gauge parameter dependence of the denominator of the propa- gator using the Nielsen identity for the two point function. This leads to a simple proof that, when infrared divergences and mass shell singularities are not present at the pole, the fermion pole mass is gauge independent, in the complete sense, to all orders in perturbation theory. Namely, the pole is not only independent of the gauge fixing parameters, but has also the same value in both covariant and non-covariant gauges.
    Physical Review D 08/2013; 88(8). · 4.69 Impact Factor
  • Ashok K. Das, J. Frenkel
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    ABSTRACT: We study the behavior of the pole of the fermion propagator, in QED in $n$-dimensions, in a general class of gauges which interpolate between the covariant, the axial and the Coulomb gauges. We use Nielsen identities, following from the BRST invariance of the theory, to determine the gauge variation of the fermion two point function in this general class of gauges. This allows us to show directly and in a simple manner, to all orders in perturbation theory, that in the absence of infrared divergences and mass shell singularities, the fermion pole mass is gauge independent.
    Physics Letters B 06/2013; 726(1). · 4.57 Impact Factor
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    R. R. Francisco, J. Frenkel
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    ABSTRACT: We study, in the long wave-length and static limits, the structure of the n-point graviton functions at high temperature. Using the gauge and Weyl invariance of the theory, we derive a simple expression for the hard thermal amplitudes in these two limits.
    Physics Letters B 01/2013; 722(s 1–3). · 4.57 Impact Factor
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    Ashok K. Das, J. Frenkel, C. Schubert
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    ABSTRACT: We study the behavior of the dynamical fermion mass when infrared divergences and mass shell singularities are present in a gauge theory. In particular, in the massive Schwinger model in covariant gauges we find that the pole of the fermion propagator is divergent and gauge dependent at one loop, but the leading singularities cancel in the quenched rainbow approximation. On the other hand, in physical gauges, we find that the dynamical fermion mass is finite and gauge independent at least up to one loop.
    Physics Letters B 12/2012; 720(s 4–5). · 4.57 Impact Factor
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    ABSTRACT: We study, in a d-dimensional space-time, the nonanalyticity of the thermal free energy in the scalar phi^4 theory as well as in QED. We find that the infrared divergent contributions induce, when d is even, a nonanalyticity in the coupling alpha of the form (alpha)^[(d-1)/2] whereas when d is odd the nonanalyticity is only logarithmic.
    Physical review D: Particles and fields 11/2012; 86(10).
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    F. T. Brandt, J. Frenkel
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    ABSTRACT: We derive a closed-form result for the leading thermal contributions which appear in the n-dimensional phi3 theory at high temperature. These contributions become local only in the long wavelength and in the static limits, being given by different expressions in these two limits.
    Brazilian Journal of Physics 07/2012; 42(5-6). · 0.60 Impact Factor
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    Ashok Das, R. R. Francisco, J. Frenkel
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    ABSTRACT: We show, in the imaginary time formalism, that the temperature dependent parts of all the retarded (advanced) amplitudes vanish in the Schwinger model. We trace this behavior to the CPT invariance of the theory and give a physical interpretation of this result in terms of forward scattering amplitudes of on-shell thermal particles.
    Physical review D: Particles and fields 06/2012; 86(4).
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    F. T.brandt, J.frenkel, A.guerra
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    ABSTRACT: We derive the classical transport equation, in scalar field theory with a g2V(ϕ) interaction, from the equation of motion for the quantum field. We obtain a very simple, but iterative, expression for the effective action Γ which generates all the n-point Green functions in the high-temperature limit. An explicit and closed form is given for Γ in the static case.
    International Journal of Modern Physics A 01/2012; 13(24). · 1.13 Impact Factor
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    J Frenkel, J C Taylor
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    ABSTRACT: From microscopic models, a Langevin equation can, in general, be derived only as an approximation. Two possible conditions to validate this approximation are studied. One is, for a linear Langevin equation, that the frequency of the Fourier transform should be close to the natural frequency of the system. The other is by the assumption of "slow" variables. We test this method by comparison with an exactly soluble model and point out its limitations. We base our discussion on two approaches. The first is a direct, elementary treatment of Senitzky. The second is via a generalized Langevin equation as an intermediate step.
    Physical Review E 01/2012; 85(1 Pt 1):011135. · 2.31 Impact Factor
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    Ashok Das, J. Frenkel
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    ABSTRACT: In the Schwinger model at finite temperature, we derive a closed form result for the chiral anomaly which arises from the long distance behavior of the electric field \cite{frenkel}. We discuss the general properties associated with this thermal anomaly as well as its relation with the "index" of the Dirac operator. We further show that the thermal anomaly, like the zero temperature anomaly which arises from the ultraviolet behavior of the theory, does not receive any contribution from higher loops. Finally, we determine the complete effective action as well as the anomaly functional on both the thermal branches in the closed time path formalism.
    Physics Letters B 07/2011; 704(1). · 4.57 Impact Factor
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    J Frenkel, J C Taylor
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    ABSTRACT: The validity of the Langevin equation (both classical and quantum) is studied in cases when not all the equations of motion are linear. In particular, a model is studied in which the interaction is bilinear in the environment variables. We conclude that the equation is valid only for frequencies close to the frequency of the system alone. If the equation of motion of the system alone is nonlinear, we are unable to find a condition for the validity of the Langevin equation.
    02/2011;
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    Ashok Das, J. Frenkel
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    ABSTRACT: We study the Schwinger model at finite temperature and show that a temperature dependent chiral anomaly may arise from the long distance behavior of the electric field. At high temperature this anomaly depends linearly on the temperature T and is present not only in the two point function, but also in all even point amplitudes.
    Physics Letters B 12/2010; · 4.57 Impact Factor
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    Ashok Das, J. Frenkel
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    ABSTRACT: In continuation of our earlier proposal for evaluating thermal effective actions, we determine the exact fermion propagator in 1+1 dimensional massive QED. This propagator is used to derive the finite temperature effective action of the theory which generates systematically all the one loop Feynman amplitudes calculated in thermal perturbation theory. Various aspects of the effective action including its imaginary part are discussed. Comment: revtex 12 pages, 6 figures
    Physical review D: Particles and fields 07/2010;
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    ABSTRACT: We investigate the dynamics of a resistively shunted Josephson junction. We compute the Josephson frequency and the generalized impedances for a variety of the parameters, particularly with relevance to predicting the measurable effects of zero-temperature current noise in the resistor. Comment: New reference added and corrected misprints. Published in Phys. Rev. B (11 pages, 8 figures and 2 tables)
    Physical review. B, Condensed matter 06/2010; · 3.77 Impact Factor
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    Ashok Das, J. Frenkel
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    ABSTRACT: This is a more detailed version of our recent paper where we proposed, from first principles, a direct method for evaluating the exact fermion propagator in the presence of a general background field at finite temperature. This can, in turn, be used to determine the finite temperature effective action for the system. As applications, we discuss the complete one loop finite temperature effective actions for 0+1 dimensional QED as well as for the Schwinger model in detail. These effective actions, which are derived in the real time (closed time path) formalism, generate systematically all the Feynman amplitudes calculated in thermal perturbation theory and also show that the retarded (advanced) amplitudes vanish in these theories. Various other aspects of the problem are also discussed in detail. Comment: 9 pages, revtex, 1 figure, references added
    Physical review D: Particles and fields 11/2009;
  • Source
    Ashok Das, J. Frenkel
    [Show abstract] [Hide abstract]
    ABSTRACT: We present, from first principles, a direct method for evaluating the exact fermion propagator in the presence of a general background field at finite temperature, which can be used to determine the finite temperature effective action for the system. As applications, we determine the complete one loop finite temperature effective actions for (0+1)-dimensional QED as well as the Schwinger model. These effective actions, which are derived in the real time (closed time path) formalism, generate systematically all the Feynman amplitudes calculated in thermal perturbation theory and also show that the retarded (advanced) amplitudes vanish in these theories.
    Physics Letters B 07/2009; · 4.57 Impact Factor

Publication Stats

1k Citations
269.93 Total Impact Points

Institutions

  • 1977–2014
    • University of São Paulo
      • São Carlos Institute of Physics
      San Paulo, São Paulo, Brazil
  • 2000
    • University of Connecticut
      • Department of Physics
      Storrs, Connecticut, United States
    • Technion - Israel Institute of Technology
      H̱efa, Haifa District, Israel
  • 1982–1995
    • University of Cambridge
      • Department of Applied Mathematics and Theoretical Physics
      Cambridge, ENG, United Kingdom
    • Instituto Geológico de São Paulo
      San Paulo, São Paulo, Brazil
  • 1990
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 1976–1981
    • University of Oxford
      Oxford, England, United Kingdom
  • 1973–1974
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States