Publications (149)570.92 Total impact
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ABSTRACT: In the context of an exactly soluble out of equilibrium (quenched) model, we study an extension of the fluctuationdissipation relation. This involves a modified differential form of this relation, with an effective temperature which may have an explicit dependence on time scales.  [Show abstract] [Hide abstract]
ABSTRACT: Using the closed time path formalism in thermal field theory, we give a derivation of the fluctuationdissipation theorem which is based on the unitarity of the $S$matrix.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: In a ddimensional field theory at finite temperature T, with a g(2) phi(4) interaction, we compute the contributions to the free energy in the ring approximation. These are calculated in a consistent manner, by evaluating the thermal selfenergy Pi(T) in a similar approximation. The complete result can be expressed in closed form in terms of the Haber and Weldon functions h(d+1)(root Pi(T)/T2). This result exhibits, to leading order, a nonanalyticity in the coupling constant of the form (g(2))((d1)/2) or log(N)(g(2)) when d is even or odd, respectively. Copyright (C) EPLA, 2014  [Show abstract] [Hide abstract]
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 energymomentum, we compute the effective Lagrangian up to twoloop order. We also obtain a nonperturbative contribution which arises from the sum of all infrared divergent ringdiagrams. 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 spacetime, with the global temperature replaced by the Tolman local temperature.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
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 noncovariant gauges.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: We study, in the long wavelength and static limits, the structure of the npoint 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. 
Article: Infrared divergences, mass shell singularities and gauge dependence of the dynamical fermion mass
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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.  [Show abstract] [Hide abstract]
ABSTRACT: We study, in a ddimensional spacetime, 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)^[(d1)/2] whereas when d is odd the nonanalyticity is only logarithmic.  [Show abstract] [Hide abstract]
ABSTRACT: We derive a closedform result for the leading thermal contributions which appear in the ndimensional 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.  [Show abstract] [Hide abstract]
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 onshell thermal particles.  [Show abstract] [Hide abstract]
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 npoint Green functions in the hightemperature limit. An explicit and closed form is given for Γ in the static case.  [Show abstract] [Hide abstract]
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 Langevn equation as an intermediate step.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: In continuation of our earlier proposal [A. Das and J. Frenkel, Phys. Lett. B 680, 195 (2009).] and [A. Das and J. Frenkel, Phys. Rev. D 80, 125039 (2009).] 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.  [Show abstract] [Hide abstract]
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 zerotemperature current noise in the resistor. Comment: New reference added and corrected misprints. Published in Phys. Rev. B (11 pages, 8 figures and 2 tables)
Publication Stats
2k  Citations  
570.92  Total Impact Points  
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Institutions

19742015

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


19891992

University of Cambridge
 Department of Applied Mathematics and Theoretical Physics
Cambridge, England, United Kingdom


1982

Instituto Geológico de São Paulo
San Paulo, São Paulo, Brazil


19761977

University of Oxford
Oxford, England, United Kingdom


19731974

University of Wisconsin–Madison
Madison, Wisconsin, United States
