57,744 research outputs found
Effective actions at finite temperature
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 adde
Effective Actions for 0+1 Dimensional Scalar QED and its SUSY Generalization at
We compute the effective actions for the 0+1 dimensional scalar field
interacting with an Abelian gauge background, as well as for its supersymmetric
generalization at finite temperature.Comment: 5 pages, Latex fil
Hard thermal effective action in QCD through the thermal operator
Through the application of the thermal operator to the zero temperature
retarded Green's functions, we derive in a simple way the well known hard
thermal effective action in QCD. By relating these functions to forward
scattering amplitudes for on-shell particles, this derivation also clarifies
the origin of important properties of the hard thermal effective action, such
as the manifest Lorentz and gauge invariance of its integrand.Comment: 6 pages, contribution of the quarks to the effective action included
and one reference added, version to be published in Phys. Rev.
Thermodynamic properties of Holstein polarons and the effects of disorder
The ground state and finite temperature properties of polarons are studied
considering a two-site and a four-site Holstein model by exact diagonalization
of the Hamiltonian. The kinetic energy, Drude weight, correlation functions
involving charge and lattice deformations, and the specific heat have been
evaluated as a function of electron-phonon (e-ph) coupling strength and
temperature. The effects of site diagonal disorder on the above properties have
been investigated. The disorder is found to suppress the kinetic energy and the
Drude weight, reduces the spatial extension of the polaron, and makes the
large-to-small polaron crossover smoother. Increasing temperature also plays
similar role. For strong coupling the kinetic energy arises mainly from the
incoherent hopping processes owing to the motion of electrons within the
polaron and is almost independent of the disorder strength. From the coherent
and incoherent contributions to the kinetic energy, the temperature above which
the incoherent part dominates is determined as a function of e-ph coupling
strength.Comment: 17 pages. 17 figure
Hard thermal effective actions in the Schwinger formulation
We derive the properties of hard thermal effective actions in gauge theories
from the point of view of Schwinger's proper time formulation. This analysis is
simplified by introducing a set of generalized energy and momenta which are
conserved and are non-local in general. These constants of motion, which embody
energy-momentum exchanges between the fields and the particles along their
trajectories, can be related to a class of gauge invariant or covariant
potentials in the hard thermal regime. We show that in this regime the
generalized energy, which is non-local in general, generates the relevant
non-local behavior of hard thermal effective actions which become local only in
the static limit.Comment: 6 pages, references added, version to appear in Physical Review
Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped PrCeCuO
We have investigated the doping dependence of the penetration depth vs.
temperature in electron doped PrCeCuO using a model
which assumes the uniform coexistence of (mean-field) antiferromagnetism and
superconductivity. Despite the presence of a pairing gap in the
underlying spectrum, we find nodeless behavior of the low- penetration depth
in underdoped case, in accord with experimental results. As doping increases, a
linear-in- behavior of the penetration depth, characteristic of d-wave
pairing, emerges as the lower magnetic band crosses the Fermi level and creates
a nodal Fermi surface pocket.Comment: Accepted in PRL for publicatio
Effects of Strain coupling and Marginal dimensionality in the nature of phase transition in Quantum paraelectrics
Here a recently observed weak first order transition in doped SrTiO3 is
argued to be a consequence of the coupling between strain and order parameter
fluctuations. Starting with a semi-microscopic action, and using
renormalization group equations for vertices, we write the free energy of such
a system. This fluctuation renormalized free energy is then used to discuss the
possibility of first order transition at zero temperature as well as at finite
temperature. An asymptotic analysis predicts small but a finite discontinuity
in the order parameter near a mean field quantum critical point at zero
temperature. In case of finite temperature transition, near quantum critical
point such a possibility is found to be extremely weak. Results are in accord
with some experimental findings on quantum paraelectrics such as SrTiO3 and
KTaO3.Comment: Revised versio
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