52,435 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
A Nonstandard Supersymmetric KP Hierarchy
We show that the supersymmetric nonlinear Schr\"odinger equation can be
written as a constrained super KP flow in a nonstandard representation of the
Lax equation. We construct the conserved charges and show that this system
reduces to the super mKdV equation with appropriate identifications. We
construct various flows generated by the general nonstandard super Lax equation
and show that they contain both the KP and mKP flows in the bosonic limits.
This nonstandard supersymmetric KP hierarchy allows us to construct a new super
KP equation which is nonlocal.Comment: 18 pages, plain TeX, UR-1367, ER-40685-81
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.
Duality, Monodromy and Integrability of Two Dimensional String Effective Action
The monodromy matrix, , is constructed for two dimensional
tree level string effective action. The pole structure of is
derived using its factorizability property. It is found that the monodromy
matrix transforms non-trivially under the non-compact T-duality group, which
leaves the effective action invariant and this can be used to construct the
monodromy matrix for more complicated backgrounds starting from simpler ones.
We construct, explicitly, for the exactly solvable
Nappi-Witten model, both when B=0 and , where these ideas can be
directly checked. We consider well known charged black hole solutions in the
heterotic string theory which can be generated by T-duality transformations
from a spherically symmetric `seed' Schwarzschild solution. We construct the
monodromy matrix for the Schwarzschild black hole background of the heterotic
string theory.Comment: 20 pages, to be published in Physical Review
Open Membranes, p-Branes and Noncommutativity of Boundary String Coordinates
We study the dynamics of an open membrane with a cylindrical topology, in the
background of a constant three form, whose boundary is attached to p-branes.
The boundary closed string is coupled to a two form potential to ensure gauge
invariance. We use the action, due to Bergshoeff, London and Townsend, to study
the noncommutativity properties of the boundary string coordinates. The
constrained Hamiltonian formalism due to Dirac is used to derive the
noncommutativity of coordinates. The chain of constraints is found to be finite
for a suitable gauge choice, unlike the case of the static gauge, where the
chain has an infinite sequence of terms. It is conjectured that the formulation
of closed string field theory may necessitate introduction of a star product
which is both noncommutative and nonassociative.Comment: 32page
Quantization in a General Light-front Frame
In this paper, we study the question of quantization of quantum field
theories in a general light-front frame. We quantize scalar, fermion as well as
gauge field theories in a systematic manner carrying out the Hamiltonian
analysis carefully. The decomposition of the fields into positive and negative
frequency terms needs to be done carefully after which we show that the (anti)
commutation relations for the quantum operators become frame independent. The
frame dependence is completely contained in the functions multiplying these
operators in the field decomposition. We derive the propagators from the vacuum
expectation values of the time ordered products of the fields.Comment: 14 pages, revtex, version to be published in Phys. Rev. D with the
discussion of Abelian field quantization replaced by the non-Abelian field
and some comments added on the Mandelstam-Liebbrandt prescriptio
Exact Effective Action for (1+1 Dimensional) Fermions in an Abelian Background at Finite Temperature
In an effort to further understand the structure of effective actions for
fermions in an external gauge background at finite temperature, we study the
example of 1+1 dimensional fermions interacting with an arbitrary Abelian gauge
field. We evaluate the effective action exactly at finite temperature. This
effective action is non-analytic as is expected at finite temperature. However,
contrary to the structure at zero temperature and contrary to naive
expectations, the effective action at finite temperature has interactions to
all (even) orders (which, however, do not lead to any quantum corrections). The
covariant structure thus obtained may prove useful in studying 2+1 dimensional
models in arbitrary backgrounds. We also comment briefly on the solubility of
various 1+1 dimensional models at finite temperature.Comment: A few clarifying remarks added;21 page
Electronic phase separation due to magnetic polaron formation in the semimetallic ferromagnet EuB - A weakly-nonlinear-transport study
We report measurements of weakly nonlinear electronic transport, as measured
by third-harmonic voltage generation , in the low-carrier density
semimetallic ferromagnet EuB, which exhibits an unusual magnetic ordering
with two consecutive transitions at \,K and \,K. Upon cooling in zero magnetic field through the ferromagnetic
transition, the dramatic drop in the linear resistivity at the upper transition
coincides with the onset of nonlinearity, and upon further cooling is
followed by a pronounced peak in at the lower transition
. Likewise, in the paramagnetic regime, a drop of the material's
magnetoresistance precedes a magnetic-field-induced peak in nonlinear
transport. A striking observation is a linear temperature dependence of
. We suggest a picture where at the upper transition
the coalescing MP form a conducting path giving rise to a strong
decrease in the resistance. The MP formation sets in at around \,K below which these entities are isolated and strongly fluctuating, while
growing in number. The MP then start to form links at , where
percolative electronic transport is observed. The MP merge and start forming a
continuum at the threshold . In the paramagnetic temperature regime
, MP percolation is induced by a magnetic field, and the
threshold accompanied by charge carrier delocalization occurs at a single
critical magnetization.Comment: to appear in J. Kor. Phys. Soc (ICM2012 conference contribution
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