13,850 research outputs found
The structure of the graviton self-energy at finite temperature
We study the graviton self-energy function in a general gauge, using a hard
thermal loop expansion which includes terms proportional to T^4, T^2 and
log(T). We verify explicitly the gauge independence of the leading T^4 term and
obtain a compact expression for the sub-leading T^2 contribution. It is shown
that the logarithmic term has the same structure as the ultraviolet pole part
of the T=0 self-energy function. We argue that the gauge-dependent part of the
T^2 contribution is effectively canceled in the dispersion relations of the
graviton plasma, and present the solutions of these equations.Comment: 27 pages, 6 figure
Non-linear electromagnetic interactions in thermal QED
We examine the behavior of the non-linear interactions between
electromagnetic fields at high temperature. It is shown that, in general, the
log(T) dependence on the temperature of the Green functions is simply related
to their UV behavior at zero-temperature. We argue that the effective action
describing the nonlinear thermal electromagnetic interactions has a finite
limit as T tends to infinity. This thermal action approaches, in the long
wavelength limit, the negative of the corresponding zero-temperature action.Comment: 7 pages, IFUSP/P-111
The theory of the reentrant effect in susceptibility of cylindrical mesoscopic samples
A theory has been developed to explain the anomalous behavior of the magnetic
susceptibility of a normal metal-superconductor () structure in weak
magnetic fields at millikelvin temperatures. The effect was discovered
experimentally by A.C. Mota et al \cite{10}. In cylindrical superconducting
samples covered with a thin normal pure metal layer, the susceptibility
exhibited a reentrant effect: it started to increase unexpectedly when the
temperature lowered below 100 mK. The effect was observed in mesoscopic
structures when the and metals were in good electric contact. The
theory proposed is essentially based on the properties of the Andreev levels in
the normal metal. When the magnetic field (or temperature) changes, each of the
Andreev levels coincides from time to time with the chemical potential of the
metal. As a result, the state of the structure experiences strong
degeneracy, and the quasiparticle density of states exhibits resonance spikes.
This generates a large paramagnetic contribution to the susceptibility, which
adds up to the diamagnetic contribution thus leading to the reentrant effect.
The explanation proposed was obtained within the model of free electrons. The
theory provides a good description for experimental results [10]
High temperature limit in static backgrounds
We prove that the hard thermal loop contribution to static thermal amplitudes
can be obtained by setting all the external four-momenta to zero before
performing the Matsubara sums and loop integrals. At the one-loop order we do
an iterative procedure for all the 1PI one-loop diagrams and at the two-loop
order we consider the self-energy. Our approach is sufficiently general to the
extent that it includes theories with any kind of interaction vertices, such as
gravity in the weak field approximation, for space-time dimensions. This
result is valid whenever the external fields are all bosonic.Comment: 15 pages, 11 figures. To be published in Physical Review
Seiberg-Witten maps and anomalies in noncommutative Yang-Mills theories
A BRST-cohomological analysis of Seiberg-Witten maps and results on gauge
anomalies in noncommutative Yang-Mills theories with general gauge groups are
reviewed.Comment: 9 pages, talk at 9th Adriatic Meeting, Dubrovnik, Croatia, 4-14 Sept.
200
Thermal Effective Lagrangian of Static Gravitational Fields
We compute the effective Lagrangian of static gravitational fields
interacting with thermal fields. Our approach employs the usual imaginary time
formalism as well as the equivalence between the static and space-time
independent external gravitational fields. This allows to obtain a closed form
expression for the thermal effective Lagrangian in space-time dimensions.Comment: Accepted for publication in the Physical Review
Thermal matter and radiation in a gravitational field
We study the one-loop contributions of matter and radiation to the
gravitational polarization tensor at finite temperatures. Using the
analytically continued imaginary-time formalism, the contribution of matter is
explicitly given to next-to-leading () order. We obtain an exact form for
the contribution of radiation fields, expressed in terms of generalized Riemann
zeta functions. A general expression is derived for the physical polarization
tensor, which is independent of the parametrization of graviton fields. We
investigate the effective thermal masses associated with the normal modes of
the corresponding graviton self-energy.Comment: 32 pages, IFUSP/P-107
"Unusual" critical states in type-II superconductors
We give a theoretical description of the general critical states in which the
critical currents in type-II superconductors are not perpendicular to the local
magnetic induction. Such states frequently occur in real situations, e.g., when
the sample shape is not sufficiently symmetric or the direction of the external
magnetic field changes in some complex way. Our study is restricted to the
states in which flux-line cutting does not occur. The properties of such
general critical states can essentially differ from the well-known properties
of the usual Bean critical states. To illustrate our approach, we analyze
several examples. In particular, we consider the critical states in a slab
placed in a uniform perpendicular magnetic field and to which two components of
the in-plane magnetic field are then applied successively. We also analyze the
critical states in a long thin strip placed in a perpendicular magnetic field
which then is tilted towards the axis of the strip.Comment: 15 pages including 11 figure
N=2 SYM Action as a BRST Exact Term, Topological Yang Mills and Instantons
By constructing a nilpotent extended BRST operator \bs that involves the
N=2 global supersymmetry transformations of one chirality, we show that the
standard N=2 off-shell Super Yang Mills Action can be represented as an exact
BRST term \bs \Psi, if the gauge fermion is allowed to depend on the
inverse powers of supersymmetry ghosts. By using this nonanalytical structure
of the gauge fermion (via inverse powers of supersymmetry ghosts), we give
field redefinitions in terms of composite fields of supersymmetry ghosts and
N=2 fields and we show that Witten's topological Yang Mills theory can be
obtained from the ordinary Euclidean N=2 Super Yang Mills theory directly by
using such field redefinitions. In other words, TYM theory is obtained as a
change of variables (without twisting). As a consequence it is found that
physical and topological interpretations of N=2 SYM are intertwined together
due to the requirement of analyticity of global SUSY ghosts. Moreover, when
after an instanton inspired truncation of the model is used, we show that the
given field redefinitions yield the Baulieu-Singer formulation of Topological
Yang Mills.Comment: Latex, 1+15 pages. Published versio
Stacking Faults, Bound States, and Quantum Hall Plateaus in Crystalline Graphite
We analyze the electronic properties of a simple stacking defect in Bernal
graphite. We show that a bound state forms, which disperses as |\bfk-\bfK|^3
in the vicinity of either of the two inequivalent zone corners \bfK. In the
presence of a strong c-axis magnetic field, this bound state develops a Landau
level structure which for low energies behaves as E\nd_n\propto |n B|^{3/2}.
We show that buried stacking faults have observable consequences for surface
spectroscopy, and we discuss the implications for the three-dimensional quantum
Hall effect (3DQHE). We also analyze the Landau level structure and chiral
surface states of rhombohedral graphite, and show that, when doped, it should
exhibit multiple 3DQHE plateaus at modest fields.Comment: 19 page
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