5,848 research outputs found

### Large-N transition temperature for superconducting films in a magnetic field

We consider the $N$-component Ginzburg-Landau model in the large $N$ limit,
the system being embedded in an external constant magnetic field and confined
between two parallel planes a distance $L$ apart from one another. On physical
grounds, this corresponds to a material in the form of a film in the presence
of an external magnetic field. Using techniques from dimensional and
$zeta$-function regularization, modified by the external field and the
confinement conditions, we investigate the behavior of the system as a function
of the film thickness $L$. This behavior suggests the existence of a minimal
critical thickness below which superconductivity is suppressed.Comment: Revtex, two column, 4 pages, 2 figure

### Non-Commutative Complete Mellin Representation for Feynman Amplitudes

We extend the complete Mellin (CM) representation of Feynman amplitudes to
the non-commutative quantum field theories. This representation is a versatile
tool. It provides a quick proof of meromorphy of Feynman amplitudes in
parameters such as the dimension of space-time. In particular it paves the road
for the dimensional renormalization of these theories. This complete Mellin
representation also allows the study of asymptotic behavior under rescaling of
arbitrary subsets of external invariants of any Feynman amplitude.Comment: 14 pages, no figur

### Gauge dependenceof the order parameter anomalous dimension in the Ginzburg-Landau model and the critical fluctuations in superconductors

The critical fluctuations of superconductors are discussed in a fixed
dimension scaling suited to describe the type II regime. The gauge dependence
of the anomalous dimension of the scalar field is stablished exactly from the
Ward-Takahashi identities. Its fixed point value gives the $\eta$ critical
exponent and it is shown that $\eta$ is gauge independent, as expected on
physical grounds. In the scaling considered, $\eta$ is found to be zero at
1-loop order, while $\nu\approx 0.63$. This result is just the 1-loop values
for the XY model obtained in the fixed dimension renormalization group
approach. It is shown that this XY behavior holds at all orders. The result
$\eta=\eta_{XY}$ should be contrasted with the negative values frequently
reported in the literature.Comment: EuroLaTex, 7 pages, 2 figures, reference updated; version to be
published in Europhysics Letter

### Thermofield Dynamics and Casimir Effect for Fermions

A generalization of the Bogoliubov transformation is developed to describe a
space compactified fermionic field. The method is the fermionic counterpart of
the formalism introduced earlier for bosons (J. C. da Silva, A. Matos Neto, F.
C. Khanna and A. E. Santana, Phys. Rev. A 66 (2002) 052101), and is based on
the thermofield dynamics approach. We analyse the energy-momentum tensor for
the Casimir effect of a free massless fermion field in a $d$-dimensional box at
finite temperature. As a particular case the Casimir energy and pressure for
the field confined in a 3-dimensional parallelepiped box are calculated. It is
found that the attractive or repulsive nature of the Casimir pressure on
opposite faces changes depending on the relative magnitude of the edges. We
also determine the temperature at which the Casimir pressure in a cubic boc
changes sign and estimate its value when the edge of the cybe is of the order
of confining lengths for baryons.Comment: 21 pages, 3 figures, to appear in Annals of Physic

### Maximum Entanglement in Squeezed Boson and Fermion States

A class of squeezed boson and fermion states is studied with particular
emphasis on the nature of entanglement. We first investigate the case of
bosons, considering two-mode squeezed states. Then we construct the fermion
version to show that such states are maximum entangled, for both bosons and
fermions. To achieve these results, we demonstrate some relations involving
squeezed boson states. The generalization to the case of fermions is made by
using Grassmann variables.Comment: 4 page

### Dressed States Approach to Quantum Systems

Using the non-perturbative method of {\it dressed} states previously
introduced in JPhysA, we study effects of the environment on a quantum
mechanical system, in the case the environment is modeled by an ensemble of non
interacting harmonic oscillators. This method allows to separate the whole
system into the {\it dressed} mechanical system and the {\it dressed}
environment, in terms of which an exact, non-perturbative approach is possible.
When applied to the Brownian motion, we give explicit non-perturbative formulas
for the classical path of the particle in the weak and strong coupling regimes.
When applied to study atomic behaviours in cavities, the method accounts very
precisely for experimentally observed inhibition of atomic decay in small
cavities PhysLA, physics0111042

### Nonrelativistic Limit of the Scalar Chern-Simons Theory and the Aharonov-Bohm Scattering

We study the nonrelativistic limit of the quantum theory of a Chern-Simons
field minimally coupled to a scalar field with quartic self-interaction. The
renormalization of the relativistic model, in the Coulomb gauge, is discussed.
We employ a procedure to calculate scattering amplitudes for low momenta that
generates their $|p|/m$ expansion and separates the contributions coming from
high and low energy intermediary states. The two body scattering amplitude is
calculated up to order $p^2/m^2$. It is shown that the existence of a critical
value of the self-interaction parameter for which the 2-particle scattering
amplitude reduces to the Aharonov-Bohm one is a strictly nonrelativistic
feature. The subdominant terms correspond to relativistic corrections to the
Aharonov-Bohm scattering. A nonrelativistic reduction scheme and an effective
nonrelativistic Lagrangian to account for the relativistic corrections are
proposed.Comment: 22 pages, 8 figures, revtex, to be published in Int. J. Mod. Phys.

### Controlling Excitations Inversion of a Cooper Pair Box Interacting with a Nanomechanical Resonator

We investigate the action of time dependent detunings upon the excitation
inversion of a Cooper pair box interacting with a nanomechanical resonator. The
method employs the Jaynes-Cummings model with damping, assuming different decay
rates of the Cooper pair box and various fixed and t-dependent detunings. It is
shown that while the presence of damping plus constant detunings destroy the
collapse/revival effects, convenient choices of time dependent detunings allow
one to reconstruct such events in a perfect way. It is also shown that the mean
excitation of the nanomechanical resonator is more robust against damping of
the Cooper pair box for convenient values of t-dependent detunings.Comment: 11 pages, 5 figure

### Large N study of extreme type II superconductors in a magnetic field

The large N analysis of an extreme type II superconductor is revisited. It is
found that the phase transition is of second-order in dimensions 4 < d < 6. For
the physical dimension d=3 no sign of phase transition is found, contrary to
early claims.Comment: Revtex, 7 pages, no figure

### Critical properties of the topological Ginzburg-Landau model

We consider a Ginzburg-Landau model for superconductivity with a Chern-Simons
term added. The flow diagram contains two charged fixed points corresponding to
the tricritical and infrared stable fixed points. The topological coupling
controls the fixed point structure and eventually the region of first order
transitions disappears. We compute the critical exponents as a function of the
topological coupling. We obtain that the value of the $\nu$ exponent does not
vary very much from the XY value, $\nu_{XY}=0.67$. This shows that the
Chern-Simons term does not affect considerably the XY scaling of
superconductors. We discuss briefly the possible phenomenological applications
of this model.Comment: RevTex, 7 pages, 8 figure

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