1,206 research outputs found
Casimir Energy and Entropy between perfect metal Spheres
We calculate the Casimir energy and entropy for two perfect metal spheres in
the large and short separation limit. We obtain nonmonotonic behavior of the
Helmholtz free energy with separation and temperature, leading to parameter
ranges with negative entropy, and also nonmonotonic behavior of the entropy
with temperature and with the separation between the spheres. The appearance of
this anomalous behavior of the entropy is discussed as well as its
thermodynamic consequences.Comment: 10 pages and 8 figures. Accepted for publication in the Proceedings
of the tenth conference on Quantum Field Theory under the influence of
external conditions - QFEXT'1
Spontaneous superconductivity and optical properties of high-Tc cuprates
We suggest that the high temperature superconductivity in cuprate compounds
may emerge due to interaction between copper-oxygen layers mediated by in-plane
plasmons. The strength of the interaction is determined by the c-axis geometry
and by the ab-plane optical properties. Without making reference to any
particular in-plane mechanism of superconductivity, we show that the interlayer
interaction favors spontaneous appearance of the superconductivity in the
layers. At a qualitative level the model describes correctly the dependence of
the transition temperature on the interlayer distance, and on the number of
adjacent layers in multilayered homologous compounds. Moreover, the model has a
potential to explain (i) a mismatch between the optimal doping levels for
critical temperature and superconducting density and (ii) a universal scaling
relation between the dc-conductivity, the superfluid density, and the
superconducting transition temperature.Comment: 4.4 pages, 2 figures; v2 matches the published version (clarifying
remarks and references are added
Roughness correction to the Casimir force : Beyond the Proximity Force Approximation
We calculate the roughness correction to the Casimir effect in the parallel
plates geometry for metallic plates described by the plasma model. The
calculation is perturbative in the roughness amplitude with arbitrary values
for the plasma wavelength, the plate separation and the roughness correlation
length. The correction is found to be always larger than the result obtained in
the Proximity Force Approximation.Comment: 7 pages, 3 figures, v2 with minor change
Of Some Theoretical Significance: Implications of Casimir Effects
In his autobiography Casimir barely mentioned the Casimir effect, but
remarked that it is "of some theortical significance." We will describe some
aspects of Casimir effects that appear to be of particular significance now,
more than half a century after Casimir's famous paper
Entanglement generation in atoms immersed in a thermal bath of external quantum scalar fields with a boundary
We examine the entanglement creation between two mutually independent
two-level atoms immersed in a thermal bath of quantum scalar fields in the
presence of a perfectly reflecting plane boundary. With the help of the master
equation that describes the evolution in time of the atom subsystem obtained,
in the weak-coupling limit, by tracing over environment (scalar fields) degrees
of freedom, we find that the presence of the boundary may play a significant
role in the entanglement creation in some circumstances and the new parameter,
the distance of the atoms from the boundary, besides the bath temperature and
the separation between the atoms, gives us more freedom in manipulating
entanglement generation. Remarkably, the final remaining entanglement in the
equilibrium state is independent of the presence of the boundary.Comment: 19 pages, 4 figures, to be published in PR
Edges and Diffractive Effects in Casimir Energies
The prototypical Casimir effect arises when a scalar field is confined
between parallel Dirichlet boundaries. We study corrections to this when the
boundaries themselves have apertures and edges. We consider several geometries:
a single plate with a slit in it, perpendicular plates separated by a gap, and
two parallel plates, one of which has a long slit of large width, related to
the case of one plate being semi-infinite. We develop a general formalism for
studying such problems, based on the wavefunctional for the field in the gap
between the plates. This formalism leads to a lower dimensional theory defined
on the open regions of the plates or boundaries. The Casimir energy is then
given in terms of the determinant of the nonlocal differential operator which
defines the lower dimensional theory. We develop perturbative methods for
computing these determinants. Our results are in good agreement with known
results based on Monte Carlo simulations. The method is well suited to
isolating the diffractive contributions to the Casimir energy.Comment: 32 pages, LaTeX, 9 figures. v2: additional discussion of
renormalization procedure, version to appear in PRD. v3: corrected a sign
error in (70
Casimir interactions in graphene systems
The non-retarded Casimir interaction (van der Waals interaction) between two
free standing graphene sheets as well as between a graphene sheet and a
substrate is determined. An exact analytical expression is given for the
dielectric function of graphene along the imaginary frequency axis within the
random phase approximation for arbitrary frequency, wave vector, and doping.Comment: 4 pages, 4 figure
Weak dispersive forces between glass-gold macroscopic surfaces in alcohols
In this work we concentrate on an experimental validation of the Lifshitz
theory for van der Waals and Casimir forces in gold-alcohol-glass systems. From
this theory weak dispersive forces are predicted when the dielectric properties
of the intervening medium become comparable to one of the interacting surfaces.
Using inverse colloid probe atomic force microscopy dispersive forces were
measured occasionally and under controlled conditions by addition of salt to
screen the electrostatic double layer force if present. The dispersive force
was found to be attractive, and an order of magnitude weaker than that in air.
Although the theoretical description of the forces becomes less precise for
these systems even with full knowledge of the dielectric properties, we find
still our results in reasonable agreement with Lifshitz theory.Comment: 19 pages, 7 figure
Casimir Forces between Compact Objects: I. The Scalar Case
We have developed an exact, general method to compute Casimir interactions
between a finite number of compact objects of arbitrary shape and separation.
Here, we present details of the method for a scalar field to illustrate our
approach in its most simple form; the generalization to electromagnetic fields
is outlined in Ref. [1]. The interaction between the objects is attributed to
quantum fluctuations of source distributions on their surfaces, which we
decompose in terms of multipoles. A functional integral over the effective
action of multipoles gives the resulting interaction. Each object's shape and
boundary conditions enter the effective action only through its scattering
matrix. Their relative positions enter through universal translation matrices
that depend only on field type and spatial dimension. The distinction of our
method from the pairwise summation of two-body potentials is elucidated in
terms of the scattering processes between three objects. To illustrate the
power of the technique, we consider Robin boundary conditions , which interpolate between Dirichlet and Neumann cases as
is varied. We obtain the interaction between two such spheres
analytically in a large separation expansion, and numerically for all
separations. The cases of unequal radii and unequal are studied. We
find sign changes in the force as a function of separation in certain ranges of
and see deviations from the proximity force approximation even at
short separations, most notably for Neumann boundary conditions.Comment: 27 pages, 9 figure
Casimir interaction between a plate and a cylinder
We find the exact Casimir force between a plate and a cylinder, a geometry
intermediate between parallel plates, where the force is known exactly, and the
plate--sphere, where it is known at large separations. The force has an
unexpectedly weak decay \sim L/(H^3 \ln(H/R)) at large plate--cylinder
separations H (L and R are the cylinder length and radius), due to transverse
magnetic modes. Path integral quantization with a partial wave expansion
additionally gives a qualitative difference for the density of states of
electric and magnetic modes, and corrections at finite temperatures.Comment: 4 pages, 3 figure
- …