745 research outputs found
Optical BCS conductivity at imaginary frequencies and dispersion energies of superconductors
We present an efficient expression for the analytic continuation to arbitrary
complex frequencies of the complex optical and AC conductivity of a homogeneous
superconductor with arbitrary mean free path. Knowledge of this quantity is
fundamental in the calculation of thermodynamic potentials and dispersion
energies involving type-I superconducting bodies. When considered for imaginary
frequencies, our formula evaluates faster than previous schemes involving
Kramers--Kronig transforms. A number of applications illustrates its
efficiency: a simplified low-frequency expansion of the conductivity, the
electromagnetic bulk self-energy due to longitudinal plasma oscillations, and
the Casimir free energy of a superconducting cavity.Comment: 20 pages, 7 figures, calculation of Casimir energy adde
Comparison among Various Expressions of Complex Admittance for Quantum System in Contact with Heat Reservoir
Relation among various expressions of the complex admittance for quantum
systems in contact with heat reservoir is studied. Exact expressions of the
complex admittance are derived in various types of formulations of equations of
motion under contact with heat reservoir. Namely, the complex admittance is
studied in the relaxation method and the external-field method. In the former
method, the admittance is calculated using the Kubo formula for quantum systems
in contact with heat reservoir in no external driving fields, while in the
latter method the admittance is directly calculated from equations of motion
with external driving terms. In each method, two types of equation of motions
are considered, i.e., the time-convolution (TC) equation and
time-convolutionless (TCL) equation. That is, the full of the four cases are
studied. It is turned out that the expression of the complex admittance
obtained by using the relaxation method with the TC equation exactly coincides
with that obtained by using the external-field method with the TC equation,
while other two methods give different forms. It is also explicitly
demonstrated that all the expressions of the complex admittance coincide with
each other in the lowest Born approximation for the systemreservoir
interaction. The formulae necessary for the higher order expansions in powers
of the system-reservoir interaction are derived, and also the expressions of
the admittance in the n-th order approximation are given. To characterize the
TC and TCL methods, we study the expressions of the admittances of two exactly
solvable models. Each exact form of admittance is compared with the results of
the two methods in the lowest Born approximation. It is found that depending on
the model, either of TC and TCL would be the better method.Comment: 34pages, no figur
Electrostatic potential in a superconductor
The electrostatic potential in a superconductor is studied. To this end
Bardeen's extension of the Ginzburg-Landau theory to low temperatures is used
to derive three Ginzburg-Landau equations - the Maxwell equation for the vector
potential, the Schroedinger equation for the wave function and the Poisson
equation for the electrostatic potential. The electrostatic and the
thermodynamic potential compensate each other to a great extent resulting into
an effective potential acting on the superconducting condensate. For the
Abrikosov vortex lattice in Niobium, numerical solutions are presented and the
different contributions to the electrostatic potential and the related charge
distribution are discussed.Comment: 19 pages, 11 figure
Deterministic and controllable photonic scattering media via direct laser writing
Photonic scattering materials, such as biological tissue and white paper, are
made of randomly positioned nanoscale inhomogeneities in refractive index that
lead to multiple scattering of light. Typically these materials, both
naturally-occurring or man-made, are formed through self assembly of the
scattering inhomogeneities, which imposes challenges in tailoring the disorder
and hence the optical properties. Here, We report on the nanofabrication of
photonic scattering media using direct laser writing with deterministic design.
These deterministic scattering media consist of submicron thick polymer
nanorods that are randomly oriented within a cubic volume. We study the total
transmission of light as a function of the number density of rods and of the
sample thickness to extract the scattering and transport mean free paths using
radiative transfer theory. Such photonic scattering media with deterministic
and controllable properties are model systems for fundamental light scattering
in particular with strong anisotropy and offer new applications in solid-state
lighting and photovoltaics.Comment: 18 pages, 9 figure
Kinetic energy driven superconductivity and superfluidity
The theory of hole superconductivity proposes that superconductivity is
driven by lowering of quantum kinetic energy and is associated with expansion
of electronic orbits and expulsion of negative charge from the interior to the
surface of superconductors and beyond. This physics provides a dynamical
explanation of the Meissner effect. Here we propose that similar physics takes
place in superfluid helium 4. Experimental manifestations of this physics in
are the negative thermal expansion of below the point
and the "Onnes effect", the fact that superfluid helium will creep up the walls
of the container and escape to the exterior. The Onnes effect and the Meissner
effect are proposed to originate in macroscopic zero point rotational motion of
the superfluids. It is proposed that this physics indicates a fundamental
inadequacy of conventional quantum mechanics
Liquid 4He near the superfluid transition in the presence of a heat current and gravity
The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.Comment: 26 pages, 9 figure
Scattering of first and second sound waves by quantum vorticity in superfluid Helium
We study the scattering of first and second sound waves by quantum vorticity
in superfluid Helium using two-fluid hydrodynamics. The vorticity of the
superfluid component and the sound interact because of the nonlinear character
of these equations. Explicit expressions for the scattered pressure and
temperature are worked out in a first Born approximation, and care is exercised
in delimiting the range of validity of the assumptions needed for this
approximation to hold. An incident second sound wave will partly convert into
first sound, and an incident first sound wave will partly convert into second
sound. General considerations show that most incident first sound converts into
second sound, but not the other way around. These considerations are validated
using a vortex dipole as an explicitely worked out example.Comment: 24 pages, Latex, to appear in Journal of Low Temperature Physic
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