28,437 research outputs found
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
Critical State in Thin Anisotropic Superconductors of Arbitrary Shape
A thin flat superconductor of arbitrary shape and with arbitrary in-plane and
out-of-plane anisotropy of flux-line pinning is considered, in an external
magnetic field normal to its plane.
It is shown that the general three-dimensional critical state problem for
this superconductor reduces to the two-dimensional problem of an infinitely
thin sample of the same shape but with a modified induction dependence of the
critical sheet current. The methods of solving the latter problem are well
known. This finding thus enables one to study the critical states in realistic
samples of high-Tc superconductors with various types of anisotropic flux-line
pinning. As examples, we investigate the critical states of long strips and
rectangular platelets of high-Tc superconductors with pinning either by the
ab-planes or by extended defects aligned with the c-axis.Comment: 13 pages including 13 figure files in the tex
Theory of Type-II Superconductors with Finite London Penetration Depth
Previous continuum theory of type-II superconductors of various shapes with
and without vortex pinning in an applied magnetic field and with transport
current, is generalized to account for a finite London penetration depth
lambda. This extension is particularly important at low inductions B, where the
transition to the Meissner state is now described correctly, and for films with
thickness comparable to or smaller than lambda. The finite width of the surface
layer with screening currents and the correct dc and ac responses in various
geometries follow naturally from an equation of motion for the current density
in which the integral kernel now accounts for finite lambda. New geometries
considered here are thick and thin strips with applied current, and `washers',
i.e. thin film squares with a slot and central hole as used for SQUIDs.Comment: 14 pages, including 15 high-resolution figure
Equivalence of the Falicov-Kimball and Brandt-Mielsch forms for the free energy of the infinite-dimensional Falicov-Kimball model
Falicov and Kimball proposed a real-axis form for the free energy of the
Falicov-Kimball model that was modified for the coherent potential
approximation by Plischke. Brandt and Mielsch proposed an imaginary-axis form
for the free energy of the dynamical mean field theory solution of the
Falicov-Kimball model. It has long been known that these two formulae are
numerically equal to each other; an explicit derivation showing this
equivalence is presented here.Comment: 4 pages, 1 figure, typeset with ReVTe
Numerical evolution of matter in dynamical axisymmetric black hole spacetimes. I. Methods and tests
We have developed a numerical code to study the evolution of self-gravitating
matter in dynamic black hole axisymmetric spacetimes in general relativity. The
matter fields are evolved with a high-resolution shock-capturing scheme that
uses the characteristic information of the general relativistic hydrodynamic
equations to build up a linearized Riemann solver. The spacetime is evolved
with an axisymmetric ADM code designed to evolve a wormhole in full general
relativity. We discuss the numerical and algorithmic issues related to the
effective coupling of the hydrodynamical and spacetime pieces of the code, as
well as the numerical methods and gauge conditions we use to evolve such
spacetimes. The code has been put through a series of tests that verify that it
functions correctly. Particularly, we develop and describe a new set of testbed
calculations and techniques designed to handle dynamically sliced,
self-gravitating matter flows on black holes, and subject the code to these
tests. We make some studies of the spherical and axisymmetric accretion onto a
dynamic black hole, the fully dynamical evolution of imploding shells of dust
with a black hole, the evolution of matter in rotating spacetimes, the
gravitational radiation induced by the presence of the matter fields and the
behavior of apparent horizons through the evolution.Comment: 42 pages, 20 figures, submitted to Phys Rev
Effect of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition
The Mott-Hubbard metal-insulator transition is one of the most important
problems in correlated electron systems. In the past decade, much progress has
been made on examining a particle-hole symmetric form of the transition in the
Hubbard model with dynamical mean field theory where it was found that the
electronic self energy develops a pole at the transition. We examine the
particle-hole asymmetric metal-insulator transition in the Falicov-Kimball
model, and find that a number of features change when the noninteracting
density of states has a finite bandwidth. Since, generically particle-hole
symmetry is broken in real materials, our results have an impact on
understanding the metal-insulator transition in real materials.Comment: 5 pages, 3 figure
Modern Observational Techniques for Comets
Techniques are discussed in the following areas: astrometry, photometry, infrared observations, radio observations, spectroscopy, imaging of coma and tail, image processing of observation. The determination of the chemical composition and physical structure of comets is highlighted
Charge-ordered ferromagnetic phase in manganites
A mechanism for charge-ordered ferromagnetic phase in manganites is proposed.
The mechanism is based on the double exchange in the presence of diagonal
disorder. It is modeled by a combination of the Ising double-exchange and the
Falicov-Kimball model. Within the dynamical mean-field theory the charge and
spin correlation function are explicitely calculated. It is shown that the
system exhibits two successive phase transitions. The first one is the
ferromagnetic phase transition, and the second one is a charge ordering. As a
result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.
A consistent interpretation of the low temperature magneto-transport in graphite using the Slonczewski--Weiss--McClure 3D band structure calculations
Magnetotransport of natural graphite and highly oriented pyrolytic graphite
(HOPG) has been measured at mK temperatures. Quantum oscillations for both
electron and hole carriers are observed with orbital angular momentum quantum
number up to . A remarkable agreement is obtained when comparing
the data and the predictions of the Slonczewski--Weiss--McClure tight binding
model for massive fermions. No evidence for Dirac fermions is observed in the
transport data which is dominated by the crossing of the Landau bands at the
Fermi level, corresponding to , which occurs away from the point
where Dirac fermions are expected.Comment: 3 figure
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
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