19,116 research outputs found
Thin superconductors and SQUIDs in perpendicular magnetic field
It is shown how the static and dynamic electromagnetic properties can be
calculated for thin flat superconducting films of any shape and size, also
multiply connected as used for SQUIDs, and for any value of the effective
magnetic London penetration depth Lambda. As examples, the distributions of
sheet current and magnetic field are obtained for rectangular and circular
films without and with slits and holes, in response to an applied perpendicular
magnetic field and to magnetic vortices moving in the film. The self energy and
interaction of vortices with each other and with an applied magnetic field
and/or transport current are given. Due to the long ranging magnetic stray
field, these energies depend on the size and shape of the film and on the
vortex position even in large films, in contrast to the situation in large bulk
superconductors. The focussing of magnetic flux into the central hole of square
films without and with a radial slit is compared.Comment: 12 pages, 10 figure
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
Vector screening masses in the quark-gluon plasma and their physical significance
Static and non-static thermal screening states that couple to the conserved
vector current are investigated in the high-temperature phase of QCD. Their
masses and couplings to the current are determined at weak coupling, as well as
using two-flavor lattice QCD simulations. A consistent picture emerges from the
comparison, providing evidence that non-static Matsubara modes can indeed be
treated perturbatively. We elaborate on the physical significance of the
screening masses.Comment: 4 pages, 3 figures. Submitted as a contribution to the proceedings of
the Quark Matter 2014 conference (talk given by H. Meyer
A relation between screening masses and real-time rates
Thermal screening masses related to the conserved vector current are
determined for the case that the current carries a non-zero Matsubara
frequency, both in a weak-coupling approach and through lattice QCD. We point
out that such screening masses are sensitive to the same infrared physics as
light-cone real-time rates. In particular, on the perturbative side, the
inhomogeneous Schrodinger equation determining screening correlators is shown
to have the same general form as the equation implementing LPM resummation for
the soft-dilepton and photon production rates from a hot QCD plasma. The static
potential appearing in the equation is identical to that whose soft part has
been determined up to NLO and on the lattice in the context of jet quenching.
Numerical results based on this potential suggest that screening masses
overshoot the free results (multiples of 2piT) more strongly than at zero
Matsubara frequency. Four-dimensional lattice simulations in two-flavour QCD at
temperatures of 250 and 340 MeV confirm the non-static screening masses at the
10% level. Overall our results lend support to studies of jet quenching based
on the same potential at T > 250 MeV.Comment: 32 pages. v2: clarifications added, typos corrected; published
versio
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
Supermetallic conductivity in bromine-intercalated graphite
Exposure of highly oriented pyrolytic graphite to bromine vapor gives rise to
in-plane charge conductivities which increase monotonically with intercalation
time toward values (for ~6 at% Br) that are significantly higher than Cu at
temperatures down to 5 K. Magnetotransport, optical reflectivity and magnetic
susceptibility measurements confirm that the Br dopes the graphene sheets with
holes while simultaneously increasing the interplanar separation. The increase
of mobility (~ 5E4 cm^2/Vs at T=300 K) and resistance anisotropy together with
the reduced diamagnetic susceptibility of the intercalated samples suggests
that the observed supermetallic conductivity derives from a parallel
combination of weakly-coupled hole-doped graphene sheets.Comment: 5 pages, 4 figure
Meissner-London currents in superconductors with rectangular cross section
Exact analytic solutions are presented for the magnetic moment and screening
currents in the Meissner state of superconductor strips with rectangular cross
section in a perpendicular magnetic field and/or with transport current. The
extension to finite London penetration is achieved by an elegant numerical
method which works also for disks. The surface current in the specimen corners
diverges as l^(-1/3) where l is the distance from the corner. This enhancement
reduces the barrier for vortex penetration and should increase the nonlinear
Meissner effect in d-wave superconductors
Current density inhomogeneity throughout the thickness of superconducting films and its effect on their irreversible magnetic properties
We calculate the distribution of the current density in superconducting
films along the direction of an external field applied perpendicular to the
film plane. Our analysis reveals that in the presence of bulk pinning is
inhomogeneous on a length scale of order the inter vortex distance. This
inhomogeneity is significantly enhanced in the presence of surface pinning. We
introduce new critical state model, which takes into account the current
density variations throughout the film thickness, and show how these variations
give rise to the experimentally observed thickness dependence of and
magnetic relaxation rate.Comment: RevTex, 9 PS figures. To appear in Phys. Rev.
The Nature of Quantum Hall States near the Charge Neutral Dirac Point in Graphene
We investigate the quantum Hall (QH) states near the charge neutral Dirac
point of a high mobility graphene sample in high magnetic fields. We find that
the QH states at filling factors depend only on the perpendicular
component of the field with respect to the graphene plane, indicating them to
be not spin-related. A non-linear magnetic field dependence of the activation
energy gap at filling factor suggests a many-body origin. We therefore
propose that the and states arise from the lifting of the spin
and sub-lattice degeneracy of the LL, respectively.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
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