28 research outputs found
Vortex lattices in strong type-II superconducting two-dimensional strips
We show how to calculate semi-analytically the dense vortex state in strong
type-II superconducting nanostructures. For the specific case of a strip, we
find vortex lattice solutions which also incorporate surface superconductivity.
We calculate the energy cost to displace individual vortex rows parallel to the
surfaces and find that this energy oscillates with the magnetic field.
Remarkably, we also find that, at a critical field below , this
''shear'' energy becomes strictly zero for the surface rows due to an
unexpected mismatch with the bulk lattice.Comment: Title, abstract, and some text paragraphs have been rewritte
Bremsstrahlung Suppression due to the LPM and Dielectric Effects in a Variety of Materials
The cross section for bremsstrahlung from highly relativistic particles is
suppressed due to interference caused by multiple scattering in dense media,
and due to photon interactions with the electrons in all materials. We present
here a detailed study of bremsstrahlung production of 200 keV to 500 MeV
photons from 8 and 25 GeV electrons traversing a variety of target materials.
For most targets, we observe the expected suppressions to a good accuracy. We
observe that finite thickness effects are important for thin targets.Comment: 52 pages, 13 figures (incorporated in the revtex LaTeX file
The ac magnetic response of mesoscopic type II superconductors
The response of mesoscopic superconductors to an ac magnetic field is
numerically investigated on the basis of the time-dependent Ginzburg-Landau
equations (TDGL). We study the dependence with frequency and dc
magnetic field of the linear ac susceptibility
in square samples with dimensions of the order of the London penetration depth.
At the behavior of as a function of agrees very well
with the two fluid model, and the imaginary part of the ac susceptibility,
, shows a dissipative a maximum at the frequency
. In the presence of a magnetic field a
second dissipation maximum appears at a frequency . The most
interesting behavior of mesoscopic superconductors can be observed in the
curves obtained at a fixed frequency. At a fixed number of
vortices, continuously increases with increasing . We
observe that the dissipation reaches a maximum for magnetic fields right below
the vortex penetration fields. Then, after each vortex penetration event, there
is a sudden suppression of the ac losses, showing discontinuities in
at several values of . We show that these
discontinuities are typical of the mesoscopic scale and disappear in
macroscopic samples, which have a continuos behavior of . We
argue that these discontinuities in are due to the effect of
{\it nascent vortices} which cause a large variation of the amplitude of the
order parameter near the surface before the entrance of vortices.Comment: 12 pages, 9 figures, RevTex
The theory of the Landau, Pomeranchuk, Migdal effect
Bremsstrahlung of photons from highly relativistic electrons is investigated.
The cross section of the processes, which is suppressed due to a multiple
scattering of an emitting electron in dense media (LPM effect) and due to
photon interaction with electrons of a medium, is calculated with an accuracy
up to "next to leading logarithm" and with the Coulomb corrections taken into
account. Making allowances for a multiple scattering and a polarization of a
medium an analysis of radiation on a target boundary is carried out. The method
of consideration of radiation in a thin target under influence of the LPM
effect is developed. Interrelation with the recent experiment is discussed.Comment: 36 pages, LaTeX, 3 eps figure
The Flux-Line Lattice in Superconductors
Magnetic flux can penetrate a type-II superconductor in form of Abrikosov
vortices. These tend to arrange in a triangular flux-line lattice (FLL) which
is more or less perturbed by material inhomogeneities that pin the flux lines,
and in high- supercon- ductors (HTSC's) also by thermal fluctuations. Many
properties of the FLL are well described by the phenomenological
Ginzburg-Landau theory or by the electromagnetic London theory, which treats
the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft
mainly because of the large magnetic penetration depth: The shear modulus of
the FLL is thus small and the tilt modulus is dispersive and becomes very small
for short distortion wavelength. This softness of the FLL is enhanced further
by the pronounced anisotropy and layered structure of HTSC's, which strongly
increases the penetration depth for currents along the c-axis of these uniaxial
crystals and may even cause a decoupling of two-dimensional vortex lattices in
the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause
thermally activated depinning of the flux lines or of the 2D pancake vortices
in the layers. Various phase transitions are predicted for the FLL in layered
HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to
interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do
not exist as postscript file
On the macroscopic formation length for GeV photons
Experimental results for the radiative energy loss of 206 and 234 GeV electrons in 5â10 ÎŒm thin Ta targets are presented. An increase in radiation emission probability at low photon energies compared to a 100 ÎŒm thick target is observed. This increase is due to the formation length of the GeV photons exceeding the thickness of the thin foils, the so-called TernovskiiâShul'gaâFomin (TSF) effect. The formation length of GeV photons from a multi-hundred GeV projectile is through the TSF effect shown directly to be a factor 1010 longer than their wavelength