1,527 research outputs found
Nucleation of Superconductivity in a Mesoscopic Loop of Finite Width
The normal/superconducting phase boundary Tc has been calculated for
mesoscopic loops, as a function of an applied perpendicular magnetic field H.
While for thin-wire loops and filled disks the Tc(H) curves are well known, the
intermediate case, namely mesoscopic loops of finite wire width, have been
studied much less. The linearized first Ginzburg-Landau equation is solved with
the proper normal/vacuum boundary conditions both at the internal and at the
external loop radius. For thin-wire loops the Tc(H) oscillations are perfectly
periodic, and the Tc(H) background is parabolic (this is the usual Little-Parks
effect). For loops of thicker wire width, there is a crossover magnetic field
above which Tc(H) becomes quasi-linear, with the period identical to the Tc(H)
of a filled disk (i.e. pseudoperiodic oscillations). This dimensional
transition is similar to the 2D-3D transition for thin films in a parallel
field, where vortices start penetrating the material as soon as the film
thickness exceeds the temperature dependent coherence length by a factor 1.8.
For the presently studied loops, the crossover point is controlled by a similar
condition. In the high field '3D' regime, a giant vortex state establishes,
where only a surface superconducting sheath near the sample's outer radius is
present.Comment: 7 pages text, 2 EPS figures, uses LaTeX's elsart.sty, proceedings of
the First Euroconference on "Vortex Matter in Superconductors", held in Crete
(18-24 september 1999
Spectral Features of the Proximity Effect
We calculate the local density of states (LDOS) of a superconductor-normal
metal sandwich at arbitrary impurity concentration. The presence of the
superconductor induces a gap in the normal metal spectrum that is proportional
to the inverse of the elastic mean free path for rather clean systems. For
a mean free path much shorter than the thickness of the normal metal, we find a
gap size proportional to that approaches the behavior predicted by the
Usadel equation (diffusive limit).Comment: LT22 proceeding
Vanishing Elasticity for Wet Foams: Equivalence With Emulsions and Role of Polydispersity
We present an experimental study of the rheology of polydisperse aqueous foams of different gas volume fractions φ. With oscillatory deformation at fixed frequency, we determine the behavior of the maximum stress as a function of the strain amplitude. At low strain, the maximum stress increases linearly, defining a shear modulus G.G. At progressively higher strains, the response eventually becomes nonlinear, defining the yield strain and the yield stress. While φ decreases toward φc=0.635±0.01,φc=0.635±0.01, GG goes to zero, and the yield stress decreases by many orders of magnitude with a quadratic behavior. The yield strain, which can be extrapolated to 0.18±0.020.18±0.02 at φ=1,φ=1, has a minimum value of 0.045±0.0100.045±0.010 at φc.φc. This behavior shows the occurrence of a melting transition located at φc,φc, which can be correlated to the random close packing of spheres. We compare these results to similar ones obtained previously for monodisperse and polydisperse emulsions. Our new experiments clarify the rheological similarities between emulsions and foams, as well as the role of polydispersity. We find that as long as polydispersity is moderate, it does not play a crucial role in the elastic response of foams and emulsions
Ground State and Tkachenko Modes of a Rapidly Rotating Bose-Einstein Condensate in the Lowest Landau Level State
The Letter considers the ground state and the Tkachenko modes for a rapidly
rotating Bose-Einstein condensate (BEC), when its macroscopic wave function is
a coherent superposition of states analogous to the lowest Landau levels of a
charge in a magnetic field. As well as in type II superconductors close to the
critical magnetic field , this corresponds to a periodic vortex
lattice. The exact value of the shear elastic modulus of the vortex lattice,
which was known from the old works on type II superconductors, essentially
exceeds the values calculated recently for BEC. This is important for
comparison with observation of the Tkachenko mode in the rapidly rotating BEC.Comment: 5 pages, 1 figure; discussion edited, references added, numerical
factors and typos correcte
Closed Abrikosov Vortices in a Superconducting Cylinder
The new type of solutions of the London equation for type-II superconductors
is obtained to describe the ring-shaped (toroidal) Abrikosov vortices. The
specific feature of these solutions is the self-consistent localization of both
the supercurrent and the magnetic field, enabling one to construct compact
magnetic structures inside a superconductor. The torus vortex contraction
caused by the vortex instability leads to the destruction of the Cooper pairing
and the formation of a normal electron stream in the vicinity of the torus
axis. The thermodynamic condition for the excitation of a small closed vortex
by a bunch of charged particles contains the fine-structure constant as a
determining parameter.Comment: LaTex using revtex, 12 pages. 5 Figures available upon request from
[email protected] Accepted for publication in Physica
Flux Confinement in Mesoscopic Superconductors
We report on flux confinement effects in superconducting submicron line, loop
and dot structures. The main idea of our study was to vary the boundary
conditions for confinement of the superconducting condensate by taking samples
of different topology and, through that, modifying the lowest Landau level
E_{LLL}(H). Since the critical temperature versus applied magnetic field
T_{c}(H) is, in fact, E_{LLL}(H) measured in temperature units, it is varied as
well when the sample topology is changed. We demonstrate that in all studied
submicron structures the shape of the T_{c}(H) phase boundary is determined by
the confinement topology in a unique way.Comment: 10 pages, 5 EPS figures, uses LaTeX's sup.sty, contribution to a
special issue of "Superlattices and Microstructures
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