734 research outputs found
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
Heat Capacity of Mesoscopic Superconducting Disks
We study the heat capacity of isolated giant vortex states, which are good
angular momentum () states, in a mesoscopic superconducting disk using the
Ginzburg-Landau (GL) theory. At small magnetic fields the =0 state
qualitatively behaves like the bulk sample characterized by a discontinuity in
heat capacity at . As the field is increased the discontinuity slowly
turns into a continuous change which is a finite size effect. The higher
states show a continuous change in heat capacity at at all fields. We
also show that for these higher states, the behavior of the peak position
with change in field is related to the paramagnetic Meissner effect
(irreversible) and can lead to an unambiguous observation of positive
magnetization in mesoscopic superconductors.Comment: Final versio
FFLO states and quantum oscillations in mesoscopic superconductors and superfluid ultracold Fermi gases
We have studied the distinctive features of the
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) instability and phase transitions in
two--dimensional (2D) mesoscopic superconductors placed in magnetic field of
arbitrary orientation and rotating superfluid Fermi gases with imbalanced state
populations. Using a generalized version of the phenomenological
Ginzburg-Landau theory we have shown that the FFLO states are strongly modified
by the effect of the trapping potential confining the condensate. The
phenomenon of the inhomogeneous state formation is determined by the interplay
of three length scales: (i) length scale of the FFLO instability; (ii) 2D
system size; (iii) length scale associated with the orbital effect caused
either by the Fermi condensate rotation or magnetic field component applied
perpendicular to the superconducting disc. We have studied this interplay and
resulting quantum oscillation effects in both superconducting and superfluid
finite -- size systems with FFLO instability and described the hallmarks of the
FFLO phenomenon in a restricted geometry. The finite size of the system is
shown to affect strongly the conditions of the observability of switching
between the states with different vorticities.Comment: 11 pages, 5 figures, Submitted to PR
Stability of the vortex lattice in D-wave superconductors
Use is made of Onsager's hydrodynamic equation to derive the vibration
spectrum of the vortex lattice in d-wave superconductor. In particular the
rhombic lattice (i.e. the tilted square lattice) is found to be
stable for . Here denotes the critical field at which
the vortex lattice transition takes place.Comment: 7 pages, Revte
Spin-Valve Effect of the Spin Accumulation Resistance in a Double Ferromagnet - Superconductor Junction
We have measured the transport properties of Ferromagnet - Superconductor
nanostructures, where two superconducting aluminum (Al) electrodes are
connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find
that, below the superconducting critical temperature of Al, the resistance
depends on the relative alignment of the ferromagnets' magnetization. This
spin-valve effect is analyzed in terms of spin accumulation in the
superconducting electrode submitted to inverse proximity effect
Paramagnetic Meissner effect in superconductors from self-consistent solutions of Ginzburg-Landau equations
The paramagnetic Meissner effect (PME) is observed in small superconducting
samples, and a number of controversial explanations of this effect are
proposed, but there is as yet no clear understanding of its nature. In the
present paper PME is considered on the base of the Ginzburg-Landau theory (GL).
The one-dimensional solutions are obtained in a model case of a long
superconducting cylinder for different cylinder radii R, the GL-parameters
\kappa and vorticities m. Acording to GL-theory, PME is caused by the presence
of vortices inside the sample. The superconducting current flows around the
vortex to screeen the vortex own field from the bulk of the sample. Another
current flows at the boundary to screen the external field H from entering the
sample. These screening currents flow in opposite directions and contribute
with opposite signs to the total magnetic moment (or magnetization) of the
sample. Depending on H, the total magnetization M may be either negative
(diamagnetism), or positive (paramagnetism). A very complicated saw-like
dependence M(H) (and other characteristics), which are obtained on the base of
self-consistent solutions of the GL-equations, are discussed.Comment: 6 pages, 5 figures, RevTex, submitted to Phys. Rev.
Geometry-dependent critical currents in superconducting nanocircuits
In this paper we calculate the critical currents in thin superconducting
strips with sharp right-angle turns, 180-degree turnarounds, and more
complicated geometries, where all the line widths are much smaller than the
Pearl length . We define the critical current as the
current that reduces the Gibbs free-energy barrier to zero. We show that
current crowding, which occurs whenever the current rounds a sharp turn, tends
to reduce the critical current, but we also show that when the radius of
curvature is less than the coherence length this effect is partially
compensated by a radius-of-curvature effect. We propose several patterns with
rounded corners to avoid critical-current reduction due to current crowding.
These results are relevant to superconducting nanowire single-photon detectors,
where they suggest a means of improving the bias conditions and reducing dark
counts. These results also have relevance to normal-metal nanocircuits, as
these patterns can reduce the electrical resistance, electromigration, and hot
spots caused by nonuniform heating.Comment: 29 pages, 24 figure
Crossed Andreev reflection and charge imbalance in diffusive NSN structures
We formulate a microscopic theory of non-local electron transport in
three-terminal diffusive normal-superconducting-normal (NSN) structures with
arbitrary interface transmissions. At low energies we predict
strong enhancement of non-local spectral conductance due to quantum interference of electrons in disordered
N-terminals. In contrast, non-local resistance remains smooth at small
and, furthermore, is found to depend neither on parameters of NS
interfaces nor on those of N-terminals. At higher temperatures
exhibits a peak caused by the trade-off between charge imbalance and Andreev
reflection. Our results are in a good agreement with recent experimental
observations and can be used for quantitative analysis of future experiments.Comment: 4 pages, 3 figure
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