2,563 research outputs found
Stable vortex-antivortex molecules in mesoscopic superconducting triangles
A thermodynamically stable vortex-antivortex pattern has been revealed in
mesoscopic type I superconducting triangles, contrary to type II
superconductors where similar patterns are unstable. The stable
vortex-antivortex "molecule" appears due to the interplay between two factors:
a repulsive vortex-antivortex interaction in type I superconductors and the
vortex confinement in the triangle.Comment: 5 pages, 4 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected]
Magnetic susceptibility of ultra-small superconductor grains
For assemblies of superconductor nanograins, the magnetic response is
analyzed as a function of both temperature and magnetic field. In order to
describe the interaction energy of electron pairs for a huge number of
many-particle states, involved in calculations, we develop a simple
approximation, based on the Richardson solution for the reduced BCS Hamiltonian
and applicable over a wide range of the grain sizes and interaction strengths
at arbitrary distributions of single-electron energy levels in a grain. Our
study is focused upon ultra-small grains, where both the mean value of the
nearest-neighbor spacing of single-electron energy levels in a grain and
variations of this spacing from grain to grain significantly exceed the
superconducting gap in bulk samples of the same material. For these ultra-small
superconductor grains, the overall profiles of the magnetic susceptibility as a
function of magnetic field and temperature are demonstrated to be qualitatively
different from those for normal grains. We show that the analyzed signatures of
pairing correlations are sufficiently stable with respect to variations of the
average value of the grain size and its dispersion over an assembly of
nanograins. The presence of these signatures does not depend on a particular
choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses:
[email protected], [email protected], [email protected]
Superconductivity in a Mesoscopic Double Square Loop: Effect of Imperfections
We have generalized the network approach to include the effects of
short-range imperfections in order to analyze recent experiments on mesoscopic
superconducting double loops. The presence of weakly scattering imperfections
causes gaps in the phase boundary or for certain intervals of
, which depend on the magnetic flux penetrating each loop. This is
accompanied by a critical temperature , showing a smooth transition
between symmetric and antisymmetric states. When the scattering strength of
imperfections increases beyond a certain limit, gaps in the phase boundary
or appear for values of magnetic flux lying in intervals
around half-integer . The critical temperature corresponding to
these values of magnetic flux is determined mainly by imperfections in the
central branch. The calculated phase boundary is in good agreement with
experiment.Comment: 9 pages, 6 figure
Polynomial kernelization for removing induced claws and diamonds
A graph is called (claw,diamond)-free if it contains neither a claw (a
) nor a diamond (a with an edge removed) as an induced subgraph.
Equivalently, (claw,diamond)-free graphs can be characterized as line graphs of
triangle-free graphs, or as linear dominoes, i.e., graphs in which every vertex
is in at most two maximal cliques and every edge is in exactly one maximal
clique.
In this paper we consider the parameterized complexity of the
(claw,diamond)-free Edge Deletion problem, where given a graph and a
parameter , the question is whether one can remove at most edges from
to obtain a (claw,diamond)-free graph. Our main result is that this problem
admits a polynomial kernel. We complement this finding by proving that, even on
instances with maximum degree , the problem is NP-complete and cannot be
solved in time unless the Exponential Time
Hypothesis fai
Short-Distance Structure of Nuclei
One of Jefferson Lab's original missions was to further our understanding of
the short-distance structure of nuclei. In particular, to understand what
happens when two or more nucleons within a nucleus have strongly overlapping
wave-functions; a phenomena commonly referred to as short-range correlations.
Herein, we review the results of the (e,e'), (e,e'p) and (e,e'pN) reactions
that have been used at Jefferson Lab to probe this short-distance structure as
well as provide an outlook for future experiments.Comment: 16 pages, 8 figures, for publication in Journal of Physics
Categorification of skew-symmetrizable cluster algebras
We propose a new framework for categorifying skew-symmetrizable cluster
algebras. Starting from an exact stably 2-Calabi-Yau category C endowed with
the action of a finite group G, we construct a G-equivariant mutation on the
set of maximal rigid G-invariant objects of C. Using an appropriate cluster
character, we can then attach to these data an explicit skew-symmetrizable
cluster algebra. As an application we prove the linear independence of the
cluster monomials in this setting. Finally, we illustrate our construction with
examples associated with partial flag varieties and unipotent subgroups of
Kac-Moody groups, generalizing to the non simply-laced case several results of
Gei\ss-Leclerc-Schr\"oer.Comment: 64 page
Phonons and thermal transport in Si/SiO multishell nanotubes: Atomistic study
Thermal transport in the Si/SiO multishell nanotubes is investigated
theoretically. The phonon energy spectra are obtained using the atomistic
Lattice Dynamics approach. Thermal conductivity is calculated using the
Boltzmann transport equation within the relaxation time approximation.
Redistribution of the vibrational spectra in multishell nanotubes leads to a
decrease of the phonon group velocity and the thermal conductivity as compared
to homogeneous Si nanowires. Phonon scattering on the Si/SiO interfaces is
another key factor of strong reduction of the thermal conductivity in these
structures (down to 0.2 W/mK at room temperature). We demonstrate that phonon
thermal transport in Si/SiO nanotubes can be efficiently suppressed by a
proper choice of nanotube's geometrical parameters: lateral cross-section,
thickness and number of shells.Comment: 14 pages, 4 figure
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