71,847 research outputs found
Universality class for bootstrap percolation with on the cubic lattice
We study the bootstrap percolation model on a cubic lattice, using
Monte Carlo simulation and finite-size scaling techniques. In bootstrap
percolation, sites on a lattice are considered occupied (present) or vacant
(absent) with probability or , respectively. Occupied sites with less
than occupied first-neighbours are then rendered unoccupied; this culling
process is repeated until a stable configuration is reached. We evaluate the
percolation critical probability, , and both scaling powers, and
, and, contrarily to previous calculations, our results indicate that the
model belongs to the same universality class as usual percolation (i.e.,
). The critical spanning probability, , is also numerically
studied, for systems with linear sizes ranging from L=32 up to L=480: the value
we found, , is the same as for usual percolation with
free boundary conditions.Comment: 11 pages; 4 figures; to appear in Int. J. Mod. Phys.
Vortex state microwave response in superconducting cuprates and MgB
We investigate the physics of the microwave response in
YBaCuO, SmBaCuO and MgB
in the vortex state. We first recall the theoretical basics of vortex-state
microwave response in the London limit. We then present a wide set of
measurements of the field, temperature, and frequency dependences of the vortex
state microwave complex resistivity in superconducting thin films, measured by
a resonant cavity and by swept-frequency Corbino disk. The combination of these
techniques allows for a comprehensive description of the microwave response in
the vortex state in these innovative superconductors. In all materials
investigated we show that flux motion alone cannot take into account all the
observed experimental features, neither in the frequency nor in the field
dependence. The discrepancy can be resolved by considering the (usually
neglected) contribution of quasiparticles to the response in the vortex state.
The peculiar, albeit different, physics of the superconducting materials here
considered, namely two-band superconductivity in MgB and superconducting
gap with lines of nodes in cuprates, give rise to a substantially increased
contribution of quasiparticles to the field-dependent microwave response. With
careful combined analysis of the data it is possible to extract or infer many
interesting quantities related to the vortex state, such as the
temperature-dependent characteristic vortex frequency and vortex viscosity, the
field dependence of the quasiparticle density, the temperature dependence of
the -band superfluid density in MgBComment: 51 pages, 27 figures, to appear as a book chapter (Nova Science
Surface impedance of superconductive thin films as a function of frequency in microwave range
We report measurements of the complex resistivity in and thin
films over a continuous frequency spectrum in the microwave range, making use
of a Corbino disk geometry. The paper mainly focuses on the extraction of the
resistivity from raw data, displaying data analisys procedure and its limits of
validity. We obtain and show resistivity curves as a function of frequency and
temperature denoting a frequency dependent widening of the superconductive
transition.Comment: 8 pages, Latex, 5 figure
New aspects of microwave properties of Nb in the mixed state
We present a study of the frequency dependence of the vortex dynamics in a
conventional superconductor. We have employed a swept-frequency, Corbino-disk
technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2
to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function
of the frequency (1-20 GHz). We have found several previously unnoticed
features: (i) a field-dependent depinning frequency in the GHz range; (ii)
deviations from the accepted frequency dependence, that can be ascribed to some
kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of
vortex instabilities. We discuss the possible origin of the features here
reported.Comment: 5 pages, 3 figures, presented at VORTEX VI Conference, to appear on
Physica
Vortex state microwave resistivity in Tl-2212 thin films
We present measurements of the field induced changes in the 47 GHz complex
resistivity, , in TlBaCaCuO
(TBCCO) thin films with 105 K, prepared on CeO buffered
sapphire substrates. At low fields (10 mT) a very small irreversible
feature is present, suggesting a little role of intergranular phenomena. Above
that level exhibits a superlinear dependence with the
field, as opposed to the expected (at high frequencies) quasilinear behaviour.
We observe a crossover between predominantly imaginary to predominantly real
(dissipative) response with increasing temperature and/or field. In addition,
we find the clear scaling property , where the scaling field maps closely the melting
field measured in single crystals. We discuss our microwave results in terms of
loss of flux lines rigidity.Comment: 8 pages, 3 figures, proceedings of 9th HTSHFF, accepted for
publication on J. Supercon
Temperature dependence of antiferromagnetic susceptibility in ferritin
We show that antiferromagnetic susceptibility in ferritin increases with
temperature between 4.2 K and 180 K (i. e. below the N\'{e}el temperature) when
taken as the derivative of the magnetization at high fields (
Oe). This behavior contrasts with the decrease in temperature previously found,
where the susceptibility was determined at lower fields ( Oe). At
high fields (up to Oe) the temperature dependence of the
antiferromagnetic susceptibility in ferritin nanoparticles approaches the
normal behavior of bulk antiferromagnets and nanoparticles considering
superantiferromagnetism, this latter leading to a better agreement at high
field and low temperature. The contrast with the previous results is due to the
insufficient field range used ( Oe), not enough to saturate the
ferritin uncompensated moment.Comment: 7 pages, 7 figures, accepted in Phys. Rev.
The Left-Right SU(3)(L)xSU(3)(R)xU(1)(X) Model with Light, keV and Heavy Neutrinos
We construct a full left-right model for the electroweak interactions based
on the gauge symmetry. The fermion
content of the model is such that anomaly cancellation restricts the number of
families to be a multiple of three. One of the most important features of the
model is the joint presence of three light active neutrinos, three additional
neutrinos at keV mass scale, and six heavy ones with masses
around\textbf{} GeV. They form a well-motivated part of the spectrum
in the sense they address challenging problems related to neutrino oscillation,
warm dark matter, and baryogenesis through leptogenesis.Comment: 11 pages. Small corrections and typos fixed. Accepted for publication
in PR
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