22,535 research outputs found
Spatially Resolved Nonlinearity Measurements of YBaCuO Bi-crystal Grain Boundaries
We have developed a near-field microwave microscope to locally excite a
superconducting film and measure second and third harmonic responses at
microwave frequencies. We study the local nonlinear response of a
YBaCuO thin film grown on a bi-crystal SrTiO substrate. The
location of the bi-crystal grain boundary is clearly identified by the
microscope through higher harmonic response, and the spatial resolution is on
the order of the magnetic loop probe size, about 500. The harmonic power
and spatial resolution are successfully modeled with a one-dimensional extended
Josephson junction simulation. From the model, the 2nd order harmonic response
is dominated by Josephson vortex generation and flow. A geometry-free nonlinear
scaling current density is also exstracted
from the data, indicating that the grain boundary weak link is the dominant
nonlinear source in this case.Comment: 4pages, 4figure
Viscous dissipative effects in isotropic brane cosmology
We consider the dynamics of a viscous cosmological fluid in the generalized
Randall-Sundrum model for an isotropic brane. To describe the dissipative
effects we use the Israel-Hiscock-Stewart full causal thermodynamic theory. In
the limiting case of a stiff cosmological fluid with pressure equal to the
energy density, the general solution of the field equations can be obtained in
an exact parametric form for a cosmological fluid with constant bulk viscosity
and with a bulk viscosity coefficient proportional to the square root of the
energy density, respectively. The obtained solutions describe generally
non-inflationary brane worlds, starting from a singular state. During this
phase of evolution the comoving entropy of the Universe is an increasing
function of time, and thus a large amount of entropy is created in the brane
world due to viscous dissipative processes.Comment: 15 pages, 11 figure
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