53,960 research outputs found
Antenna and radome loss measurements for MFMR and PMIS with appendix on MFMR/PMIS computer programs
The NMSU/PSL radiometer antenna calibration facility is described, and the antenna and radome loss measurements made on the passive microwave imaging system and the multifrequency microwave radiometer are summarized. Antenna/radome data reduction techniques, estimation of sky brightness temperatures, and bucket performance tests are presented along with radiometer computer programs
Breakdown of counterflow superfluidity in a disordered quantum Hall bilayer
We present a theory for the regime of coherent interlayer tunneling in a
disordered quantum Hall bilayer at total filling factor one, allowing for the
effect of static vortices. We find that the system consists of domains of
polarized superfluid phase. Injected currents introduce phase slips between the
polarized domains which are pinned by disorder. We present a model of saturated
tunneling domains that predicts a critical current for the breakdown of
coherent tunneling that is extensive in the system size. This theory is
supported by numerical results from a disordered phase model in two dimensions.
We also discuss how our picture might be used to interpret experiments in the
counterflow geometry and in two-terminal measurements.Comment: 7 pages, 3 figure
Coupled Ferromagnetic and Nematic Ordering of Fermions in an Optical Flux Lattice
Ultracold atoms in Raman-dressed optical lattices allow for effective
momentum-dependent interactions among single-species fermions originating from
short-range s-wave interactions. These dressed-state interactions combined with
very flat bands encountered in the recently introduced optical flux lattices
push the Stoner instability towards weaker repulsive interactions, making it
accessible with current experiments. As a consequence of the coupling between
spin and orbital degrees of freedom, the magnetic phase features Ising nematic
order.Comment: 5 pages, 4 figures (published version
Inhomogeneities and iron diffusion in a Thailand tektite Semiannual report, Apr. 1969
Electron microscopic examination of Thailand tektit
Diamagnetism and flux creep in bilayer exciton superfluids
We discuss the diamagnetism induced in an isolated quantum Hall bilayer with
total filling factor one by an in-plane magnetic field. This is a signature of
counterflow superfluidity in these systems. We calculate magnetically induced
currents in the presence of pinned vortices nucleated by charge disorder, and
predict a history-dependent diamagnetism that could persist on laboratory
timescales. For current samples we find that the maximum in-plane moment is
small, but with stronger tunneling the moments would be measurable using torque
magnetometry. Such experiments would allow the persistent currents of a
counterflow superfluid to be observed in an electrically isolated bilayer.Comment: 8 pages, 2 figures. v2: updated to accepted version, extended
presentatio
Dissipation and Tunnelling in Quantum Hall Bilayers
We discuss the interplay between transport and intrinsic dissipation in
quantum Hall bilayers, within the framework of a simple thought experiment. We
compute, for the first time, quantum corrections to the semiclassical dynamics
of this system. This allows us to re-interpret tunnelling measurements on these
systems. We find a strong peak in the zero-temperature tunnelling current that
arises from the decay of Josephson-like oscillations into incoherent charge
fluctuations. In the presence of an in-plane field, resonances in the
tunnelling current develop an asymmetric lineshape.Comment: 4 pages, 3 figure
Effects of Disorder and Momentum Relaxation on the Intertube Transport of Incommensurate Carbon Nanotube Ropes and Multiwall Nanotubes
We study theoretically the electrical transport between aligned carbon
nanotubes in nanotube ropes, and between shells in multiwall carbon nanotubes.
We focus on transport between two metallic nanotubes (or shells) of different
chiralities with mismatched Fermi momenta and incommensurate periodicities. We
perform numerical calculations of the transport properties of such systems
within a tight-binding formalism. For clean (disorder-free) nanotubes the
intertube transport is strongly suppressed as a result of momentum
conservation. For clean nanotubes, the intertube transport is typically
dominated by the loss of momentum conservation at the contacts. We discuss in
detail the effects of disorder, which also breaks momentum conservation, and
calculate the effects of localised scatterers of various types. We show that
physically relevant disorder potentials lead to very dramatic enhancements of
the intertube conductance. We show that recent experimental measurements of the
intershell transport in multiwall nanotubes are consistent with our theoretical
results for a model of short-ranged correlated disorder.Comment: References adde
Chaos in Time Dependent Variational Approximations to Quantum Dynamics
Dynamical chaos has recently been shown to exist in the Gaussian
approximation in quantum mechanics and in the self-consistent mean field
approach to studying the dynamics of quantum fields. In this study, we first
show that any variational approximation to the dynamics of a quantum system
based on the Dirac action principle leads to a classical Hamiltonian dynamics
for the variational parameters. Since this Hamiltonian is generically nonlinear
and nonintegrable, the dynamics thus generated can be chaotic, in distinction
to the exact quantum evolution. We then restrict attention to a system of two
biquadratically coupled quantum oscillators and study two variational schemes,
the leading order large N (four canonical variables) and Hartree (six canonical
variables) approximations. The chaos seen in the approximate dynamics is an
artifact of the approximations: this is demonstrated by the fact that its onset
occurs on the same characteristic time scale as the breakdown of the
approximations when compared to numerical solutions of the time-dependent
Schrodinger equation.Comment: 10 pages (12 figures), RevTeX (plus macro), uses epsf, minor typos
correcte
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