28 research outputs found
Undersampled Phase Retrieval with Outliers
We propose a general framework for reconstructing transform-sparse images
from undersampled (squared)-magnitude data corrupted with outliers. This
framework is implemented using a multi-layered approach, combining multiple
initializations (to address the nonconvexity of the phase retrieval problem),
repeated minimization of a convex majorizer (surrogate for a nonconvex
objective function), and iterative optimization using the alternating
directions method of multipliers. Exploiting the generality of this framework,
we investigate using a Laplace measurement noise model better adapted to
outliers present in the data than the conventional Gaussian noise model. Using
simulations, we explore the sensitivity of the method to both the
regularization and penalty parameters. We include 1D Monte Carlo and 2D image
reconstruction comparisons with alternative phase retrieval algorithms. The
results suggest the proposed method, with the Laplace noise model, both
increases the likelihood of correct support recovery and reduces the mean
squared error from measurements containing outliers. We also describe exciting
extensions made possible by the generality of the proposed framework, including
regularization using analysis-form sparsity priors that are incompatible with
many existing approaches.Comment: 11 pages, 9 figure
Unified order-disorder vortex phase transition in high-Tc superconductors
The diversity of vortex melting and solid-solid transition lines measured in
different high-T superconductors is explained, postulating a unified
order-disorder phase transition driven by both thermally- and disorder-induced
fluctuations. The temperature dependence of the transition line and the nature
of the disordered phase (solid, liquid, or pinned liquid) are determined by the
relative contributions of these fluctuations and by the pinning mechanism. By
varying the pinning mechanism and the pinning strength one obtains a spectrum
of monotonic and non-monotonic transition lines similar to those measured in
BiSrCaCuO, YBaCuO,
NdCeCuO,
BiPbSrCaCuO and (LaSr)CuOComment: To be published in Phys. Rev. B Rapid Com
Metastable vortex states in YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB> crystal
Magnetization measurements in an untwinned YBa2Cu3O7-δ crystal reveal a range of temperatures and fields for which the magnetization exhibits unusual history-dependent behavior. This range envelopes the transition line, Bss(T), separating between the vortex quasiordered and disordered phases. The observed history effects indicate that a disordered (quasiordered) vortex state can exist as a metastable state below (above) the Bss(T) line. The fields, defining the phase transition and the borders of the metastability region, are correlated to sharp features in the magnetization loop
Disorder and thermally driven vortex-lattice melting in La{2-x}Sr{x}CuO{4} crystals
Magnetization measurements in La{2-x}Sr{x}CuO{4} crystals indicate vortex
order-disorder transition manifested by a sharp kink in the second
magnetization peak. The transition field exhibits unique temperature
dependence, namely a strong decrease with temperature in the entire measured
range. This behavior rules out the conventional interpretation of a
disorder-driven transition into an entangled vortex solid phase. It is shown
that the transition in La{2-x}Sr{x}CuO{4} is driven by both thermally- and
disorder-induced fluctuations, resulting in a pinned liquid state. We conclude
that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions
are different manifestations of the same thermodynamic order-disorder
transition, distinguished by the relative contributions of thermal and
disorder-induced fluctuations.Comment: To be published in phys. Rev. B Rapid Com
Calculation of minor hysteresis loops under metastable to stable transformations in vortex matter
We present a model in which metastable supercooled phase and stable
equilibrium phase of vortex matter coexist in different regions of a sample.
Minor hysteresis loops are calculated with the simple assumption of the two
phases of vortex matter having field-independent critical current densities. We
use our earlier published ideas that the free energy barrier separating the
metastable and stable phases reduces as the magnetic induction moves farther
from the first order phase transition line, and that metastable to stable
transformations occur in local regions of the sample when the local energy
dissipation exceeds a critical value. Previously reported anomalous features in
minor hysteresis loops are reproduced, and calculated field profiles are
presented.Comment: 9pages, 7 figure
Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBaCuO superconductor
By means of local Hall probe ac and dc permeability measurements we
investigated the phase diagram of vortex matter for the HgBaCuO superconductor in the regime near the critical temperature. The second peak
line, , in contrast to what is usually assumed, doesn't terminate
at the critical temperature. Our local ac permeability measurements revealed
pronounced hysteretic behavior and thermomagnetic history effects near the
onset of the second peak, giving evidence for a phase transition of vortex
matter from an ordered qausilattice state to a disordered glass
Peak effect in a superconducting DyBa2Cu3O7-y film at microwave frequencies
We report the observation of a peak in the microwave (9.55 GHz) surface
resistance in an epitaxial DyBa2Cu3O7-y superconducting film in magnetic fields
(parallel to the c axis) ranging between 0.2 to 0.9 Tesla. Such a peak is
absent in the measurements done in zero-field. The temperature and field
dependence of the peak suggests that this peak could be associated with the
peak effect phenomenon reflecting the order-disorder transformation in the
flux-line lattice. A strong dependence of this peak effect at frequencies close
to the depinning frequency of the flux line lattice is observed.Comment: 1 text, 4 figures (all postscript) to be published in Phys. Rev.
An Assessment of Multilevel Governance in Cohesion Policy 2007-2013 : Volume 1, Study
This study provides an in-depth analysis of Multi-Level Governance in Cohesion Policy in the programming period of 2007-2013. It begins by examining the evolution of the concept and previous practices of implementing Multi-Level Governance in Cohesion Policy across the EU. It then provides a comparative analysis of partnership-working in nine programme case studies during the programming period and across the various stages of the policy cycle. The study identifies the advantages and disadvantages of partnership-working and formulates strategic and operational recommendations in the context of the preparation of the 2014-2020 programming period