306 research outputs found
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
Effect of mesoscopic inhomogeneities on local tunnelling density of states
We carry out a theoretical analysis of the momentum dependence of the
Fourier-transformed local density of states (LDOS) in the superconducting
cuprates within a model considering the interference of quasiparticles
scattering on quenched impurities. The impurities introduce an external
scattering potential, which is either nearly local in space or it can acquire a
substantial momentum dependence due to a possible strong momentum dependence of
the electronic screening near a charge modulation instability. The key new
effect that we introduce is an additional mesoscopic disorder aiming to
reproduce the inhomogeneities experimentally observed in scanning tunnelling
microscopy. The crucial effect of this mesoscopic disorder is to give rise to
point-like spectroscopic features, to be contrasted with the curve-like shape
of the spectra previously calculated within the interfering-quasiparticle
schemes. It is also found that stripe-like charge modulations play a relevant
role to correctly reproduce all the spectral features of the experiments.Comment: 11 pages and 5 figure
Phase diagram for Coulomb-frustrated phase separation in systems with negative short-range compressibility
Using numerical techniques and asymptotic expansions we obtain the phase
diagram of a paradigmatic model of Coulomb frustrated phase separation in
systems with negative short-range compressibility. The transition from the
homogeneous phase to the inhomogeneous phase is generically first order in
isotropic three-dimensional systems except for a critical point. Close to the
critical point, inhomogeneities are predicted to form a BCC lattice with
subsequent transitions to a triangular lattice of rods and a layered structure.
Inclusion of a strong anisotropy allows for second- and first-order transition
lines joined by a tricritical point.Comment: 4 pages, 3 figures. Improved figures and presentatio
Screening effects in Coulomb frustrated phase separation
We solve a model of phase separation among two competing phases frustrated by
the long-range Coulomb interaction in two and three dimensions (2D/3D) taking
into account finite compressibility effects. In the limit of strong frustration
in 2D, we recover the results of R. Jamei, S. Kivelson, and B. Spivak, Phys.
Rev. Lett. 94, 056805 (2005) and the system always breaks into domains in a
narrow range of densities, no matter how big is the frustration. For weak
frustration in 2D and for arbitrary frustration in 3D the finite
compressibility of the phases is shown to play a fundamental role. Our results
clarify the different role of screening in 2D and 3D systems. We discuss the
thermodynamic stability of the system near the transition to the phase
separated state and the possibility to observe it in real systems.Comment: 8 pages, 8 figure
One Step Non SUSY Unification
We show that it is possible to achieve one step gauge coupling unification in
a general class of non supersymmetric models which at low energies have only
the standard particle content and extra Higgs fields doublets. The constraints
are the experimental values of , and
at , and the lower bounds for FCNC and proton decay rates. Specific
example are pointed out.Comment: 10 pages, Latex file,, uses epsf style, Two Postscript figures
included. To appear in Europhysics Letter
Branon search in hadronic colliders
In the context of the brane-world scenarios with compactified extra
dimensions, we study the production of brane fluctuations (branons) in hadron
colliders (, and ) in terms of the brane tension
parameter , the branon mass and the number of branons . From the
absence of monojets events at HERA and Tevatron (run I), we set bounds on these
parameters and we also study how such bounds could be improved at Tevatron (run
II) and the future LHC. The single photon channel is also analyzed for the two
last colliders.Comment: 17 pages, 10 figures, LaTeX. New comments and figures included. Final
version to appear in Phys. Rev.
Universal scaling of the order-parameter distribution in strongly disordered superconductors
We investigate theoretically and experimentally the statistical properties of
the inhomogeneous order-parameter distribution (OPD) at the verge of the
superconductor-insulator transition (SIT). We find within two prototype
fermionic and bosonic models for disordered superconductors that one can
identify a universal rescaling of the OPD. By performing scanning-tunneling
microscopy experiments in three samples of NbN with increasing disorder we show
that such a rescaling describes also with an excellent accuracy the
experimental data. These results can provide a breakthrough in our
understanding of the SIT.Comment: 11 pages, 8 figures, revised version submitted to PR
The assessment of disability related to vision performance-based measure in diabetic retinopathy.
PURPOSE: To validate a third-generation performance-based measure of visual function titled Assessment of Disability Related to Vision (ADREV) in a study population of patients with diabetic retinopathy.
DESIGN: Prospective, cross-sectional study.
METHODS: Patients with nonproliferative or proliferative diabetic retinopathy, free from ocular comorbidity, were recruited from a single institute and completed the ADREV, the 25-Item National Eye Institute Visual Functioning Questionnaire (VFQ-25), and a clinical ophthalmic examination. Correlation, regression, and bootstrap analysis were conducted to determine the relationship between ADREV scoring and each of the study\u27s clinical and self-report measures of visual ability, while controlling for potential confounders.
RESULTS: Ninety-one patients with diabetic retinopathy completed the study and analysis showed that the ADREV total and subscale scores shared a stronger relationship with the clinical measures of visual function than did the VFQ total and subscale scores. Regression analysis revealed that binocular visual acuity, contrast sensitivity, and better eye visual field were the best predictors of ADREV performance.
CONCLUSIONS: The ADREV performance measure is a valid instrument for the assessment of disability related to vision in patients with diabetic retinopathy. Furthermore, the assessments provided by ADREV were more related to traditional clinical indicators of visual impairment than were the results of the self-report measure, specifically the VFQ-25
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