17,472 research outputs found
Effective inter-band coupling in MgB2 due to anharmonic phonons
We investigate the origin of the inter-band coupling in MgB2 by focusing on
its unusual phononic features, namely, the strong anharmonicity of the phonons
and the presence of both linear and quadratic electron-phonon interactions of
the Su-Schrieffer-Heeger (SSH) type. The bare electronic Hamiltonian has two
bands with intra- and inter-band hopping, which lead to two decoupled
hybridized bands. The phonon Hamiltonian including the anharmonic terms is
diagonalized approximately by a squeezing transformation, which causes the
softening of the phonon frequency. The linear SSH coupling amplitude is
reduced, consistently with the estimates from first-principle calculations.
Additionally, the quadratic coupling generates an effective phonon-induced
interaction between the hybridized bands, which is non-vanishing even in the
limit of vanishing inter-bare-band hopping amplitude.Comment: 11 page
A model Hamiltonian for MgB2 which takes into account its unusual phononic features
Taking as a starting point the results of LDA calculations, which show that
in MgB2 the phonons have a strong quartic anharmonicity and that the
bond-stretching electron-phonon interaction (EPI) has both a linear and a large
quadratic component, we propose a model Hamiltonian which succesfully matches a
number of experimental evidences. We relate the single critical temperature for
both superconducting gaps to a phonon-induced inter-band coupling whose
amplitude increases with temperature. We also obtain phonon frequencies and
linewidths depending on the band filling, as well as band energies and
hybridization amplitudes depending on the phonon number.Comment: 19 pages, no figures, accepted on The European Physical Journal
Role of the particle's stepping cycle in an asymmetric exclusion process: A model of mRNA translation
Messenger RNA translation is often studied by means of statistical-mechanical
models based on the Asymmetric Simple Exclusion Process (ASEP), which considers
hopping particles (the ribosomes) on a lattice (the polynucleotide chain). In
this work we extend this class of models and consider the two fundamental steps
of the ribosome's biochemical cycle following a coarse-grained perspective. In
order to achieve a better understanding of the underlying biological processes
and compare the theoretical predictions with experimental results, we provide a
description lying between the minimal ASEP-like models and the more detailed
models, which are analytically hard to treat. We use a mean-field approach to
study the dynamics of particles associated with an internal stepping cycle. In
this framework it is possible to characterize analytically different phases of
the system (high density, low density or maximal current phase). Crucially, we
show that the transitions between these different phases occur at different
parameter values than the equivalent transitions in a standard ASEP, indicating
the importance of including the two fundamental steps of the ribosome's
biochemical cycle into the model.Comment: 9 pages, 9 figure
Control of spin relaxation in semiconductor double quantum dots
We propose a scheme to manipulate the spin relaxation in vertically coupled
semiconductor double quantum dots. Up to {\em twelve} orders of magnitude
variation of the spin relaxation time can be achieved by a small gate voltage
applied vertically on the double dot. Different effects such as the dot size,
barrier height, inter-dot distance, and magnetic field on the spin relaxation
are investigated in detail. The condition to achieve a large variation is
discussed.Comment: 5 pages, 4 figures, to be published in PR
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Improving Visual Field Examination of the Macula Using Structural Information
Purpose: To investigate a novel approach for structure-function modeling in glaucoma to improve visual field testing in the macula.
Methods: We acquired data from the macular region in 20 healthy eyes and 31 with central glaucomatous damage. Optical coherence tomography (OCT) scans were used to estimate the local macular ganglion cell density. Perimetry was performed with a fundus-tracking device using a 10-2 grid. OCT scans were matched to the retinal image from the fundus perimeter to accurately map the tested locations onto the structural damage. Binary responses from the subjects to all presented stimuli were used to calculate the structure-function model used to generate prior distributions for a ZEST (Zippy Estimation by Sequential Testing) Bayesian strategy. We used simulations based on structural and functional data acquired from an independent dataset of 20 glaucoma patients to compare the performance of this new strategy, structural macular ZEST (MacS-ZEST), with a standard ZEST.
Results: Compared to the standard ZEST, MacS-ZEST reduced the number of presentations by 13% in reliable simulated subjects and 14% with higher rates (≥20%) of false positive or false negative errors. Reduction in mean absolute error was not present for reliable subjects but was gradually more important with unreliable responses (≥10% at 30% error rate).
Conclusions: Binary responses can be modeled to incorporate detailed structural information from macular OCT into visual field testing, improving overall speed and accuracy in poor responders.
Translational Relevance: Structural information can improve speed and reliability for macular testing in glaucoma practice
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