910 research outputs found
Absolutely Continuous Spectrum for Parabolic Flows/Maps
We provide an abstract framework for the study of certain spectral properties
of parabolic systems; specifically, we determine under which general conditions
to expect the presence of absolutely continuous spectral measures. We use these
general conditions to derive results for spectral properties of time-changes of
unipotent flows on homogeneous spaces of semisimple groups regarding absolutely
continuous spectrum as well as maximal spectral type; the time-changes of the
horocycle flow are special cases of this general category of flows. In addition
we use the general conditions to derive spectral results for twisted horocycle
flows and to rederive certain spectral results for skew products over
translations and Furstenberg transformations
Parabolic Flows Renormalized by Partially Hyperbolic Maps
We consider parabolic flows on 3-dimensional manifolds which are renormalized
by circle extensions of Anosov diffeormorphisms. This class of flows includes
nilflows on the Heisenberg nilmanifold which are renormalized by partially
hyperbolic automorphisms. The transfer operators associated to the
renormalization maps, acting on anisotropic Sobolev spaces, are known to have
good spectral properties (this relies on ideas which have some resemblance to
representation theory but also apply to non-algebraic systems). The spectral
information is used to describe the deviation of ergodic averages and solutions
of the cohomological equation for the parabolic flow.Comment: Comments welcom
De-excitation spectroscopy of strongly interacting Rydberg gases
We present experimental results on the controlled de-excitation of Rydberg
states in a cold gas of Rb atoms. The effect of the van der Waals interactions
between the Rydberg atoms is clearly seen in the de-excitation spectrum and
dynamics. Our observations are confirmed by numerical simulations. In
particular, for off-resonant (facilitated) excitation we find that the
de-excitation spectrum reflects the spatial arrangement of the atoms in the
quasi one-dimensional geometry of our experiment. We discuss future
applications of this technique and implications for detection and controlled
dissipation schemes.Comment: 6 pages, 5 figure
Local structure of REFeAsO (RE=La, Pr, Nd, Sm) oxypnictides studied by Fe K-edge EXAFS
Local structure of REOFeAs (RE=La, Pr, Nd, Sm) system has been studied as a
function of chemical pressure varied due to different rare-earth size. Fe
K-edge extended X-ray absorption fine structure (EXAFS) measurements in the
fluorescence mode has permitted to compare systematically the inter-atomic
distances and their mean square relative displacements (MSRD). We find that the
Fe-As bond length and the corresponding MSRD hardly show any change, suggesting
the strongly covalent nature of this bond, while the Fe-Fe and Fe-RE bond
lengths decrease with decreasing rare earth size. The results provide important
information on the atomic correlations that could have direct implication on
the superconductivity and magnetism of REOFeAs system, with the chemical
pressure being a key ingredient
Isotope effect on the E2g phonon and mesoscopic phase separation near the electronic topological transition in Mg1-xAlxB2
We report the boron isotope effect on the E2g phonon mode by micro-Raman
spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes
10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the
full range decreasing near x = 0. The intraband electron-phonon (e-ph)
coupling, for the electrons in the sigma band, has been extracted from the E2g
line-width and frequency softening. Tuning the Fermi energy near the electronic
topological transition (ETT), where the sigma Fermi surface changes from 2D to
3D topology the E2g mode, shows the known Kohn anomaly on the 2D side of the
ETT and a splitting of the E2g phonon frequency into a hard and soft component
from x = 0 to x = 0.28. The results suggest a minor role of the intraband
phonon mediated pairing in the control of the high critical temperature in
Mg1-xAlxB2. The common physical features of diborides with the novel multigap
FeAs-based superconductors and cuprates is discussed.Comment: 19 pages, 6 figure
Pupillometry via smartphone for low-resource settings
The photopupillary reflex regulates the pupil reaction to changing light conditions. Being controlled by the autonomic nervous system, it is a proxy for brain trauma and for the conditions of patients in critical care. A prompt evaluation of brain traumas can save lives. With a simple penlight, skilled clinicians can do that, whereas less specialized ones have to resort to a digital pupilometer. However, many low-income countries lack both specialized clinicians and digital pupilometers. This paper presents the early results of our study aiming at designing, prototyping and validating an app for testing the photopupillary reflex via Android, following the European Medical Device Regulation and relevant standards. After a manual validation, the prototype underwent a technical validation against a commercial Infrared pupilometer. As a result, the proposed app performed as well as the manual measurements and better than the commercial solution, with lower errors, higher and significant correlations, and significantly better Bland-Altman plots for all the pupillometry-related measures. The design of this medical device was performed based on our expertise in low-resource settings. This kind of environments imposes more stringent design criteria due to contextual challenges, including the lack of specialized clinicians, funds, spare parts and consumables, poor maintenance, and harsh environmental conditions, which may hinder the safe operationalization of medical devices. This paper provides an overview of how these unique contextual characteristics are cascaded into the design of an app in order to contribute to the Sustainable Development Goal 3 of the World Health Organization: Good health and well-being
Orbital occupancies and the putative jeff = 1/2 groundstate in Ba2IrO4: a combined oxygen K edge XAS and RIXS study
The nature of the electronic groundstate of Ba2IrO4 has been addressed using
soft X-ray absorption and inelastic scattering techniques in the vicinity of
the oxygen K edge. From the polarization and angular dependence of XAS we
deduce an approximately equal superposition of xy, yz and zx Ir4+ 5d orbitals.
By combining the measured orbital occupancies, with the value of the spin-orbit
coupling provided by RIXS, we estimate the crystal field splitting associated
with the tetragonal distortion of the IrO6 octahedra to be small, \Delta=50(50)
meV. We thus conclude definitively that Ba2IrO4 is a close realization of a
spin-orbit Mott insulator with a jeff = 1/2 groundstate, thereby overcoming
ambiguities in this assignment associated with the interpretation of X-ray
resonant scattering experiments.Comment: 5 pages, 5 figure
- …