268 research outputs found
Mott transition in Cr-doped V2O3 studied by ultrafast reflectivity: electron correlation effects on the transient response
The ultrafast response of the prototype Mott-Hubbard system (V1-xCrx)2O3 was
systematically studied with fs pump-probe reflectivity, allowing us to clearly
identify the effects of the metal-insulator transition on the transient
response. The isostructural nature of the phase transition in this material
made it possible to follow across the phase diagram the behaviour of the
detected coherent acoustic wave, whose average value and lifetime depend on the
thermodynamic phase and on the correlated electron density of states. It is
also shown how coherent lattice oscillations can play an important role in some
changes affecting the ultrafast electronic peak relaxation at the phase
transition, changes which should not be mistakenly attributed to genuine
electronic effects. These results clearly show that a thorough understanding of
the ultrafast response of the material over several tenths of ps is necessary
to correctly interpret its sub-ps excitation and relaxation regime, and appear
to be of general interest also for other strongly correlated materials.Comment: 6 pages, 3 figures. Europhysics Letters (in press
Automatic classification of endoscopic images for premalignant conditions of the esophagus
Barrett’s esophagus (BE) is a precancerous complication of gastroesophageal reflux disease in which normal stratified squamous epithelium lining the esophagus is replaced by intestinal metaplastic columnar epithelium. Repeated endoscopies and multiple biopsies are often necessary to establish the presence of intestinal metaplasia. Narrow Band Imaging (NBI) is an imaging technique commonly used with endoscopies that enhances the contrast of vascular pattern on the mucosa. We present a computer-based method for the automatic normal/metaplastic classification of endoscopic NBI images. Superpixel segmentation is used to identify and cluster pixels belonging to uniform regions. From each uniform clustered region of pixels, eight features maximizing differences among normal and metaplastic epithelium are extracted for the classification step. For each superpixel, the three mean intensities of each color channel are firstly selected as features. Three added features are the mean intensities for each superpixel after separately applying to the red-channel image three different morphological filters (top-hatfiltering, entropy filtering and range filtering). The last two features require the computation of the Grey-Level Co-Occurrence Matrix (GLCM), and are re ective of the contrast and the homogeneity of each superpixel. The classification step is performed using an ensemble of 50 classification trees, with a 10-fold cross-validation scheme by training the classifier at each step on a random 70% of the images and testing on the remaining 30% of the dataset. Sensitivity and Specificity are respectively of 79.2% and 87.3%, with an overall accuracy of 83.9%. © 2016 SPIE
Superpixel-based automatic segmentation of villi in confocal endomicroscopy
Confocal Laser Endomicroscopy (CLE) is a technique permitting on-site microscopy of the gastrointestinal mucosa after the application of a fluorescent agent, allowing the evaluation of mucosa alterations. These are used as features by skilled technicians to stage the severity of multiple diseases, celiac disease or irritable bowel syndrome among the others. We present an automatic method for villi detection from confocal endoscopy images, whose appearance changes with mucosal alterations. Superpixel segmentation, a well-known technique originating from computer vision, is used to identify and cluster together pixels belonging to uniform regions. Each image in the dataset is analyzed in a multiscale fashion (scale 1, 0.5 and 0.25). From each superpixel, 37 features are extracted at multiple image scales. Each superpixel is classified using a random forest, and a post-processing step is performed to refine the final output. Results in the test set (70 images, 30870 superpixels) show 85.87% accuracy, 92.88% sensitivity, 76.99% specificity in the superpixel space, and 86.36% of accuracy and 87.44% Dice score in the pixel domain. © 2016 IEEE
Small Atomic displacements Recorded in Bismuth by the Optical Reflectivity of Femtosecond Laser-Pulse Excitations
Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10-5. Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling
Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry
The advent of Dirac materials has made it possible to realize two dimensional
gases of relativistic fermions with unprecedented transport properties in
condensed matter. Their photoconductive control with ultrafast light pulses is
opening new perspectives for the transmission of current and information. Here
we show that the interplay of surface and bulk transient carrier dynamics in a
photoexcited topological insulator can control an essential parameter for
photoconductivity - the balance between excess electrons and holes in the Dirac
cone. This can result in a strongly out of equilibrium gas of hot relativistic
fermions, characterized by a surprisingly long lifetime of more than 50 ps, and
a simultaneous transient shift of chemical potential by as much as 100 meV. The
unique properties of this transient Dirac cone make it possible to tune with
ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that
is impossible with conventional optoelectronic materials.Comment: Nature Communications, in press (12 pages, 6 figures
Ultrafast surface carrier dynamics in the topological insulator Bi2Te3
We discuss the ultrafast evolution of the surface electronic structure of the
topological insulator BiTe following a femtosecond laser excitation.
Using time and angle resolved photoelectron spectroscopy, we provide a direct
real-time visualisation of the transient carrier population of both the surface
states and the bulk conduction band. We find that the thermalization of the
surface states is initially determined by interband scattering from the bulk
conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary
for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac
distribution, while their relaxation extends over more than 10 ps. The surface
sensitivity of our measurements makes it possible to estimate the range of the
bulk-surface interband scattering channel, indicating that the process is
effective over a distance of 5 nm or less. This establishes a correlation
between the nanoscale thickness of the bulk charge reservoir and the evolution
of the ultrafast carrier dynamics in the surface Dirac cone
Reply to: Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material
International audienceReplying to D. Moreno-MencĂa et al. Nature Communicationshttps://doi.org/10.1038/s41467-019-11743-3 (2019)
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