68 research outputs found
Supercollision cooling in undoped graphene
Carrier mobility in solids is generally limited by electron-impurity or
electron-phonon scattering depending on the most frequently occurring event.
Three body collisions between carriers and both phonons and impurities are
rare; they are denoted supercollisions (SCs). Elusive in electronic transport
they should emerge in relaxation processes as they allow for large energy
transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in
undoped graphene where the small Fermi surface drastically restricts the
allowed phonon energy in ordinary collisions. Using electrical heating and
sensitive noise thermometry we report on SC-cooling in diffusive monolayer
graphene. At low carrier density and high phonon temperature the Joule power
obeys a law as a function of electronic temperature .
It overrules the linear law expected for ordinary collisions which has recently
been observed in resistivity measurements. The cubic law is characteristic of
SCs and departs from the dependence recently reported for metallic
graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are
important for applications of graphene in bolometry and photo-detection
Geometrical Dependence of High-Bias Current in Multiwalled Carbon Nanotubes
We have studied the high-bias transport properties of the different shells
that constitute a multiwalled carbon nanotube. The current is shown to be
reduced as the shell diameter is decreased or the length is increased. We
assign this geometrical dependence to the competition between electron-phonon
scattering process and Zener tunneling.Comment: 4 pages, 4 figure
Single electron quantum tomography in quantum Hall edge channels
We propose a quantum tomography protocol to measure single electron coherence
in quantum Hall edge channels and therefore access for the first time the wave
function of single electron excitations propagating in ballistic quantum
conductors. Its implementation would open the way to quantitative studies of
single electron decoherence and would provide a quantitative tool for analyzing
single to few electron sources. We show how this protocol could be implemented
using ultrahigh sensitivity noise measurement schemes.Comment: Version 3: long version (7 figures): corrections performed and
references have been added. Figures reprocessed for better readabilit
Regional scale land use/land cover classification using temporal series of MODIS data.
This paper describes a methodology for systematic land use/land cover classification on a regional scale, with emphasis on a low cost and highly automatized approach. This methodology is based on multitemporal analyses of surface reflectance data from the Moderate Resolution Imaging Spectroradiometer (MODIS), which is located on board NASA's Terra and Aqua satellites and features high temporal frequency, extensive coverage, and extremely low costs for data acquisition. A sequence of automatized procedures were developed for MODIS data pre-processing, as well as for the training and execution of a supervised classification algorithm, where temporal profiles are fitted to smooth polynomial curves and intelligent curve features are then computed in order to reduce data dimensionality and improve profile interpretability, thus providing a more robust classification approach. A case study was performed in the High Taquari Basin, in the states of Mato Grosso do Sul and Mato Grosso, Brazil, which showed that the method was indeed capable of generalizing well over the entire region of study (over 25,000km2), effectively discriminating between areas of agriculture, pasture, and savannah. The methodology was also seen to be quite successful in identifying areas of deforestation, which is of particular interest for the monitoring of land use and land use change in the region
Epidural blood patch for refractory low CSF pressure headache: a pilot study
Once believed an exceedingly rare disorder, recent evidence suggests that low cerebrospinal fluid (CSF) pressure headache has to be considered an important cause of new daily persistent headaches, particularly among young and middle-aged individuals. Treatment of low CSF pressure headache consists of non-invasive/conservative measures and invasive measures with epidural blood patch providing the cornerstone of the invasive measures. In the present pilot study we therefore aimed to evaluate the treatment efficacy of epidural blood patch (EBP) in treatment-refractory low-pressure headache. Our primary effect parameter was total headache burden defined as area under the curve (AUC: intensity × duration) and as secondary effect parameters we identified: intensity (VAS 0-10), frequency (days per month), duration in hours (total hours/month) and also medication days (days on medication/month). In our primary effect parameter we found a significant reduction in AUC with more than 25% and this is considered to be clinically relevant. We found also a significant and relevant reduction at −22% in intensity. A trend towards reduction in duration was seen. We found no statistically significant reduction in frequency. An increase in days with use of medication was found. Increased awareness of low CSF pressure headache is emphasized and a controlled larger randomized study is needed to confirm the results. However the present results, allows us to conclude that EBP in treatment-refractory low CSF pressure headache can be considered as a treatment option
Noisy Kondo impurities
The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with
the conduction electrons spins of a host metal is the basic mechanism
responsible for the increase of the resistance of an alloy such as
CuFe at low temperature, as originally suggested by
Kondo . This coupling has emerged as a very generic property of localized
electronic states coupled to a continuum . The possibility to design artificial
controllable magnetic impurities in nanoscopic conductors has opened a path to
study this many body phenomenon in unusual situations as compared to the
initial one and, in particular, in out of equilibrium situations. So far,
measurements have focused on the average current. Here, we report on
\textit{current fluctuations} (noise) measurements in artificial Kondo
impurities made in carbon nanotube devices. We find a striking enhancement of
the current noise within the Kondo resonance, in contradiction with simple
non-interacting theories. Our findings provide a test bench for one of the most
important many-body theories of condensed matter in out of equilibrium
situations and shed light on the noise properties of highly conductive
molecular devices.Comment: minor differences with published versio
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