433 research outputs found
Theory of Coulomb drag for massless Dirac fermions
Coulomb drag between two unhybridized graphene sheets separated by a
dielectric spacer has recently attracted considerable theoretical interest. We
first review, for the sake of completeness, the main analytical results which
have been obtained by other authors. We then illustrate pedagogically the
minimal theory of Coulomb drag between two spatially-separated two-dimensional
systems of massless Dirac fermions which are both away from the
charge-neutrality point. This relies on second-order perturbation theory in the
screened interlayer interaction and on Boltzmann transport theory. In this
theoretical framework and in the low-temperature limit, we demonstrate that, to
leading (i.e. quadratic) order in temperature, the drag transresistivity is
completely insensitive to the precise intralayer momentum-relaxation mechanism
(i.e. to the functional dependence of the scattering time on energy). We also
provide analytical results for the low-temperature drag transresistivity for
both cases of "thick" and "thin" spacers and for arbitrary values of the
dielectric constants of the media surrounding the two Dirac-fermion layers.
Finally, we present numerical results for the low-temperature drag
transresistivity in the case in which one of the media surrounding the
Dirac-fermion layers has a frequency-dependent dielectric constant. We conclude
by suggesting an experiment that can potentially allow for the observation of
departures from the canonical Fermi-liquid quadratic-in-temperature behavior of
the transresistivity.Comment: 20 pages, 4 figure
On Coulomb drag in double layer systems
We argue, for a wide class of systems including graphene, that in the low
temperature, high density, large separation and strong screening limits the
drag resistivity behaves as d^{-4}, where d is the separation between the two
layers. The results are independent of the energy dispersion relation, the
dependence on momentum of the transport time, and the wave function structure
factors. We discuss how a correct treatment of the electron-electron
interactions in an inhomogeneous dielectric background changes the theoretical
analysis of the experimental drag results of Ref. [1]. We find that a
quantitative understanding of the available experimental data [1] for drag in
graphene is lacking.Comment: http://iopscience.iop.org/0953-8984/24/33/335602
Measuring Hall Viscosity of Graphene's Electron Fluid
Materials subjected to a magnetic field exhibit the Hall effect, a phenomenon
studied and understood in fine detail. Here we report a qualitative breach of
this classical behavior in electron systems with high viscosity. The viscous
fluid in graphene is found to respond to non-quantizing magnetic fields by
producing an electric field opposite to that generated by the classical Hall
effect. The viscous contribution is large and identified by studying local
voltages that arise in the vicinity of current-injecting contacts. We analyze
the anomaly over a wide range of temperatures and carrier densities and extract
the Hall viscosity, a dissipationless transport coefficient that was long
identified theoretically but remained elusive in experiment. Good agreement
with theory suggests further opportunities for studying electron
magnetohydrodynamics.Comment: 18 pages, 9 figure
Silence of the lambs: The immunological and molecular mechanisms of covid-19 in children in comparison with adults
Children infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can suffer from severe coronavirus disease 2019 (COVID-19). However, compared to adults and the elderly, susceptibility to SARS-CoV-2 infection in children seems to be lower; when infection does develop, most infected children remain asymptomatic or develop a mild disease. Understanding why children seem generally protected from severe COVID-19 and only rarely develop clinical conditions that can cause hospitalization, admission to the pediatric intensive care unit and death can be important. More details on the mechanism of action of SARS-CoV-2 could be defined. Moreover, the role played by children in virus diffusion should be better analyzed, and the development of effective preventive and therapeutic measures against COVID-19 could be favored. The main aim of this paper is to discuss the present knowledge on immunological and molecular mechanisms that could explain differences in COVID-19 clinical manifestations between children and adults. Literature analysis showed that although most children are clearly protected from the development of severe COVID-19, the reasons for this peculiarity are not fully understood. Developmental variations in immune system function together with the potential role of repeated antigen stimulation in the first periods of life on innate immunity are widely studied. As the few children who develop the most severe form of pediatric COVID-19 have certain alterations in the immune system response to SARS-CoV-2 infection, studies about the relationships between SARS-CoV-2 and the immune system of the host are essential to understand the reasons for the age-related differences in the severity of COVID-19
A young infant with transient severe hypertriglyceridemia temporarily associated with meropenem administration : A case report and review of the literature
Background: Slight changes in the lipid profile can be observed over the acute phase of infectious diseases. Moreover, some antiinfective drugs can modify serum lipid concentrations, although antibiotics do not seem to have a relevant, direct, or acute effect on the lipid profile. Methods:A 75-day-old breastfed Caucasian female, born at term after a regular pregnancy, was hospitalized for osteomyelitis. She was immediately treated with intravenous meropenem and vancomycin. Therapy was effective, but after 22 days of treatment, her blood was found to be viscous with a purple shade. Results: A fasting blood sample showed serum triglycerides of 966mg/dL, total cholesterol of 258mg/dL, and high-density lipoprotein cholesterol of 15mg/dL. Secondary causes of hyperlipidemia and primary hereditary disorders were ruled out. Thereafter, the possibility that antibiotics may have had a role in the hypertriglyceridemia was considered, and meropenem was discontinued. After 72 hours of meropenem discontinuation, a sharp modification of lipid variables was observed, and further testing showed a complete normalization of the lipid profile. Conclusion: In this child with osteomyelitis, the increase in serum triglycerides appeared suddenly after 3 weeks of meropenem treatment and resolved quickly after meropenem discontinuation, thus highlighting the possible association between meropenem and lipid profile alterations. Monitoring the lipid profile should be considered in cases of long-term treatment with meropenem, and further studies on meropenem safety should include evaluation of the lipid profile
Proposal of standardization of every step of angiographic procedure in bleeding patients from pelvic trauma
Trauma accounts for a third of the deaths in Western countries, exceeded only by cardiovascular disease and cancer. The high risk of massive bleeding, which depends not only on the type of fractures, but also on the severity of any associated parenchymal injuries, makes pelvic fractures one of the most life-threatening skeletal injuries, with a high mortality rate. Therefore, pelvic trauma represents an important condition to correctly and early recognize, manage, and treat. For this reason, a multidisciplinary approach involving trauma surgeons, orthopedic surgeons, emergency room physicians and interventional radiologists is needed to promptly manage the resuscitation of pelvic trauma patients and ensure the best outcomes, both in terms of time and costs. Over the years, the role of interventional radiology in the management of patient bleeding due to pelvic trauma has been increasing. However, the current guidelines on the management of these patients do not adequately reflect or address the varied nature of injuries faced by the interventional radiologist. In fact, in the therapeutic algorithm of these patients, after the word “ANGIO”, there are no reports on the different possibilities that an interventional radiologist has to face during the procedure. Furthermore, variations exist in the techniques and materials for performing angioembolization in bleeding patients with pelvic trauma. Due to these differences, the outcomes differ among different published series. This article has the aim to review the recent literature on optimal imaging assessment and management of pelvic trauma, defining the role of the interventional radiologist within the multidisciplinary team, suggesting the introduction of common and unequivocal terminology in every step of the angiographic procedure. Moreover, according to these suggestions, the present paper tries to expand the previously drafted algorithm exploring the role of the interventional radiologist in pelvic trauma, especially given the multidisciplinary setting
Drude weight, plasmon dispersion, and a.c. conductivity in doped graphene sheets
We demonstrate that the plasmon frequency and Drude weight of the electron
liquid in a doped graphene sheet are strongly renormalized by electron-electron
interactions even in the long-wavelength limit. This effect is not captured by
the Random Phase Approximation (RPA), commonly used to describe electron fluids
and is due to coupling between the center of mass motion and the pseudospin
degree of freedom of the graphene's massless Dirac fermions. Making use of
diagrammatic perturbation theory to first order in the electron-electron
interaction, we show that this coupling enhances both the plasmon frequency and
the Drude weight relative to the RPA value. We also show that interactions are
responsible for a significant enhancement of the optical conductivity at
frequencies just above the absorption threshold. Our predictions can be checked
by far-infrared spectroscopy or inelastic light scattering.Comment: 15 pages, 8 figures, submitte
Graphene plasmonics
Two rich and vibrant fields of investigation, graphene physics and
plasmonics, strongly overlap. Not only does graphene possess intrinsic plasmons
that are tunable and adjustable, but a combination of graphene with noble-metal
nanostructures promises a variety of exciting applications for conventional
plasmonics. The versatility of graphene means that graphene-based plasmonics
may enable the manufacture of novel optical devices working in different
frequency ranges, from terahertz to the visible, with extremely high speed, low
driving voltage, low power consumption and compact sizes. Here we review the
field emerging at the intersection of graphene physics and plasmonics.Comment: Review article; 12 pages, 6 figures, 99 references (final version
available only at publisher's web site
Resonant Terahertz Detection Using Graphene Plasmons
Plasmons, collective oscillations of electron systems, can efficiently couple
light and electric current, and thus can be used to create sub-wavelength
photodetectors, radiation mixers, and on-chip spectrometers. Despite
considerable effort, it has proven challenging to implement plasmonic devices
operating at terahertz frequencies. The material capable to meet this challenge
is graphene as it supports long-lived electrically-tunable plasmons. Here we
demonstrate plasmon-assisted resonant detection of terahertz radiation by
antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot
cavities and rectifying elements. By varying the plasmon velocity using gate
voltage, we tune our detectors between multiple resonant modes and exploit this
functionality to measure plasmon wavelength and lifetime in bilayer graphene as
well as to probe collective modes in its moir\'e minibands. Our devices offer a
convenient tool for further plasmonic research that is often exceedingly
difficult under non-ambient conditions (e.g. cryogenic temperatures and strong
magnetic fields) and promise a viable route for various photonic applications.Comment: 19 pages, 12 figure
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