Violation of the Wiedemann-Franz law in clean graphene layers

The Wiedemann-Franz law, connecting the electronic thermal conductivity to the electrical conductivity of a disordered metal, is generally found to be well satisfied even when electron-electron (e-e) interactions are strong. In ultra-clean conductors, however, large deviations from the standard form of the law are expected, due to the fact that e-e interactions affect the two conductivities in radically different ways. Thus, the standard Wiedemann-Franz ratio between the thermal and the electric conductivity is reduced by a factor $1+\tau/\tau_{\rm th}^{\rm ee}$, where $1/\tau$ is the momentum relaxation rate, and $1/\tau_{\rm th}^{\rm ee}$ is the relaxation time of the thermal current due to e-e collisions. Here we study the density and temperature dependence of $1/\tau_{\rm th}^{\rm ee}$ in the important case of doped, clean single layers of graphene, which exhibit record-high thermal conductivities. We show that at low temperature $1/\tau_{\rm th}^{\rm ee}$ is $8/5$ of the quasiparticle decay rate. We also show that the many-body renormalization of the thermal Drude weight coincides with that of the Fermi velocity.Comment: 6 pages, 5 appendices (13 pages

Many-body orbital paramagnetism in doped graphene sheets

The orbital magnetic susceptibility (OMS) of a gas of noninteracting massless Dirac fermions is zero when the Fermi energy is away from the Dirac point. Making use of diagrammatic perturbation theory, we calculate exactly the OMS of massless Dirac fermions to first order in the Coulomb interaction demonstrating that it is finite and positive. Doped graphene sheets are thus unique systems in which the OMS is completely controlled by many-body effects.Comment: 4 pages, 2 figures, submitte

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

Linear response of doped graphene sheets to vector potentials

URL:http://link.aps.org/doi/10.1103/PhysRevB.80.075418 DOI:10.1103/PhysRevB.80.075418A two-dimensional gas of massless Dirac fermions (MDFs) is a very useful model to describe low-energy electrons in monolayer graphene. Because the MDF current operator is directly proportional to the (sublattice) pseudospin operator, the MDF current-current response function, which describes the response to a vector potential, happens to coincide with the pseudospin-pseudospin response function. In this work, we present analytical results for the wave vector- and frequency-dependent longitudinal and transverse pseudospin-pseudospin response functions of noninteracting MDFs. The transverse response in the static limit is then used to calculate the noninteracting orbital magnetic susceptibility. These results are a starting point for the construction of approximate pseudospin-pseudospin response functions that would take into account electron-electron interactions (for example at the random-phase-approximation level). They also constitute a very useful input for future applications of current-density-functional theory to graphene sheets subjected to time and spatially varying vector potentials.M.P. was partially supported by the CNR-INFM “SeedProjects.” G.V. acknowledges support from NSF Grant No.DMR-0705460

New antibiotics for the treatment of acute bacterial skin and soft tissue infections in pediatrics

Acute bacterial skin and soft tissue infections (aSSTIs) are a large group of diseases that can involve exclusively the skin or also the underlying subcutaneous tissues, fascia, or muscles. Despite differences in the localization and severity, all these diseases are due mainly to Gram-positive bacteria, especially Staphylococcus aureus and Streptococcus pyogenes. aSSTI incidence increased considerably in the early years of this century due to the emergence and diffusion of community-acquired methicillin-resistant S. aureus (CA-MRSA). Despite the availability of antibiotics effective against CA-MRSA, problems of resistance to these drugs and risks of significant adverse events have emerged. In this paper, the present knowledge on the potential role new antibiotics for the treatment of pediatric aSSTIs is discussed. The most recent molecules that have been licensed for the treatment of aSSTIs include ozenoxacin (OZ), ceftaroline fosamil (CF), dalbavancin (DA), oritavancin (OR), tedizolid (TD), delafloxacin (DL), and omadacycline (OM). However, only OZ and CF have been licensed for use in children with aSSTIs, although the superiority of these antibiotics to those routinely used for the treatment of aSSTIs should be further demonstrated. Waiting for additional studies, OZ and CF should be prescribed for aSSTI treatment in the presence of the potential failure of old molecules

Dysbiosis in pediatrics is associated with respiratory infections: Is there a place for bacterial-derived products?

Respiratory tract infections (RTIs) are common in childhood because of the physiologic immaturity of the immune system, a microbial community under development in addition to other genetic, physiological, environmental and social factors. RTIs tend to recur and severe lower viral RTIs in early childhood are not uncommon and are associated with increased risk of respiratory disorders later in life, including recurrent wheezing and asthma. Therefore, a better understanding of the main players and mechanisms involved in respiratory morbidity is necessary for a prompt and improved care as well as for primary prevention. The inter-talks between human immune com-ponents and microbiota as well as their main functions have been recently unraveled; nevertheless, more is still to be discovered or understood in the above medical conditions. The aim of this review paper is to provide the most up-to-date overview on dysbiosis in pre-school children and its association with RTIs and their complications. The potential role of non-harmful bacterial-derived prod-ucts, according to the old hygiene hypothesis and the most recent trained-innate immunity concept, will be discussed together with the need of proof-of-concept studies and larger clinical trials with immunological and microbiological endpoints

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

THE LATE EARLY CRETACEOUS TRANSGRESSION ON THE LATERITES IN VOURINOS AND VERMION MASSIFS (WESTERN MACEDONIA, GREECE)

Τρεις στρωματογραφικές τομές, από τον ανατολικό Βούρινο (περιοχή Ροδιανής) και το ανατολικό Βέρμιο, απεκάλυψαν την ίδια ηλικία των φαινομένων που σχετίζονται με τις λατεριτιώσεις που προσέβαλαν το σερπεντινιωμένο οφιολιθικό σύμπλεγμα μετά την τοποθέτηση του πάνω στην πελαγονική επικράτεια. Στις τομές παρατηρούνται τεμάχη σερπεντινιωμένων χαρτσβουργιτών που φέρουν λατεριτικές ασυμφωνίες στην κορυφή, καλυπτόμενες από επικλυσιγενείς υστερο-ηωκρητιδικούς νηριτικούς ασβεστόλιθους. Κοντά στο χωρίο Κτένι (περιοχή Ροδιανής), ένας ορίζοντας λατερίτη, που κάθεται στην κορυφή σερπεντινιτών, καλύπτεται από επικλυσιγενείς νηριτικούς ασβεστόλιθους με Salpingoporella urladanasi, η οποία προσδιορίζει ηλικία Βαρρέμιο - Άλβιο, ακολουθούμενους από ασβεστόλιθους με Orbitolinidae. Στη θέση Τσιμοδιά (ΒΒΔ της προηγουμένης), ο λατεριτικός ορίζοντας, που κάθεται πάνω σε καρστικοποιημένους ανωιουρασικούς υφαλογενείς ασβεστόλιθους (οι οποίοι είναι το κορυφαίο μέλος ενός σώματος ανθρακικής πλατφόρμας που κάθεται τεκτονικός πάνω στους οφιολίθους), καλύπτεται επικλυσιγενώς από ένα σιδηρούχο πισσολιθικό στρωματίδιο και ασβεστόλιθους του Απτίου, που έχουν υφή wackes tone-muds tone και επίσης περιέχουν Salpingoporella urladanasi, ακολουθούμενους από ασβεστόλιθους με Orbitolina του Κενομανίου. Τέλος, η τρίτη εξετασθείσα τοποθεσία, αρκετά ΒΑ των προηγουμένων, βρίσκεται στις ανατολικές παρυφές του Βερμίου και συγκεκριμένα στα ΒΔ του χωρίου Κουμαριά. Εκεί, ομοίως παρατηρείται ότι τα λατεριτιωμένα τεμάχη σερπεντινιτών καλύπτονται επικλυσιγενώς από νηριτικούς ασβεστόλιθους με Salpingoporella urladanasi, που περνούν προς τα πάνω σε ανακρυσταλλωμένους ανωκρητιδικούς ασβεστόλιθους με Orbitolinidae και θραύσματα ρουδιστών και, εν τέλει, στο φλύσχη. Οι παραπάνω χαρακτήρες οδηγούν στο συμπέρασμα ότι η ανάδυση και η επακόλουθη λατεριτίωση των επωθημένων οφιολίθων του Βούρινου και του Βερμίου, στη βόρεια πελαγονική επικράτεια, που έλαβε χώρα αρχής γενομένης από το ύστατο Ιουρασικό, ακολουθήθηκε από μια θαλάσσια επίκλυση που άρχισε μέσα στο διάστημα Βαρρέμιο - Άλβιο, σε συνθήκες ανθρακικής πλατφόρμας, αρχικώς κλειστής και υφάλμυρης, που σημαδεύονται από την παρουσία του φύκους Salpingoporella urladanasi, ακολουθούμενες από κανονικής αλμυρότητας συνθήκες ανθρακικής πλατφόρμας. Η νηριτική ιζηματογένεση έμεινε ακλόνητη μέχρι το κατώτερο Κενομάνιο. Στη συνέχεια, μία βάθυνση, νωρίτερα στο Βούρινο και αργότερα στο Βέρμιο, οδήγησε στην απόθεση πελαγικών και τουρβιδιτικών ασβεστόλιθων και στη συνέχεια του φλύσχη.Three stratigraphical sections from eastern Vourinos (Rhodiani area) to eastern Vermion massifs revealed the same age of the latérite events affecting the serpentinized ophiolite complex after its emplacement on the Pelagonian domain. All of them consist from their base upwards of serpentinized harzburgite slivers with lateritic unconformities on the top, followed by transgressive upper Lower Cretaceous neritic limestones. At Kteni locality (Rhodiani area), a laterite horizon, lying on top of serpentinites, is covered by transgressive neritic limestones with Salpingoporella urladanasi, assigning a Barremian - Albian age, followed by Orbitolinidae limestones. At Tsimodia locality (NNW to the previous), the latente horizon, lying on karstified Upper Jurassic reef limestones (which are the top member of a carbonate platform body tectonically lying on the ophiolites), is trans gres s ively overlain by iron-rich pisolith levels and Aptian limestones of the wackes tone-muds tone type, also containing Salpingoporella urladanasi, followed by Cenomanian Orbitolina limestones. Finally, the third examined locality, further north-eastward to the previous, is situated at the eastern slopes of Vermion massif and more precisely at the NWpart of Koumaria village. There, it can again be observed that the lateritized serpentinite slivers are overlain transgress ively by neritic limestones with Salpingoporella urladanasi, passing upwards into Upper Cretaceous recrystallized limestones with Orbitolinidae and rudist fragments and, finally, toflysch deposition. These features allow to recognize that the emersion and the consecutive lateritization of the thrust-emplaced ophiolites in Vourinos and Vermion massifs in the northern Pelagonian domain, starting from the Latest Jurassic, was followed by a marine transgression beginning from the Barremian - Albian, firstly under restricted and brackish carbonate platform conditions, marked by the presence of the dasycladalean alga Salpingoporella urladanasi, followed by normal salinity carbonate platform conditions. The neritic sedimentation was stable until the Early Cenomanian. Subsequently, a deepening, earlier at Vourinos and later at Vermion, resulted in deposition of pelagic and turbiditic carbonates and then offlysch

Negative local resistance caused by viscous electron backflow in graphene

Graphene hosts a unique electron system in which electron-phonon scattering is extremely weak but electron-electron collisions are sufficiently frequent to provide local equilibrium above liquid nitrogen temperature. Under these conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids. Here we report strong evidence for this transport regime. We find that doped graphene exhibits an anomalous (negative) voltage drop near current injection contacts, which is attributed to the formation of submicrometer-size whirlpools in the electron flow. The viscosity of graphene's electron liquid is found to be ~0.1 m$^2$ /s, an order of magnitude larger than that of honey, in agreement with many-body theory. Our work shows a possibility to study electron hydrodynamics using high quality graphene