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1.
Nature ; 609(7926): 276-281, 2022 09.
Artículo en Inglés | MEDLINE | ID: mdl-36071191

RESUMEN

Electrical resistance usually originates from lattice imperfections. However, even a perfect lattice has a fundamental resistance limit, given by the Landauer1 conductance caused by a finite number of propagating electron modes. This resistance, shown by Sharvin2 to appear at the contacts of electronic devices, sets the ultimate conduction limit of non-interacting electrons. Recent years have seen growing evidence of hydrodynamic electronic phenomena3-18, prompting recent theories19,20 to ask whether an electronic fluid can radically break the fundamental Landauer-Sharvin limit. Here, we use single-electron-transistor imaging of electronic flow in high-mobility graphene Corbino disk devices to answer this question. First, by imaging ballistic flows at liquid-helium temperatures, we observe a Landauer-Sharvin resistance that does not appear at the contacts but is instead distributed throughout the bulk. This underpins the phase-space origin of this resistance-as emerging from spatial gradients in the number of conduction modes. At elevated temperatures, by identifying and accounting for electron-phonon scattering, we show the details of the purely hydrodynamic flow. Strikingly, we find that electron hydrodynamics eliminates the bulk Landauer-Sharvin resistance. Finally, by imaging spiralling magneto-hydrodynamic Corbino flows, we show the key emergent length scale predicted by hydrodynamic theories-the Gurzhi length. These observations demonstrate that electronic fluids can dramatically transcend the fundamental limitations of ballistic electrons, with important implications for fundamental science and future technologies.

2.
Nature ; 593(7860): 528-534, 2021 05.
Artículo en Inglés | MEDLINE | ID: mdl-34040212

RESUMEN

Van der Waals heterostructures display numerous unique electronic properties. Nonlocal measurements, wherein a voltage is measured at contacts placed far away from the expected classical flow of charge carriers, have been widely used in the search for novel transport mechanisms, including dissipationless spin and valley transport1-9, topological charge-neutral currents10-12, hydrodynamic flows13 and helical edge modes14-16. Monolayer1-5,10,15-19, bilayer9,11,14,20 and few-layer21 graphene, transition-metal dichalcogenides6,7 and moiré superlattices8,10,12 have been found to display pronounced nonlocal effects. However, the origin of these effects is hotly debated3,11,17,22-24. Graphene, in particular, exhibits giant nonlocality at charge neutrality1,15-19, a striking behaviour that has attracted competing explanations. Using a superconducting quantum interference device on a tip (SQUID-on-tip) for nanoscale thermal and scanning gate imaging25, here we demonstrate that the commonly occurring charge accumulation at graphene edges23,26-31 leads to giant nonlocality, producing narrow conductive channels that support long-range currents. Unexpectedly, although the edge conductance has little effect on the current flow in zero magnetic field, it leads to field-induced decoupling between edge and bulk transport at moderate fields. The resulting giant nonlocality at charge neutrality and away from it produces exotic flow patterns that are sensitive to edge disorder, in which charges can flow against the global electric field. The observed one-dimensional edge transport is generic and nontopological and is expected to support nonlocal transport in many electronic systems, offering insight into the numerous controversies and linking them to long-range guided electronic states at system edges.

3.
Nature ; 576(7786): E6, 2019 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-31772389

RESUMEN

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

4.
Nature ; 575(7784): 628-633, 2019 11.
Artículo en Inglés | MEDLINE | ID: mdl-31634903

RESUMEN

Topology is a powerful recent concept asserting that quantum states could be globally protected against local perturbations1,2. Dissipationless topologically protected states are therefore of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often susceptible to various dissipative mechanisms, which are difficult to study directly because of their microscopic origins. Here we use scanning nanothermometry3 to visualize and investigate the microscopic mechanisms that undermine dissipationless transport in the quantum Hall state in graphene. Simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries as a result of edge reconstruction. Instead of local Joule heating, however, the dissipation mechanism comprises two distinct and spatially separated processes. The work-generating process that we image directly, which involves elastic tunnelling of charge carriers between the quantum channels, determines the transport properties but does not generate local heat. By contrast, the heat and entropy generation process-which we visualize independently-occurs nonlocally upon resonant inelastic scattering from single atomic defects at graphene edges, and does not affect transport. Our findings provide an insight into the mechanisms that conceal the true topological protection, and suggest routes towards engineering more robust quantum states for device applications.

5.
Nat Commun ; 10(1): 3334, 2019 Jul 26.
Artículo en Inglés | MEDLINE | ID: mdl-31350410

RESUMEN

Van der Waals materials and their heterostructures offer a versatile platform for studying a variety of quantum transport phenomena due to their unique crystalline properties and the exceptional ability in tuning their electronic spectrum. However, most experiments are limited to devices that have lateral dimensions of only a few micrometres. Here, we perform magnetotransport measurements on graphene/hexagonal boron-nitride Hall bars and show that wider devices reveal additional quantum effects. In devices wider than ten micrometres we observe distinct magnetoresistance oscillations that are caused by resonant scattering of Landau-quantised Dirac electrons by acoustic phonons in graphene. The study allows us to accurately determine graphene's low energy phonon dispersion curves and shows that transverse acoustic modes cause most of phonon scattering. Our work highlights the crucial importance of device width when probing quantum effects and also demonstrates a precise, spectroscopic method for studying electron-phonon interactions in van der Waals heterostructures.

6.
Sci Adv ; 5(12): eaay8897, 2019 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-32064323

RESUMEN

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

7.
Rev Esp Quimioter ; 21 Spec No 1: 9-13, 2008.
Artículo en Español | MEDLINE | ID: mdl-18752075

RESUMEN

The number of new antimicrobial drugs in the health care clinical practice has decreased gradually and significantly in the last 15 years. At the same time, there has been an increase in the appearance of microorganisms with resistance to conventional antibiotics, above all in intensive care units (ICU). Within this group, Methicillin-resistant Staphylococcus aureus (MSRA) and methicillin-resistant coagulase- negative staphylococci, vancomycin-resistant enterococci, Pseudomonas aeruginosa and Acinetobacter baumanii resistant to carbapenemics and extended-spectrum betalactamase-producing (ESBL) Enterobacteria are the most important. These pathogens are frequently also resistant to other groups of antibiotics such as aminoglycosides, fluoroquinolones and macrolides. New recently introduced antimicrobial agents are available to combat these resistances. These are active mainly against gram positive bacteria resistant strains and in a more timely way against gram negative ones or both. Among the first group, the following stand out: daptomycin (a lipopeptide bactericide for parenteral use) and linezolid (oxazolidinone with bacteriostatic activity for parenteral and oral use). On its part, ertapenem (a carbapenem parenteral bactericide) and tigecyclin (a parenteral bacteriostatic tetracycline) are active against ESBL enterobacteria, the latter also being active against non-fermented gram positives and gram negatives, except for P. aeruginosa. Possibly, the introduction of these new compounds and other futures ones pending introduction will not only improve antimicrobial diversification but also serve to limit the spreading of these microorganisms.


Asunto(s)
Antibacterianos/uso terapéutico , Infecciones Bacterianas/tratamiento farmacológico , Farmacorresistencia Bacteriana Múltiple , Unidades de Cuidados Intensivos , Humanos
8.
Rev. esp. quimioter ; 21(extr.1): 9-13, jul. 2008.
Artículo en Español | IBECS | ID: ibc-77617

RESUMEN

El número de nuevos antimicrobianos en la clínica asistencialha descendido de forma gradual y significativa enlos últimos 15 años. Paralelamente se ha producido un incrementoen la aparición de microorganismos con resistenciaa los antibióticos convencionales, sobre todo en las Unidadesde Cuidados Intensivos (UCI). Dentro de este grupo, laresistencia a la meticilina de Staphylococcus aureus (SARM) yde estafilococos coagulasa negativos resistentes a la meticilina,enterococos resistentes a la vancomicina, Pseudomonasaeruginosa y Acinetobacter baumannii resistentes a carbapenémicosy Enterobacterias productoras de betalactamasasde espectro extendido (BLEE) son las más destacables. Estospatógenos con frecuencia son además resistentes a otrosgrupos de antibióticos, como aminoglucósidos, fluorquinolonasy macrólidos. Para hacer frente a estas resistenciasdisponemos de nuevos antimicrobianos de reciente introducción.Éstos son activos principalmente frente a cepas resistentesde bacterias grampositivas, y de manera más puntualfrente a gramnegativos o a ambos. Entre los primerosdestacan: daptomicina (lipopéptido bactericida de uso parenteral)y linezolid (oxazolidinona bacteriostática de usoparenteral y oral). Por su parte, el ertapenem (carbapenembactericida parenteral) y la tigeciclina (tetraciclina bacteriostáticaparenteral) son activos frente a enterobacterias BLEE,siendo esta última además activa frente a grampositivos ygramnegativos no fermentadores, excepto P. aeruginosa. Posiblementela introducción de estos nuevos compuestos yotros futuros pendientes salir, aparte de mejorar la diversificaciónantimicrobiana, sirva para limitar la dispersión de estosmicroorganismos (AU)


The number of new antimicrobial drugs in the healthcare clinical practice has decreased gradually and significantlyin the last 15 years. At the same time, there hasbeen an increase in the appearance of microorganisms withresistance to conventional antibiotics, above all in intensivecare units (ICU). Within this group, Methicillin-resistantStaphylococcus aureus (MSRA) and methicillin-resistant coagulase-negative staphylococci, vancomycin-resistant enterococci,Pseudomonas aeruginosa and Acinetobacter baumaniiresistant to carbapenemics and extended-spectrumß-lactamase-producing (ESBL) Enterobacteria are themost important. These pathogens are frequently also resistantto other groups of antibiotics such as aminoglycosides,fluoroquinolones and macrolides. New recentlyintroduced antimicrobial agents are available to combatthese resistances. These are active mainly against grampositive bacteria resistant strains and in a more timelyway against gram negative ones or both. Among the firstgroup, the following stand out: daptomycin (a lipopeptidebactericide for parenteral use) and linezolid (oxazolidinonewith bacteriostatic activity for parenteral andoral use). On its part, ertapenem (a carbapenem parenteralbactericide) and tigecyclin (a parenteral bacteriostatictetracycline) are active against ESBL enterobacteria, thelatter also being active against non-fermented gram positivesand gram negatives, except for P. aeruginosa.Possibly, the introduction of these new compounds andother futures ones pending introduction will not onlyimprove antimicrobial diversification but also serve tolimit the spreading of these microorganisms (AU)


Asunto(s)
Humanos , Masculino , Femenino , Antibacterianos/síntesis química , Antibacterianos/historia , Antibacterianos/farmacología , Antibacterianos/farmacocinética , Antibacterianos/uso terapéutico , Cuidados Críticos/tendencias , Farmacorresistencia Microbiana/inmunología , Farmacorresistencia Microbiana/fisiología , Daptomicina/administración & dosificación , Daptomicina/farmacología , Daptomicina/uso terapéutico
9.
An Med Interna ; 22(2): 59-64, 2005 Feb.
Artículo en Español | MEDLINE | ID: mdl-15896110

RESUMEN

BACKGROUND: Currently growing medical and social significance of nosocomial infection by multiresistant pathogens (NIMP) prompted us to establish its incidence, nosology, presenting forms in admission areas, and mortality in a secondary hospital, Lleida (Spain). METHOD: For that purpose, we analyzed the first year experience of a unit for the control of nosocomial infection (NI) created in our hospital. From January to December 2000, 79 patients with a NIMP admitted to the University Hospital Arnau de Vilanova entered in this prospective, descriptive study. RESULTS: The overall annual incidence of NIMP was 4.0 per 103 patients admitted. Acinetobacter baumannii showed the highest individual rate of incidence, particularly, at the Intensive Care Unit (15.4 per 103 patients admitted; p < 0.001). By nosologies, infection prevailed over colonization (69.6% vs 30.4%; p < 0.001). Mean hospital stay length increased in colonized patients (38.9 days). Finally, overall mortality was high (29.1%); again, A. baumannii was the agent most frequently detected in death cases (66.6%; p < 0.001). CONCLUSIONS: Surveillance and control measures are required for the prevention of NIMP. Incidence studies how this, can be useful to create a database to establish the distribution and occurrence of NI, including the detection of multiresistant pathogen outbreaks.


Asunto(s)
Infección Hospitalaria/epidemiología , Infección Hospitalaria/microbiología , Farmacorresistencia Bacteriana Múltiple , Adolescente , Adulto , Anciano , Anciano de 80 o más Años , Infección Hospitalaria/tratamiento farmacológico , Femenino , Hospitales , Humanos , Incidencia , Masculino , Persona de Mediana Edad , Estudios Prospectivos , Factores de Tiempo
10.
An. med. interna (Madr., 1983) ; 22(2): 59-64, feb. 2005. ilus, tab
Artículo en Es | IBECS | ID: ibc-038390

RESUMEN

Fundamento: Dada la gran relevancia actual, médica y social, de la infección nosocomial por gérmenes multirresistentes (INGM), se pretendió determinar su incidencia, nosología, formas de presentación en las áreas de ingreso y mortalidad en un hospital de 2º nivel en Lleida (España). Método: Se utilizó la experiencia de una unidad para el control de la Infección Nosocomial (IN), en su primer año de funcionamiento en este hospital. Por medio de la unidad, se practicó un estudio descriptivo prospectivo sobre todos los pacientes ingresados, en el Hospital Universitario Arnau de Vilanova, entre enero y diciembre de 2000 en los que se detectó una INGM. Resultados: En total fueron 79 pacientes los que presentaron una INGM. La tasa de incidencia global anual de INGM fue de 4,0 por 103 pacientes ingresados. A. baumannii fue el germen que obtuvo la tasa de incidencia individual más elevada, y en especial en la Unidad de Cuidados Intensivos (15,4 por 103 pacientes ingresados; p < 0,001). Por nosologías,la infección predominó sobre la colonización (69,6 frente 30,4%; p < 0,001). En general, se observó una estancia media más prolongada en los pacientes colonizados 38,9 días. Por último, la mortalidad total fue elevada (29,1%), destacando de nuevo A. baumannii, como el agente más frecuentemente detectado en los pacientes que fallecieron (66,6%; p < 0,001). Conclusiones: En la prevención de la INGM son necesarias medidas de vigilancia y control. Los estudios de incidencia como éste, pueden ser útiles a la hora de establecer una base de datos con la que observar la distribución y ocurrencia de la IN, incluyendo la detección de brotes epidémicos por gérmenes multirresistentes


Background: Currently growing medical and social significance of nosocomial infection by multiresistant pathogens (NIMP) prompted us to establish its incidence, nosology, presenting forms in admission areas, and mortality in a secondary hospital, Lleida (Spain). Method: For that purpose, we analyzed the first year experience of a unit for the control of nosocomial infection (NI) created in our hospital. From January to December 2000, 79 patients with a NIMP admitted to the University Hospital Arnau de Vilanova entered in this prospective, descriptive study. Results: The overall annual incidence of NIMP was 4.0 per 103 patients admitted. Acinetobacter baumannii showed the highest individual rate of incidence, particularly, at the Intensive Care Unit (15.4 per 103 patients admitted; p < 0.001). By nosologies, infection prevailed over colonization (69.6% vs 30.4%; p < 0.001). Mean hospital stay length increased in colonized patients (38.9 days). Finally, overall mortality was high (29.1%); again, A. baumannii was the agent most frequently detected in death cases (66.6%; p < 0.001). Conclusions: Surveillance and control measures are required for the prevention of NIMP. Incidence studies how this, can be useful to create adatabase to establish the distribution and occurrence of NI, including the detection of multiresistant pathogen outbreaks


Asunto(s)
Masculino , Femenino , Adulto , Anciano , Persona de Mediana Edad , Humanos , Infección Hospitalaria/epidemiología , Resistencia a Múltiples Medicamentos , Atención Secundaria de Salud , Indicadores de Morbimortalidad , Hospitales/estadística & datos numéricos
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