Imaging hydrodynamic electrons flowing without Landauer-Sharvin resistance.

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.

Long-range nontopological edge currents in charge-neutral graphene.

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.

Imaging work and dissipation in the quantum Hall state in graphene.

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.

Publisher Correction: Imaging work and dissipation in the quantum Hall state in graphene.

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

Strong magnetophonon oscillations in extra-large graphene.

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.

Composite super-moiré lattices in double-aligned graphene heterostructures.

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.

[New therapeutic options for the treatment of multiresistant bacteria in the ICU]. / Nuevas opciones terapéuticas para el tratamiento de las bacterias multirresistentes en Unidades de Cuidados Intensivos.

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.

[Nosocomial infection by multiresistant pathogens during one year in a secondary hospital: clinical an epidemiological analysis]. / Infección nosocomial por gérmenes multirresistentes durante 1 año en un hospital de segundo nivel: análisis clínico y microbiológico.

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.