Atomic-scale images of charge ordering in a mixed-valence manganite

Transition-metal perovskite oxides exhibit a wide range of extraordinary but imperfectly understood phenomena. Charge, spin, orbital, and lattice degrees of freedom all undergo order-disorder transitions in regimes not far from where the best-known of these phenomena, namely high-temperature superconductivity of the copper oxides, and the 'colossal' magnetoresistance of the manganese oxides, occur. Mostly diffraction techniques, sensitive either to the spin or the ionic core, have been used to measure the order. Unfortunately, because they are only weakly sensitive to valence electrons and yield superposition of signals from distinct mesoscopic phases, they cannot directly image mesoscopic phase coexistence and charge ordering, two key features of the manganites. Here we describe the first experiment to image charge ordering and phase separation in real space with atomic-scale resolution in a transition metal oxide. Our scanning tunneling microscopy (STM) data show that charge order is correlated with structural order, as well as with whether the material is locally metallic or insulating, thus giving an atomic-scale basis for descriptions of the manganites as mixtures of electronically and structurally distinct phases.Comment: 8 pages, 4 figures, 19 reference

Polarons and confinement of electronic motion to two dimensions in a layered transition metal oxide

A very remarkable feature of the layered transition metal oxides (TMOs), whose most famous members are the high-temperature superconductors (HTSs), is that even though they are prepared as bulk three-dimensional single crystals, they display hugely anisotropic electrical and optical properties, seeming to be insulating perpendicular to the layers and metallic within them. This is the phenomenon of confinement, a concept at odds with the conventional theory of solids and recognized as due to magnetic and electron-lattice interactions in the layers which must be overcome at a substantial energy cost if electrons are to be transferred between layers. The associated energy gap or 'pseudogap' is particularly obvious in experiments where charge is moved perpendicular to the planes, most notably scanning tunneling microscopy (STM) and polarized infrared spectroscopy. Here, using the same experimental tools, we show that there is a second family of TMOs - the layered manganites La2-2xSr1+2xMn2O7 (LSMO) - with even more extreme confinement and pseudogap effects. The data, which are the first to resolve atoms in any metallic manganite, demonstrate quantitatively that because they are attached to polarons - lattice and spin textures within the planes -, it is equally difficult to remove carriers from the planes via vacuum tunneling into a conventional metallic tip, as it is for them to move between Mn-rich layers within the material itself

Incidence of influenza A(H3N2) virus infections in Hong Kong in a longitudinal sero-epidemiological study, 2009-2015

BACKGROUND: Many serologic studies were done during and after the 2009 influenza pandemic, to estimate the cumulative incidence of influenza A(H1N1)pdm09 virus infections, but there are few comparative estimates of the incidence of influenza A(H3N2) virus infections during epidemics. METHODS: We conducted a longitudinal serologic study in Hong Kong. We collected sera annually and tested samples from 2009-13 by HAI against the A/Perth/16/2009(H3N2) virus, and samples from 2013-15 against the A/Victoria/361/2011(H3N2) virus using the hemagglutination inhibition (HAI) assay. We estimated the cumulative incidence of infections based on 4-fold or greater rises in HAI titers in consecutive sera. RESULTS: There were four major H3N2 epidemics: (1) Aug-Oct 2010; (2) Mar-Jun 2012; (3) Jul-Oct 2013; and (4) Jun-Jul 2014. Between 516 and 619 relevant pairs of sera were available for each epidemic. We estimated that 9%, 19%, 7% and 7% of the population were infected in each epidemic, respectively, with higher incidence in children in epidemics 1 and 4. CONCLUSIONS: We found that re-infections in each of the four H3N2 epidemics that occurred from 2010 through 2014 were rare. The largest H3N2 epidemic occurred with the lowest level of pre-epidemic immunity

Nuclear spin relaxation of Li-8 in a thin film of La0.67Ca0.33MnO3

We report beta-NMR measurements of the nuclear spin relaxation rate (1/T-1) in a thin film of La0.67Ca0.33MnO3 (LCMO) using a low-energy beam of spin-polarized Li-8. In a small magnetic field of 150 G, there is a broad peak in 1/T-1 near the Curie temperature (T-c = 259 K) and a dramatic decrease in 1/T-1 at lower temperatures. This is attributed to a critical slowing down of the spin fluctuations near T-c and freezing of the magnetic excitations at low temperatures, respectively. In addition, there is a small amplitude, slow relaxing component at high temperatures, which we attribute to Li-8 in the SrTiO3 substrate. There is an indication that the spin relaxation rate in the substrate is also peaked at T-c due to close proximity to the magnetic film. These results establish that low-energy beta-NMR can be used as a probe of magnetic fluctuations in magnetic thin films over a wide range of temperatures. (c) 2005 Elsevier B.V. All rights reserved

Strong nonlinear focusing of light in nonlinearly controlled electromagnetic active metamaterial field concentrators

The idea of nonlinear 'transformation optics-inspired' [1–6] electromagnetic cylindrical field concentrators has been taken up in a preliminary manner in a number of conference reports [7–9]. Such a concentrator includes both external linear region with a dielectric constant increased towards the centre and internal region with nonlinearity characterized by constant coefficients. Then, in the process of farther investigations we realized the following factors considered neither in [7–9] nor in the recent paper [10]: saturation of nonlinearity, nonlinear losses, linear gain, numerical convergence, when nonlinear effect becomes very strong and formation of 'hotspots' starts. It is clearly demonstrated here that such a strongly nonlinear process starts when the nonlinear amplitude of any incident beam(s) exceeds some 'threshold' value. Moreover, it is shown that the formation of hotspots may start as the result of any of the following processes: an increase of the input amplitude, increasing the linear amplification in the central nonlinear region, decreasing the nonlinear losses, a decrease in the saturation of the nonlinearity. Therefore, a tendency to a formation of 'hotspots' is a rather universal feature of the strongly nonlinear behaviour of the 'nonlinear resonator' system, while at the same time the system is not sensitive to the 'prehistory' of approaching nonlinear threshold intensity (amplitude). The new proposed method includes a full-wave nonlinear solution analysis (in the nonlinear region), a new form of complex geometric optics (in the linear inhomogeneous external cylinder), and new boundary conditions, matching both solutions. The observed nonlinear phenomena will have a positive impact upon socially and environmentally important devices of the future. Although a graded-index concentrator is used here, it is a direct outcome of transformation optics. Numerical evaluations show that for known materials these nonlinear effects could be readily achieved

Tunable Klein-like tunnelling of high-temperature superconducting pairs into graphene

El texto completo de este trabajo no se encuentra disponible por no haber sido facilitado aún por su autor, por restricciones de copyright, o por no existir una versión digitalSuperconductivity can be induced in a normal material via the ‘leakage’ of superconducting pairs of charge carriers from an adjacent superconductor. This so-called proximity e ect is markedly influenced by graphene’s unique electronic structure, both in fundamental and technologically relevant ways. These include an unconventional form1,2 of the ‘leakage’ mechanism— the Andreev reflection3—and the potential of supercurrent modulation through electrical gating4 . Despite the interest of high-temperature superconductors in that context5,6 , realizations have been exclusively based on low-temperature ones. Here we demonstrate a gate-tunable, high-temperature superconducting proximity e ect in graphene. Notably, gating e ects result from the perfect transmission of superconducting pairs across an energy barrier—a form of Klein tunnelling7,8 , up to now observed only for non-superconducting carriers9,10— and quantum interferences controlled by graphene doping. Interestingly, we find that this type of interference becomes dominant without the need of ultraclean graphene, in stark contrast to the case of low-temperature superconductors11. These results pave the way to a new class of tunable, high-temperature Josephson devices based on large-scale graphene.Agence Nationale de la Recherche (ANR)EU Work ProgrammeEuropean Union (EU) Marie Curie ActionsUK Research & Innovation (UKRI)Engineering & Physical Sciences Research Council (EPSRC)Institut Universitaire de FranceDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu