Surveying colloid sedimentation by coplanar waveguides.

By using coplanar waveguides, direct access to the dielectric properties of aqueous solutions of polystyrene beads with different diameters from 330 nm to 10 µm is provided. The relative variation of the transmission parameter with respect to water is monitored, ranging from [Formula: see text] obtained for a 9.5% solution with 330 nm diameter beads to ∼22% for 10 µm diameter particles at the same concentration. To highlight its applicability in biosensing, the technique was further employed to survey the clustering between biotin and streptavidin-coated beads. The transmission parameter displays a ∼50% increase for mixtures containing nine volumes of biotin and one volume of streptavidin-modified beads (4.5 ng µl(-1) of streptavidin) and reaches ∼400% higher values when equal volumes of biotin and streptavidin-coated beads (22.5 ng µl(-1) of streptavidin) were mixed.

Vertical single nanowire devices based on conducting polymers.

A simple scheme for single conducting polymer nanowire fabrication and device integration is presented. We discuss a combined top-down and bottom-up approach for the sequential, precise manufacture of vertical polyaniline nanowires. The method is scalable and can be applied on rigid as well as on flexible substrates. The kinetics of the template-confined growth is presented and discussed. We further study the electrical behavior of single vertical polyaniline nanowires and address the fabrication of crossbar latches using a criss-cross arrangement of electrodes. The as-synthesized polyaniline nanowires display electric conductivities reaching values as high as 0.4 S cm⁻¹.

Publisher Correction: Statistics of thermomagnetic breakdown in Nb superconducting films.

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

Statistics of thermomagnetic breakdown in Nb superconducting films.

Superconductors are well known for their ability to screen out magnetic fields. In type-II superconductors, as the magnetic field pressure is progressively increased, magnetic flux accumulates at the periphery of the sample, very much like charges accumulate in a capacitor when voltage is increased. As for capacitors, exceeding certain threshold field causes the blocked magnetic flux to abruptly penetrate into the sample. This phenomenon, triggered by a thermomagnetic instability, is somewhat analogous to the dielectric breakdown of the capacitor and leaves behind a similar Lichtenberg imprinting. Even though electrical breakdown threshold has been extensively studied in dielectrics, little information is known about the statistical distribution of the thermomagnetic breakdown in superconductors. In this work, we address this problem by performing magneto-optical imaging experiments on a Nb film where nanometric heating elements are used to rapidly erase the magnetic history of the sample. We demonstrate that the size and shape distributions of avalanches permits to unambiguously identify the transition between two regimes where either thermal diffusivity or magnetic diffusivity dominates. Clear criteria for discriminating athermal dynamic avalanches from thermally driven avalanches are introduced. This allows us to provide the first precise determination of the threshold field of the thermomagnetic breakdown and unveil the details of the transition from finger-like magnetic burst to dendritic branching morphology. These findings open a new avenue in the interdisciplinary exploration of catastrophic avalanches through non destructive repeatable experiments.

Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures.

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data are complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.

Carbon Redox-Polymer-Gel Hybrid Supercapacitors.

Energy storage devices that provide high specific power without compromising on specific energy are highly desirable for many electric-powered applications. Here, we demonstrate that polymer organic radical gel materials support fast bulk-redox charge storage, commensurate to surface double layer ion exchange at carbon electrodes. When integrated with a carbon-based electrical double layer capacitor, nearly ideal electrode properties such as high electrical and ionic conductivity, fast bulk redox and surface charge storage as well as excellent cycling stability are attained. Such hybrid carbon redox-polymer-gel electrodes support unprecedented discharge rate of 1,000C with 50% of the nominal capacity delivered in less than 2 seconds. Devices made with such electrodes hold the potential for battery-scale energy storage while attaining supercapacitor-like power performances.

Charge and spin transport in single and packed ruthenium-terpyridine molecular devices: Insight from first-principles calculations.

Using first-principles calculations, we study the electronic and transport properties of rutheniumterpyridine molecules sandwiched between two Au(111) electrodes. We analyse both single and packed molecular devices, more amenable to scaling and realistic integration approaches. The devices display all together robust negative differential resistance features at low bias voltages. Remarkably, the electrical control of the spin transport in the studied systems implies a subtle distribution of the magnetisation density within the biased devices and highlights the key role of the Au(111) electrical contacts.

Anomalous thermopower in the metalliclike phase of a 2D hole system

We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism.

NMR determination of 2D electron spin polarization at nu = 1/2

Using a "standard" NMR spin-echo technique we determined the spin polarization P of two-dimensional electrons, confined to GaAs quantum wells, from the hyperfine shift of Ga nuclei located in the wells. Concentrating on the temperature ( 0.05 less, similarT less, similar10 K) and magnetic field ( 7 less, similarB less, similar17 T) dependencies of P at Landau level filling factor nu = 1/2, we find that the results are described well by a simple model of noninteracting composite fermions, although some inconsistencies remain when the two-dimensional electron system is tilted in the magnetic field.

Hybrid supercapacitor-battery materials for fast electrochemical charge storage.

High energy and high power electrochemical energy storage devices rely on different fundamental working principles--bulk vs. surface ion diffusion and electron conduction. Meeting both characteristics within a single or a pair of materials has been under intense investigations yet, severely hindered by intrinsic materials limitations. Here, we provide a solution to this issue and present an approach to design high energy and high power battery electrodes by hybridizing a nitroxide-polymer redox supercapacitor (PTMA) with a Li-ion battery material (LiFePO4). The PTMA constituent dominates the hybrid battery charge process and postpones the LiFePO4 voltage rise by virtue of its ultra-fast electrochemical response and higher working potential. We detail on a unique sequential charging mechanism in the hybrid electrode: PTMA undergoes oxidation to form high-potential redox species, which subsequently relax and charge the LiFePO4 by an internal charge transfer process. A rate capability equivalent to full battery recharge in less than 5 minutes is demonstrated. As a result of hybrid's components synergy, enhanced power and energy density as well as superior cycling stability are obtained, otherwise difficult to achieve from separate constituents.

Thermopower of interacting GaAs bilayer hole systems in the reentrant insulating phase near nu=1.

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor nu=1. Our data show that, as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.

Spin polarization and g factor of a dilute GaAs two-dimensional electron system.

The effective g factor (g(*)) of a dilute interacting two-dimensional electron system is expected to increase with respect to its bare value as the density is lowered, and to eventually diverge as the system makes a transition to a ferromagnetic state. We report here measurements of g(*) in dilute (density 0.8 to 6.5x10(10) cm(-2)), high-mobility GaAs two-dimensional electrons from their spin polarization in a parallel magnetic field. The data reveal a surprising trend. While g(*) is indeed significantly enhanced with respect to the band g factor of GaAs, the enhancement factor decreases from about 6 to 3 as the density is reduced.

Ventricular refractory periods in relation to rate and test-site VT intervals in anesthetized and conscious dogs: a canine model for conscious state measurements.

The functional relationship of ventricular effective refractory period (ERP) with basic cycle length (BCL) of stimulated ventricular depolarization and VT intervals of test-site unipolar ventricular electrograms were studied in five conscious and eight anesthetized (sodium pentobarbital, 30 mg/kg) open-chested dogs. The range of BCLs studied was 300 to 1000 ms, achieved through ventricular stimulation following chemically-induced complete AV block. In the conscious animal model developed for this study, two-to-four ventricular electrode leads were exteriorized and the ventricular rate was maintained by an implanted programmable VVI pacemaker. In all animals studied, the BCL-ERP relation was closely represented by the empirical equation ERP = A-B.Exp(-k.BCL), and the VT-ERP relation over the same range of BCLs was linear: ERP = C + D.VT. The correlation coefficients were in the range of 0.991 to 0.999. The mean values of the parameters in the above equations determined by the appropriate non-linear or linear regression analysis showed significant differences between the two groups of animals studied. In three conscious animals a strong linear correlation between the test-site VT intervals and simultaneously measured QT intervals measured from the lead II surface electrogram was demonstrated (r = 0.993 to 0.998). For a fixed site of stimulation, the morphology of ventricular depolarization complexes as well as the corresponding T-waves remained essentially unaltered with BCL for both myocardial and surface electrograms. The possible applications of developed canine model and the results of the present study include: (1) the study of the rate-dependent effects of cardioactive drugs on ventricular electrophysiology and, (2) the improved design of electronic refractory periods of rate programmable pacemakers.

Role of density imbalance in an interacting bilayer hole system.

We study interacting GaAs hole bilayers in the limit of zero interlayer tunneling. When the layers have equal density, we observe a phase-coherent bilayer quantum Hall state (QHS) at a total filling factor nu=1, flanked by a reentrant insulating phase at nearby fillings which suggests the formation of a pinned, bilayer Wigner crystal. As we transfer charge from one layer to another, the phase-coherent QHS becomes stronger, evincing its robustness against charge imbalance, but the insulating phase disappears, suggesting that its stability requires the commensurability of the two layers.

Laterally modulated 2D electron system in the extreme quantum limit.

We report on magnetotransport of a two-dimensional electron system (2DES), located 32 nm below the surface, with a surface superlattice gate structure of periodicity 39 nm imposing a periodic modulation of its potential. For low Landau level fillings nu, the diagonal resistivity displays a rich pattern of fluctuations, even though the disorder dominates over the periodic modulation. Theoretical arguments based on the combined effects of the long-wavelength, strong disorder and the short-wavelength, weak periodic modulation present in the 2DES qualitatively explain the data.