Have We Crossed the Bridge Yet?

Last test week, David Remnick's book, The Bridge, was published. It is a book ostensibly about the life and rise of our President, Barack Obama. The Bridge is a reference to the bridge in Selma, Alabama where civil rights demonstrators were violently attacked by state troopers on March 7, 1965 on their way to Montgomery . . . Although deterred that day, the marchers, invoking the rule of law, eventually prevailed and were permitted to complete their historic march to Montgomery. That confrontation galvanized our nation and helped lead to the passage of the Voting Rights Act later that year. . . . During the festivities relating to President Obama's inauguration, when asked to comment on the historic significance of the impending inauguration, Congressman [John] Lewis commented that Barack Obama is what comes at the end of that Bridge in Selma. It is a powerful, yet intriguing, metaphor that is susceptible to many interpretations. Is having an African American President reaching the promised land? Is he what we, as African Americans, have hoped for in our quest for freedom? Is President Obama the proverbial pot of gold at the end of the rainbow

Resonant control of cold-atom transport through two optical lattices with a constant relative speed

We show theoretically that the dynamics of cold atoms in the lowest energy band of a stationary optical lattice can be transformed and controlled by a second, weaker, periodic potential moving at a constant speed along the axis of the stationary lattice. The atom trajectories exhibit complex behavior, which depends sensitively on the amplitude and speed of the propagating lattice. When the speed and amplitude of the moving potential are low, the atoms are dragged through the static lattice and perform drifting orbits with frequencies an order of magnitude higher than that corresponding to the moving potential. Increasing either the speed or amplitude of the moving lattice induces Bloch-like oscillations within the energy band of the static lattice, which exhibit complex resonances at critical values of the system parameters. In some cases, a very small change in these parameters can reverse the atom's direction of motion. In order to understand these dynamics we present an analytical model, which describes the key features of the atom transport and also accurately predicts the positions of the resonant features in the atom's phase space. The abrupt controllable transitions between dynamical regimes, and the associated set of resonances, provide a mechanism for transporting atoms between precise locations in a lattice: as required for using cold atoms to simulate condensed matter or as a stepping stone to quantum information processing. The system also provides a direct quantum simulator of acoustic waves propagating through semiconductor nanostructures in sound analogs of the optical laser (SASER)

Estimating oil concentration and flow rate with calibrated vessel-mounted acoustic echo sounders

As part of a larger program aimed at evaluating acoustic techniques for mapping the distribution of subsurface oil and gas associated with the Deepwater Horizon-Macondo oil spill, observations were made on June 24 and 25, 2010 using vessel-mounted calibrated single-beam echo sounders on the National Oceanic and Atmospheric Administration ship Thomas Jefferson. Coincident with visual observations of oil at the sea surface, the 200-kHz echo sounder showed anomalously high-volume scattering strength in the upper 200 m on the western side of the wellhead, more than 100 times higher than the surrounding waters at 1,800-m distance from the wellhead, and weakening with increasing distance out to 5,000 m. Similar high-volume scattering anomalies were not observed at 12 or 38 kHz, although observations of anomalously low-volume scattering strength were made in the deep scattering layer at these frequencies at approximately the same locations. Together with observations of ocean currents, the acoustic observations are consistent with a rising plume of small (\u3c 1-mm radius) oil droplets. Using simplistic but reasonable assumptions about the properties of the oil droplets, an estimate of the flow rate was made that is remarkably consistent with those made at the wellhead by other means. The uncertainty in this acoustically derived estimate is high due to lack of knowledge of the size distribution and rise speed of the oil droplets. If properly constrained, these types of acoustic measurements can be used to rapidly estimate the flow rate of oil reaching the surface over large temporal and spatial scales

Controlling high-frequency collective electron dynamics via single-particle complexity

We demonstrate, through experiment and theory, enhanced high-frequency current oscillations due to magnetically-induced conduction resonances in superlattices. Strong increase in the ac power originates from complex single-electron dynamics, characterized by abrupt resonant transitions between unbound and localized trajectories, which trigger and shape propagating charge domains. Our data demonstrate that external fields can tune the collective behavior of quantum particles by imprinting configurable patterns in the single-particle classical phase space.Comment: 5 pages, 4 figure

Photoresponsive type III porous liquids

Porous materials are the subject of extensive research because of potential applications in areas such as gas adsorption and molecular separations. Until recently, most porous materials were solids, but there is now an emerging class of materials known as porous liquids. The incorporation of intrinsic porosity or cavities in a liquid can result in free-flowing materials that are capable of gas uptakes that are significantly higher than conventional non-porous liquids. A handful of porous liquids have also been investigated for gas separations. Until now, the release of gas from porous liquids has relied on molecular displacement (e.g., by adding small solvent molecules), pressure or temperature swings, or sonication. Here, we explore a new method of gas release which involves photoisomerisable porous liquids comprising a photoresponsive MOF dispersed in an ionic liquid. This results in the selective uptake of CO2 over CH4 and allows gas release to be controlled by using UV light

The Universal Cloud and Aerosol Sounding System (UCASS): a low-cost miniature optical particle counter for use in dropsonde or balloon-borne sounding systems

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. An earlier version of this work was published in Atmospheric Measurement Techniques Discussions: https://dx.doi.org/10.5194/amt-2019-70.A low-cost miniaturized particle counter has been developed by The University of Hertfordshire (UH) for the measurement of aerosol and droplet concentrations and size distributions. The Universal Cloud and Aerosol Sounding System (UCASS) is an optical particle counter (OPC), which uses wide-angle elastic light scattering for the high-precision sizing of fluid-borne particulates. The UCASS has up to 16 configurable size bins, capable of sizing particles in the range 0.4–40 µm in diameter. Unlike traditional particle counters, the UCASS is an open-geometry system that relies on an external air flow. Therefore, the instrument is suited for use as part of a dropsonde, balloon-borne sounding system, as part of an unmanned aerial vehicle (UAV), or on any measurement platform with a known air flow. Data can be logged autonomously using an on-board SD card, or the device can be interfaced with commercially available meteorological sondes to transmit data in real time. The device has been deployed on various research platforms to take measurements of both droplets and dry aerosol particles. Comparative results with co-located instrumentation in both laboratory and field settings show good agreement for the sizing and counting ability of the UCASS.Peer reviewe

Understanding the Dynamics of Fluorescence Emission During Zeolite Detemplation Using Time Resolved Photoluminescence Spectroscopy

Time-resolved photoluminescence spectroscopy (TRPS) shows potential as a sensitive, non-destructive, high throughput, label-free laser-based spectroscopy technique capable of analysing low concentrations of organic species adsorbed on and within zeolite pores. Here we report the results from a study that uses TRPS to characterise photoluminescence (PL) arising from synthesised chabazite framework zeolites at three different stages of the detemplation process (from an uncalcined, partially calcined, and calcined zeolite). Temporal resolution was used to demonstrate the steric confinement effects of OSDA within a zeolite framework and therefore to establish a signature region for determining the presence of the template. Gated spectra comparisons between an uncalcined and a partially calcined zeolite demonstrated the presence of template alongside the proliferation of template-derived combustion products. An analysis of lifetime values demonstrated the ability for TRPS to track depletion of OSDA and establish a characteristic PL spectrum for a clean zeolite

Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators

We assess the potential of two-terminal graphene-hexagonal boron nitride-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable