Showing papers in "Bulletin of the American Physical Society in 2013"

High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt

Abstract: Fuel cell catalysts synthesized from abundant metals approach the performance and durability of platinum at lower cost. The prohibitive cost of platinum for catalyzing the cathodic oxygen reduction reaction (ORR) has hampered the widespread use of polymer electrolyte fuel cells. We describe a family of non–precious metal catalysts that approach the performance of platinum-based systems at a cost sustainable for high-power fuel cell applications, possibly including automotive power. The approach uses polyaniline as a precursor to a carbon-nitrogen template for high-temperature synthesis of catalysts incorporating iron and cobalt. The most active materials in the group catalyze the ORR at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supported platinum, combining their high activity with remarkable performance stability for non–precious metal catalysts (700 hours at a fuel cell voltage of 0.4 volts) as well as excellent four-electron selectivity (hydrogen peroxide yield <1.0%).

Hydrogen Bonding in the Electronic Excited State

Abstract: Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated.Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtos...

Protein typing of circulating microvesicles allows real-time monitoring of glioblastoma therapy

Abstract: Glioblastomas shed large quantities of small, membrane-bound microvesicles into the circulation. Although these hold promise as potential biomarkers of therapeutic response, their identification and quantification remain challenging. Here, we describe a highly sensitive and rapid analytical technique for profiling circulating microvesicles directly from blood samples of patients with glioblastoma. Microvesicles, introduced onto a dedicated microfluidic chip, are labeled with target-specific magnetic nanoparticles and detected by a miniaturized nuclear magnetic resonance system. Compared with current methods, this integrated system has a much higher detection sensitivity and can differentiate glioblastoma multiforme (GBM) microvesicles from nontumor host cell–derived microvesicles. We also show that circulating GBM microvesicles can be used to analyze primary tumor mutations and as a predictive metric of treatment-induced changes. This platform could provide both an early indicator of drug efficacy and a potential molecular stratifier for human clinical trials.

Effect of Defects on the Intrinsic Strength and Stiffness of Graphene

Abstract: It is important from a fundamental standpoint and for practical applications to understand how the mechanical properties of graphene are influenced by defects. Here we report that the two-dimensional elastic modulus of graphene is maintained even at a high density of sp(3)-type defects. Moreover, the breaking strength of defective graphene is only ~14% smaller than its pristine counterpart in the sp(3)-defect regime. By contrast, we report a significant drop in the mechanical properties of graphene in the vacancy-defect regime. We also provide a mapping between the Raman spectra of defective graphene and its mechanical properties. This provides a simple, yet non-destructive methodology to identify graphene samples that are still mechanically functional. By establishing a relationship between the type and density of defects and the mechanical properties of graphene, this work provides important basic information for the rational design of composites and other systems utilizing the high modulus and strength of graphene.

Synthetic gauge fields in synthetic dimensions

Abstract: We describe a simple technique for generating a cold-atom lattice pierced by a uniform magnetic field. Our method is to extend a one-dimensional optical lattice into the \"dimension\" provided by the internal atomic degrees of freedom, yielding a synthetic two-dimensional lattice. Suitable laser coupling between these internal states leads to a uniform magnetic flux within the two-dimensional lattice. We show that this setup reproduces the main features of magnetic lattice systems, such as the fractal Hofstadter-butterfly spectrum and the chiral edge states of the associated Chern insulating phases.

Ultrafast Magnetization Enhancement in Metallic Multilayers Driven by Superdiffusive Spin Current

Abstract: Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro- or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.

Physical solutions to the public goods dilemma in bacterial biofilms

Abstract: Bacteria frequently live in densely populated surface-bound communities, termed biofilms [1-4]. Biofilm-dwelling cells rely on secretion of extracellular substances to construct their communities and to capture nutrients from the environment [5]. Some secreted factors behave as cooperative public goods: they can be exploited by nonproducing cells [6-11]. The means by which public-good-producing bacteria avert exploitation in biofilm environments are largely unknown. Using experiments with Vibrio cholerae, which secretes extracellular enzymes to digest its primary food source, the solid polymer chitin, we show that the public goods dilemma may be solved by two very different mechanisms: cells can produce thick biofilms that confine the goods to producers, or fluid flow can remove soluble products of chitin digestion, denying access to nonproducers. Both processes are unified by limiting the distance over which enzyme-secreting cells provide benefits to neighbors, resulting in preferential benefit to nearby clonemates and allowing kin selection to favor public good production. Our results demonstrate new mechanisms by which the physical conditions of natural habitats can interact with bacterial physiology to promote the evolution of cooperation.

Manifestation of topological protection in transport properties of epitaxial Bi$_2$Se$_3$ thin films

Abstract: The massless Dirac fermions residing on the surface of three-dimensional topological insulators are protected from backscattering and cannot be localized by disorder, but such protection can be lifted in ultrathin films when the three-dimensionality is lost. By measuring the Shubnikov-de Haas oscillations in a series of high-quality Bi2Se3 thin films, we revealed a systematic evolution of the surface conductance as a function of thickness and found a striking manifestation of the topological protection: The metallic surface transport abruptly diminishes below the critical thickness of ~6 nm, at which an energy gap opens in the surface state and the Dirac fermions become massive. At the same time, the weak antilocalization behavior is found to weaken in the gapped phase due to the loss of π Berry phase.

Induced Seismicity Potential of Energy Technologies

Abstract: Although the vast majority of earthquakes that occur in the world each year have natural causes, some of these earthquakes and a number of lesser magnitude seismic events are related to human activities and are called “induced seismic events” or “induced earthquakes.” Induced seismic activity has been documented since at least the 1920s and has been attributed to a range of human activities including the impoundment of large reservoirs behind dams, controlled explosions related to mining or construction, and underground nuclear tests. In addition, energy technologies that involve injection or withdrawal of fluids from the subsurface can also create induced seismic events that can be measured and felt. Historically known induced seismicity has generally been small in both magnitude and intensity of ground shaking. Recently, several induced seismic events related to energy technology development projects in the United States have drawn heightened public attention. Although none of these events resulted in loss of life or significant structural damage, their effects were felt by local residents, some of whom also experienced minor property damage. Particularly in areas where tectonic (natural) seismic activity is uncommon and energy development is ongoing, these induced seismic events, though small in scale, can be disturbing to the public and raise concern about increased seismic activity and its potential consequences. This report addresses induced seismicity that may be related to four energy development technologies that involve fluid injection or withdrawal: geothermal energy; conventional oil and gas development including enhanced oil recovery (EOR); shale gas recovery; and carbon capture and storage (CCS). These broad categories of energy technologies are divided into finer categories herein (including underground waste water disposal), to show details of induced seismic events as they relate to specific aspects of each energy technology. The study arose through a request by Senator Bingaman of New Mexico to Department of Energy (DOE) Secretary Stephen Chu. The DOE was asked to engage the National Research Council (NRC) to examine the scale, scope, and consequences of seismicity induced during the injection of fluids related to energy production; to identify gaps in knowledge and research needed to advance the understanding of induced seismicity; to identify gaps in induced seismic hazard assessment methodologies and the research needed to close those gaps; and to assess options for interim steps toward best practices with regard to energy development and induced seismicity potential. The report responds to this charge and aims to provide an understanding of the nature and scale of induced seismicity caused by or likely related to energy development and guidance as to how best to proceed with safe development of these technologies while minimizing their potential to induce earthquakes that can be felt by the public.

Universal current-velocity relation of skyrmion motion in chiral magnets

Abstract: Current-induced motion of skyrmions is attracting attention due to its low critical current density, however, its microscopic mechanisms have not been elucidated yet. Using numerical simulations, the authors demonstrate a universal current-velocity relation of skyrmion motion, independent of disorder or nonadiabatic effects.

The Use of Research-Based Instructional Strategies in Introductory Physics: Where do Faculty Leave the Innovation-Decision Process?

Abstract: Department of Statistics, Western Michigan University, Kalamazoo, Michigan 49008, USA(Received 20 February 2012; published 31 July 2012)During the fall of 2008 aweb survey, designed to collect information about pedagogical knowledge andpractices, was completed by a representative sample of 722 physics faculty across the United States(50.3% response rate). This paper presents partial results to describe how 20 potential predictor variablescorrelate with faculty knowledge about and use of research-based instructional strategies (RBIS). Theinnovation-decision process was conceived of in terms of four stages: knowledge versus no knowledge,trial versus no trial, continuationversus discontinuation, and high versus low use. The largest losses occurat the continuation stage, with approximately 1=3 offaculty discontinuing use of all RBIS after trying oneor more of these strategies. Nine of the predictor variables were statistically significant for at least one ofthese stages when controlling for other variables. Knowledge and/or use of RBIS are significantlycorrelated with reading teaching-related journals, attending talks and workshops related to teaching,attending the physics and astronomy new faculty workshop, having an interest in using more RBIS, beingfemale, being satisfied with meeting instructional goals, and having a permanent, full-time position. Thetypes of variables that are significant at each stage vary substantially. These results suggest that commondissemination strategies are good at creating knowledge about RBIS and motivation to try a RBIS, butmore work is needed to support faculty during implementation and continued use of RBIS. Also, contraryto common assumptions, faculty age, institutional type, and percentage of job related to teaching were notfoundtobebarrierstoknowledgeoruseatanystage.Highresearchproductivityandlargeclasssizeswerenot found to be barriers to use of at least some RBIS.

Universal Slow Growth of Entanglement in Interacting Strongly Disordered Systems

Abstract: Recent numerical work by Bardarson, Pollmann, and Moore revealed a slow, logarithmic in time, growth of the entanglement entropy for initial product states in a putative many-body localized phase. We show that this surprising phenomenon results from the dephasing due to exponentially small interaction-induced corrections to the eigenenergies of different states. For weak interactions, we find that the entanglement entropy grows as ξln(Vt/ℏ), where V is the interaction strength, and ξ is the single-particle localization length. The saturated value of the entanglement entropy at long times is determined by the participation ratios of the initial state over the eigenstates of the subsystem. Our work shows that the logarithmic entanglement growth is a universal phenomenon characteristic of the many-body localized phase in any number of spatial dimensions, and reveals a broad hierarchy of dephasing time scales present in such a phase.

Electron Ion Collider: The next QCD frontier

Abstract: Abstract.This White Paper presents the science case of an Electron-Ion Collider (EIC), focused on the structure and interactions of gluon-dominated matter, with the intent to articulate it to the broader nuclear science community. It was commissioned by the managements of Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility (JLab) with the objective of presenting a summary of scientific opportunities and goals of the EIC as a follow-up to the 2007 NSAC Long Range plan. This document is a culmination of a community-wide effort in nuclear science following a series of workshops on EIC physics over the past decades and, in particular, the focused ten-week program on “Gluons and quark sea at high energies” at the Institute for Nuclear Theory in Fall 2010. It contains a brief description of a few golden physics measurements along with accelerator and detector concepts required to achieve them. It has been benefited profoundly from inputs by the users’ communities of BNL and JLab. This White Paper offers the promise to propel the QCD science program in the US, established with the CEBAF accelerator at JLab and the RHIC collider at BNL, to the next QCD frontier.

Tuning the Electronic and Chemical Properties of Monolayer MoS$_2$ Adsorbed on Transition Metal Substrates

Abstract: Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS(2) adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS(2) overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS(2) layer. The enhanced binding of hydrogen, by as much as ~0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS(2) overlayer, and in part to a stronger MoS(2)-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS(2)-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

The space group classification of topological band insulators

Abstract: Topological band insulators (TBIs) are bulk insulating materials which feature topologically protected metallic states on their boundary. The existing classification departs from time-reversal symmetry, but the role of the crystal lattice symmetries in the physics of these topological states remained elusive. Here we provide the classification of TBIs protected not only by time-reversal, but also by crystalline symmetries. We find three broad classes of topological states: (a) Γ states robust against general time-reversal invariant perturbations; (b) Translationally-active states protected from elastic scattering, but susceptible to topological crystalline disorder; (c) Valley topological insulators sensitive to the effects of non-topological and crystalline disorder. These three classes give rise to 18 different two-dimensional, and, at least 70 three-dimensional TBIs, opening up a route for the systematic search for new types of TBIs.

Structure and Dynamics of a Phase-Separating Active Colloidal Fluid

Abstract: We examine a minimal model for an active colloidal fluid in the form of self-propelled Brownian spheres that interact purely through excluded volume with no aligning interaction. Using simulations and analytic modeling, we quantify the phase diagram and separation kinetics. We show that this nonequilibrium active system undergoes an analog of an equilibrium continuous phase transition, with a binodal curve beneath which the system separates into dense and dilute phases whose concentrations depend only on activity. The dense phase is a unique material that we call an active solid, which exhibits the structural signatures of a crystalline solid near the crystal-hexatic transition point, and anomalous dynamics including superdiffusive motion on intermediate time scales.