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Item Evaluation of electropolished stainless steel electrodes for use in DC high voltage photoelectron guns(2015-01-01) BastaniNejad, Mahzad; Elmustafa, Abdelmageed A.; Forman, Eric; Das, Lopa; Kelley, MichaelDC high voltage photoelectron guns are used to produce polarized electron beams for accelerator-based nuclear and high-energy physics research. Low-level field emission (similar to nA) from the cathode electrode degrades the vacuum within the photogun and reduces the photoelectron yield of the delicate GaAs-based photocathode used to produce the electron beams. High-level field emission (> mu A) can cause significant damage the photogun. To minimize field emission, stainless steel electrodes are typically diamond-paste polished, a labor-intensive process often yielding field emission performance with a high degree of variability, sample to sample. As an alternative approach and as comparative study, the performance of electrodes electropolished by conventional commercially available methods is presented. Our observations indicate the electropolished electrodes exhibited less field emission upon the initial application of high voltage, but showed less improvement with gas conditioning compared to the diamond-paste polished electrodes. In contrast, the diamond-paste polished electrodes responded favorably to gas conditioning, and ultimately reached higher voltages and field strengths without field emission, compared to electrodes that were only electropolished. The best performing electrode was one that was both diamond-paste polished and electropolished, reaching a field strength of 18.7 MV/m while generating less than 100 pA of field emission. The authors speculate that the combined processes were the most effective at reducing both large and small scale topography. However, surface science evaluation indicates topography cannot be the only relevant parameter when it comes to predicting field emission performance. (C) 2015 American Vacuum Society.Item A Framework for Inferring Unobserved Multistrain Epidemic Subpopulations Using Synchronization Dynamics(2015-01-01) Forgoston, Eric; Shaw, Leah B.; Schwartz, Ira B.A new method is proposed to infer unobserved epidemic subpopulations by exploiting the synchronization properties of multistrain epidemic models. A model for dengue fever is driven by simulated data from secondary infective populations. Primary infective populations in the driven system synchronize to the correct values from the driver system. Most hospital cases of dengue are secondary infections, so this method provides a way to deduce unobserved primary infection levels. We derive center manifold equations that relate the driven system to the driver system and thus motivate the use of synchronization to predict unobserved primary infectives. Synchronization stability between primary and secondary infections is demonstrated through numerical measurements of conditional Lyapunov exponents and through time series simulations.Item Synaptic Depression Influences Inspiratory-Expiratory Phase Transition in Dbx1 Interneurons of the preBotzinger Complex in Neonatal Mice(2015-01-01) Kottick, Andrew; Del Negro, Christopher A.The brainstem preBotzinger complex (preBotC) generates the rhythm underlying inspiratory breathing movements and its core interneurons are derived from Dbx1-expressing precursors. Recurrent synaptic excitation is required to initiate inspiratory bursts, but whether excitatory synaptic mechanisms also contribute to inspiratory-expiratory phase transition is unknown. Here, we examined the role of short-term synaptic depression using a rhythmically active neonatal mouse brainstem slice preparation. We show that afferent axonal projections to Dbx1 preBotC neurons undergo activity-dependent depression and we identify a refractory period (similar to 2 s) after endogenous inspiratory bursts that precludes light-evoked bursts in channelrhodopsin-expressing Dbx1 preBotC neurons. We demonstrate that the duration of the refractory period-but neither the cycle period nor the magnitude of endogenous inspiratory bursts-is sensitive to changes in extracellular Ca2+. Further, we show that postsynaptic factors are unlikely to explain the refractory period or its modulation by Ca2+. Our findings are consistent with the hypothesis that short-term synaptic depression in Dbx1 preBotC neurons influences the inspiratory-expiratory phase transition during respiratory rhythmogenesis.Item Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes(2015-01-01) Liao, Yunlong; Lin, Yi; Tu, Kaixiong; Lin, YiLateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole-and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics.Item Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes(2015-01-01) Ma, X.; Fang, F.; Luepke, G.; Li, Q.; Zhu, J.; Yang, Y.Optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.Item Nonlinear magneto-plasmonics(2015-01-01) Zheng, Wei; Liu, Xiao; Luepke, Gunter; Hanbicki, Aubrey T.; Jonker, Berend T.Nonlinear magneto-plasmonics (NMP) describes systems where nonlinear optics, magnetics and plasmonics are all involved. In such systems, nonlinear magneto-optical Kerr effect (nonlinear MOKE) plays an important role as a characterization method, and Surface Plasmons (SPs) work as catalyst to induce many new effects. Magnetization-induced second-harmonic generation (MSHG) is the major nonlinear magneto-optical process involved. The new effects include enhanced MSHG, controlled and enhanced magnetic contrast, etc. Nanostructures such as thin films, nanoparticles, nanogratings, and nanoarrays are critical for the excitation of SPs, which makes NMP an interdisciplinary research field in nanoscience and nanotechnology. In this review article, we organize recent work in this field into two categories: surface plasmon polaritons (SPPs) representing propagating surface plasmons, and localized surface plasmons (LSPs), also called particle plasmons. We review the structures, experiments, findings, and the applications of NMP from various groups. (C) 2015 Optical Society of AmericaItem Oscillation Regularity in Noise-Driven Excitable Systems with Multi-Time-Scale Adaptation(2008-08-22) Nesse, William H; Del Negro, Christopher A; Bressloff, Paul C; University of UtahWe investigate oscillation regularity of a noise-driven system modeled with a slow after-hyperpolarizing adaptation current (AHP) composed of multiple-exponential relaxation time scales. Sufficiently separated slow and fast AHP time scales (biphasic decay) cause a peak in oscillation irregularity for intermediate input currents I, with relatively regular oscillations for small and large currents. An analytic formulation of the system as a stochastic escape problem establishes that the phenomena is distinct from standard forms of coherence resonance. Our results explain data on the oscillation regularity of the pre-Bötzinger complex, a neural oscillator responsible for inspiratory breathing rhythm generation in mammals.Item Outward currents contributing to inspiratory burst termination in preBotzinger Complex neurons of neonatal mice studied in vitro(2010-11-29) Kray, Rebecca; Goodreau, Adam M; Arnold, Thomas B; Del Negro, Christopher AWe studied pre-Botzinger(preBotC) inspiratory interneurons to determine the cellular mechanisms that influence burst termination in a mammalian central pattern generator. Neonatal mouse slice preparations that retain pre-BoTC neurons generate respiratory motor rhythms in vitro. Inspiratory-related bursts rely on inward currents that flux Na+, thus outward currents coupled to Na+ accumulation are logical candidates for assisting in, or causing, burst termination. We examined Na+/K+/ATPase electrogenic pump current (Ipump), Na+-dependent K+ current, and ATP-dependent K+ current (IK-ATP). The pharmacological blockade of IPUMP, IK-Na, or IK-ATP caused pathological depolarization akin to a burst that cannot terminate, which impeded respiratory rhythm generation and reversibly stopped motor output. By simulating inspiratory bursts with current-step commands in synaptically isolated preBoTC neurons, we determined that each current generates approximately 3–8 mV of transient post-burst hyperpolarization that decays in 50–1600 ms. Ipump, IK–Na, and – to a lesser extent – IK–ATP contribute to terminating inspiratory bursts in the context of respiratory rhythm generation by responding to activity dependent cues such as Na+ accumulation.Item Dendritic A-Current in Rhythmically Active PreBo¨tzinger Complex Neurons in Organotypic Cultures from Newborn Mice(2018-03-21) Phillips, Wiktor S; Del Negro, Christopher A; Rekling, Jens K; University of CopenhagenBrainstem neurons in the preBo¨tC generate the oscillatory activity that underlies breathing. PreBo¨tC neurons express voltagedependent currents that can influence inspiratory activity, among which is a transient potassium current (IA) previously identified in a rhythmogenic excitatory subset of type-1 preBo¨tC neurons. We sought to determine whether IA is expressed in the dendrites of preBo¨tC. We found that dendrites of type-1 preBo¨tC neurons indeed express IA, which may aid in shunting sparse non-summating synaptic inputs, while enabling strong summating excitatory inputs to readily pass and thus influence somatic membrane potential trajectory. The subcellular distribution of IA in rhythmically active neurons of the preBo¨tC may thus be critical for producing well coordinated ensemble activity during inspiratory burst formation.Item Dbx1 Pre-Bötzinger Complex Interneurons Comprise the Core Inspiratory Oscillator for Breathing in Unanesthetized Adult Mice(2018-05-11) Del Negro, Christopher A; Pham, FrancisThe brainstem pre-Bötzinger complex (preBötC) generates inspiratory breathing rhythms, but which neurons comprise its rhythmogenic core? Dbx1-derived neurons may play the preeminent role in rhythm generation, an idea well founded at perinatal stages of development but incompletely evaluated in adulthood. We expressed archaerhodopsin or channelrhodopsin in Dbx1 preBötC neurons in intact adult mice to interrogate their function. Prolonged photoinhibition slowed down or stopped breathing, whereas prolonged photostimulation sped up breathing. Brief inspiratory-phase photoinhibition evoked the next breath earlier than expected, whereas brief expiratory-phase photoinhibition delayed the subsequent breath. Conversely, brief inspiratory-phase photostimulation increased inspiratory duration and delayed the subsequent breath, whereas brief expiratory-phase photostimulation evoked the next breath earlier than expected. Because they govern the frequency and precise timing of breaths in awake adult mice with sensorimotor feedback intact, Dbx1 preBötC neurons constitute an essential core component of the inspiratory oscillator, knowledge directly relevant to human health and physiology.Item Climate change as migration driver from rural and urban Mexico(IOP Publishing, 2015-11-23) Nawrotzki, Raphael J.; Hunter, Lori M.; Runfola, Daniel; Riosmena, FernandoStudies investigating migration as a response to climate variability have largely focused on rural locations to the exclusion of urban areas. This lack of urban focus is unfortunate given the sheer numbers of urban residents and continuing high levels of urbanization. To begin filling this empirical gap, this study investigates climate change impacts on US-bound migration from rural and urban Mexico, 1986–1999. We employ geostatistical interpolation methods to construct two climate change indices, capturing warm and wet spell duration, based on daily temperature and precipitation readings for 214 weather stations across Mexico. In combination with detailed migration histories obtained from the Mexican Migration Project, we model the influence of climate change on household-level migration from 68 rural and 49 urban municipalities. Results from multilevel event-history models reveal that a temperature warming and excessive precipitation significantly increased international migration during the study period. However, climate change impacts on international migration is only observed for rural areas. Interactions reveal a causal pathway in which temperature (but not precipitation) influences migration patterns through employment in the agricultural sector. As such, climate-related international migration may decline with continued urbanization and the resulting reductions in direct dependence of households on rural agriculture.Item Undocumented migration in response to climate change(ACCSCIENCE Publishing, 2015-01-01) Nawrotzki, Raphael J.; Riosmena, Fernando; Hunter, Lori M.; Runfola, DanielIn the face of climate change-induced economic uncertainties, households may em-ploy migration as an adaptation strategy to diversify their livelihood portfolio through remit-tances. However, it is unclear whether such climate-related migration will be documented or undocumented. In this study we combined detailed migration histories with daily temperature and precipitation information from 214 weather stations to investigate whether climate change more strongly impacted undocumented or documented migrations from 68 rural Mexican mu-nicipalities to the U.S. from 1986−1999. We employed two measures of climate change, the warm spell duration index (WSDI) and precipitation during extremely wet days (R99PTOT). Results from multi-level event-history models demonstrated that climate-related international migration from rural Mexico was predominantly undocumented. We conclude that programs to facilitate climate change adaptations in rural Mexico may be more effective in reducing undo-cumented border crossings than increasing border fortification.Item Organotypic slice cultures containing the preBotzinger complex generate respiratory-like rhythms(2016-02-19) Phillips, Wiktor S.; Herly, Mikkel; Rekling, Jens C.; Del Negro, Christopher A.Study of acute brain stem slice preparations in vitro has advanced our understanding of the cellular and synaptic mechanisms of respiratory rhythm generation, but their inherent limitations preclude long-term manipulation and recording experiments. In the current study, we have developed an organotypic slice culture preparation containing the preBotzinger complex (preBotC), the core inspiratory rhythm generator of the ventrolateral brain stem. We measured bilateral synchronous network oscillations, using calcium-sensitive fluorescent dyes, in both ventrolateral (presumably the preBotC) and dorsomedial regions of slice cultures at 7-43 days in vitro. These calcium oscillations appear to be driven by periodic bursts of inspiratory neuronal activity, because whole cell recordings from ventrolateral neurons in culture revealed inspiratory-like drive potentials, and no oscillatory activity was detected from glial fibrillary associated protein-expressing astrocytes in cultures. Acute slices showed a burst frequency of 10.9 +/- 4.2 bursts/min, which was not different from that of brain stem slice cultures (13.7 +/- 10.6 bursts/min). However, slice cocultures that include two cerebellar explants placed along the dorsolateral border of the brainstem displayed up to 193% faster burst frequency (22.4 +/- 8.3 bursts/min) and higher signal amplitude (340%) compared with acute slices. We conclude that preBotC-containing slice cultures retain inspiratory-like rhythmic function and therefore may facilitate lines of experimentation that involve extended incubation (e.g., genetic transfection or chronic drug exposure) while simultaneously being amenable to imaging and electrophysiology at cellular, synaptic, and network levels.Item The Influence of Internal Migration on Exposure to Extreme Weather Events in Mexico(2015-11-01) Runfola, Daniel; Romero-Lankao, Patricia; Jiang, LeiwenBetween 2005 and 2010, 6.3 million migrants (approximately 6% of the population) moved domestically within Mexico. These shifts have potential implications for exposure to natural disasters. To examine this relationship, we use census microdata in conjunction with information on natural disaster events. The populations exposed to extreme weather events are first calculated based on observed disasters and demographic change between 2005 and 2010. This is compared to a hypothetical scenario with no migration between 2005 and 2010. The results presented in this research note demonstrate that while migration has slightly decreased overall exposure within Mexico, this influence is highly localized in select areas, with internal migration increasing exposure in key urban destinations. This highlights the need to better understand the interacting roles of household-scale migratory decision making and economic/urban growth policy in climate change mitigation, and provides guidance on geographic regions to target for more detailed analysis.Item Solid-state electric double layer capacitors for ac line-filtering(2016-01-01) Gao, Han; Li, Jak; Lian, Keryn; Outlaw, Ronald A.Ultra-fast solid electric double layer capacitors (EDLCs) have been developed in both sandwich and planar interdigitated configurations using vertically-oriented graphene nanosheet (VOGN) electrodes with a hydroxide ion-conducting tetraethylammonium hydroxide (TEAOH)-polyvinyl alcohol (PVA) polymer electrolyte. These solid-state EDLCs could be scanned at a rate of 1000 Vs(-1) in cyclic voltammetry and demonstrated response times of less than 1 ms. They retained high performance over 18 months of shelf storage and after 100,000 charge/discharge cycles with limited packaging, demonstrating the high stability of TEAOH-PVA electrolyte. The solid-state capacitors are capable of performing at elevated temperatures and have demonstrated a response time of 0.35 ms at 90 degrees C. Given their ultra-fast rate capability, excellent shelf-life and cycle life, and excellent temperature stability, these solid-state EDLCs are promising smaller and lighter alternatives to the bulky electrolytic capacitors now used for ac line-filtering. (C) 2016 Elsevier B.V. All rights reserved.Item Buffer- and Diffusion-Mediated Calcium Concentration Fluctuations Accelerate the Stochastic Dynamics of Calcium-Triggered Events(2014-01-01) Weinberg, Seth H.; Smith, Gregory D.Item The TRPM4 channel inhibitor 9-phenanthrol(2014-01-01) Guinamard, R.; Hof, T.; Del Negro, C. A.The phenanthrene-derivative 9-phenanthrol is a recently identified inhibitor of the transient receptor potential melastatin (TRPM) 4 channel, a Ca2+-activated non-selective cation channel whose mechanism of action remains to be determined. Subsequent studies performed on other ion channels confirm the specificity of the drug for TRPM4. In addition, 9-phenanthrol modulates a variety of physiological processes through TRPM4 current inhibition and thus exerts beneficial effects in several pathological conditions. 9-Phenanthrol modulates smooth muscle contraction in bladder and cerebral arteries, affects spontaneous activity in neurons and in the heart, and reduces lipopolysaccharide-induced cell death. Among promising potential applications, 9-phenanthrol exerts cardioprotective effects against ischaemia-reperfusion injuries and reduces ischaemic stroke injuries. In addition to reviewing the biophysical effects of 9-phenanthrol, here we present information about its appropriate use in physiological studies and possible clinical applications.Item Atoh1-dependent rhombic lip neurons are required for temporal delay between independent respiratory oscillators in embryonic mice(2014-05-14) Tupal, Srinivasan; Ling, Guang-Yi; Gray, Paul A.; Picardo, Maria Cristina D.; Del Negro, Christopher A.All motor behaviors require precise temporal coordination of different muscle groups. Breathing, for example, involves the sequential activation of numerous muscles hypothesized to be driven by a primary respiratory oscillator, the preBotzinger Complex, and at least one other as-yet unidentified rhythmogenic population. We tested the roles of Atoh1-, Phox2b-, and Dbx1-derived neurons (three groups that have known roles in respiration) in the generation and coordination of respiratory output in embryonic mice. We found that Dbx1-derived neurons are necessary for all respiratory behaviors, whereas independent but coupled respiratory rhythms persist from at least three different motor pools after eliminating or silencing Phox2b-or Atoh1-expressing hindbrain neurons. Without Atoh1 neurons, however, the motor pools become temporally disorganized and coupling between independent respiratory oscillators decreases. We propose Atoh1 neurons tune the sequential activation of independent oscillators essential for the fine control of different muscles during breathing.Item The Influence of Ca2+ Buffers on Free [Ca2+] Fluctuations and the Effective Volume of Ca2+ Microdomains(2014-06-01) Weinberg, Seth H.; Smith, Gregory D.Intracellular calcium (Ca2+) plays a significant role in many cell signaling pathways, some of which are localized to spatially restricted microdomains. Ca2+ binding proteins (Ca2+ buffers) play an important role in regulating Ca2+ concentration ([Ca2+]). Buffers typically slow [Ca2+] temporal dynamics and increase the effective volume of Ca2+ domains. Because fluctuations in [Oa(2+)] decrease in proportion to the square-root of a domain's physical volume, one might conjecture that buffers decrease [Ca2+] fluctuations and, consequently, mitigate the significance of small domain volume concerning Ca2+ signaling. We test this hypothesis through mathematical and computational analysis of idealized buffer-containing domains and their stochastic dynamics during free Ca2+ influx with passive exchange of both Ca2+ and buffer with bulk concentrations. We derive Langevin equations for the fluctuating dynamics of Ca2+ and buffer and use these stochastic differential equations to determine the magnitude of [Ca2+] fluctuations for different buffer parameters (e.g., dissociation constant and concentration). In marked contrast to expectations based on a naive application of the principle of effective volume as employed in deterministic models of Ca2+ signaling, we find that mobile and rapid buffers typically increase the magnitude of domain [Ca2+] fluctuations during periods of Ca2+ influx, whereas stationary (immobile) Ca2+ buffers do not. Also contrary to expectations, we find that in the absence of Ca2+ influx, buffers influence the temporal characteristics, but not the magnitude, of [Ca2+] fluctuations. We derive an analytical formula describing the influence of rapid Ca2+ buffers on [Ca2+] fluctuations and, importantly, identify the stochastic analog of (deterministic) effective domain volume. Our results demonstrate that Ca2+ buffers alter the dynamics of [Ca2+] fluctuations in a nonintuitive manner. The finding that Ca2+ buffers do not suppress intrinsic domain [Ca2+] fluctuations raises the intriguing question of whether or not [Ca2+] fluctuations are a physiologically significant aspect of local Ca2+ signaling.Item The Contribution of Microbunching Instability to Solar Flare Emission in the GHz to THz Range of Frequencies(2014-01-01) Klopf, J. Michael; Kaufmann, Pierre; Raulin, Jean-Pierre; Szpigel, SergioRecent solar flare observations in the sub-terahertz range have provided evidence of a new spectral component with fluxes increasing for larger frequencies, separated from the well-known microwave emission that maximizes in the gigahertz range. Suggested interpretations explain the terahertz spectral component but do not account for the simultaneous microwave component. We present a mechanism for producing the observed "double spectra." Based on coherent enhancement of synchrotron emission at long wavelengths in laboratory accelerators, we consider how similar processes may occur within a solar flare. The instability known as microbunching arises from perturbations that produce electron beam density modulations, giving rise to broadband coherent synchrotron emission at wavelengths comparable to the characteristic size of the microbunch structure. The spectral intensity of this coherent synchrotron radiation (CSR) can far exceed that of the incoherent synchrotron radiation (ISR), which peaks at a higher frequency, thus producing a double-peaked spectrum. Successful CSR simulations are shown to fit actual burst spectral observations, using typical flaring physical parameters and power-law energy distributions for the accelerated electrons. The simulations consider an energy threshold below which microbunching is not possible because of Coulomb repulsion. Only a small fraction of the radiating charges accelerated to energies above the threshold is required to produce the microwave component observed for several events. The ISR/CSR mechanism can occur together with other emission processes producing the microwave component. It may bring an important contribution to microwaves, at least for certain events where physical conditions for the occurrence of the ISR/CSR microbunching mechanism are possible.