Measuring Group Velocity in Seismic Noise Correlation Studies Based on Phase Coherence and Resampling Strategies

Seismic noise cross correlation studies are of increasing importance in the seismological research community due to the ubiquity of noise sources and advances on how to use the seismic noise wave field for structural imaging and monitoring purposes. Stacks of noise cross correlations are now routinely used to extract empirical Green's functions between station pairs. In regional and global scale studies, mostly surface waves are extracted due to their dominance in seismic noise wave fields. Group arrival times measured from the time-frequency representation of frequency dispersive surface waves are further used in tomographic inversions to image seismic structure. Often, the group arrivals are not clearly identified or ambiguous depending on the signal and noise characteristics. Here, we present a procedure to robustly measure group velocities using the time-frequency domain phase-weighted stack (PWS) combined with data resampling and decision strategies. The time-frequency PWS improves signal extraction through incoherent signal attenuation during the stack of the noise cross correlations. Resampling strategies help to identify signals robust against data variations and to assess their errors. We have gathered these ingredients in an algorithm where the decision strategies and tuning parameters are reduced for semiautomated processing schemes. Our numerical and field data examples show a robust assignment of surface-wave group arrivals. The method is computational efficient thanks to an implementation based on pseudoanalytic frames of wavelets and enables processing large amounts of data.This work was supported in part by the Project MISTERIOS under Grant CGL2013-48601-C2-1-R, in part by the MIMOSA under Grant ANR-14-CE01-0012, in part by the COST Action ES1401 TIDES, in part by AGAUR, and in part by the FP7 Marie Curie Project through SV's Beatriu de Pinos Fellowship under Contract 600385. This is IPGP contribution 3814.Peer reviewe

Detection of microseismic compressional (P) body waves aided by numerical modeling of oceanic noise sources

Among the different types of waves embedded in seismic noise, body waves present appealing properties but are still challenging to extract. Here we first validate recent improvements in numerical modeling of microseismic compressional (P) body waves and then show how this tool allows fast detection and location of their sources. We compute sources at ~0.2 Hz within typical P teleseismic distances (30-90°) from the Southern California Seismic Network and analyze the most significant discrete sources. The locations and relative strengths of the computed sources are validated by the good agreement with beam-forming analysis. These 54 noise sources exhibit a highly heterogeneous distribution, and cluster along the usual storm tracks in the Pacific and Atlantic oceans. They are mostly induced in the open ocean, at or near water depths of 2800 and 5600 km, most likely within storms or where ocean waves propagating as swell meet another swell or wind sea. We then emphasize two particularly strong storms to describe how they generate noise sources in their wake. We also use these two specific noise bursts to illustrate the differences between microseismic body and surface waves in terms of source distribution and resulting recordable ground motion. The different patterns between body and surface waves result from distinctive amplification of ocean wave-induced pressure perturbation and different seismic attenuation. Our study demonstrates the potential of numerical modeling to provide fast and accurate constraints on where and when to expect microseismic body waves, with implications for seismic imaging and climate studies. © 2013. American Geophysical Union. All Rights Reserved.This work was supported by the European Research Council (IOWAGA project), the Program >Investment for the future” Labex Mer (grant ANR-10-LABX-19-01), and the Consolider-Ingeno (Topo-Iberia). M.O. performed the data analysis while visiting the Domaines Océanique laboratorPeer Reviewe

Bipolar polaron pair recombination in P3HT/PCBM solar cells

The unique properties of organic semiconductors make them versatile base materials for many applications ranging from light emitting diodes to transistors. The low spin-orbit coupling typical for carbon-based materials and the resulting long spin lifetimes give rise to a large influence of the electron spin on charge transport which can be exploited in spintronic devices or to improve solar cell efficiencies. Magnetic resonance techniques are particularly helpful to elucidate the microscopic structure of paramagnetic states in semiconductors as well as the transport processes they are involved in. However, in organic devices the nature of the dominant spin-dependent processes is still subject to considerable debate. Using multi-frequency pulsed electrically detected magnetic resonance (pEDMR), we show that the spin-dependent response of P3HT/PCBM solar cells at low temperatures is governed by bipolar polaron pair recombination involving the positive and negative polarons in P3HT and PCBM, respectively, thus excluding a unipolar bipolaron formation as the main contribution to the spin-dependent charge transfer in this temperature regime. Moreover the polaron-polaron coupling strength and the recombination times of polaron pairs with parallel and antiparallel spins are determined. Our results demonstrate that the pEDMR pulse sequences recently developed for inorganic semiconductor devices can very successfully be transferred to the study of spin and charge transport in organic semiconductors, in particular when the different polarons can be distinguished spectrally

Lock-in detection for pulsed electrically detected magnetic resonance

We show that in pulsed electrically detected magnetic resonance (pEDMR) signal modulation in combination with a lock-in detection scheme can reduce the low-frequency noise level by one order of magnitude and in addition removes the microwave-induced non-resonant background. This is exemplarily demonstrated for spin-echo measurements in phosphorus-doped Silicon. The modulation of the signal is achieved by cycling the phase of the projection pulse used in pEDMR for the read-out of the spin state.Comment: 4 pages, 2 figure

Repeated cocaine exposure decreases dopamine D2-like receptor modulation of Ca2+ homeostasis in rat nucleus accumbens neurons

The nucleus accumbens is a limbic structure in the forebrain which plays a critical role in cognitive function and addiction.  Dopamine modulates activity of medium spiny neurons in the nucleus accumbens.  Both dopamine D1-like and D2-like receptors (including D1R or D1,5R, and D2R or D2,3,4R, respectively) are thought to play critical roles in cocaine addiction.  Our previous studies demonstrated that repeated cocaine exposure (which alters dopamine transmission) decreases excitability of nucleus accumbens medium spiny neurons in cocaine-sensitized, withdrawn rats.  This decrease is characterized by a reduction in voltage-sensitive Na+ currents and high voltage-activated Ca2+ currents, along with increased voltage-gated K+ currents.  These changes are associated with enhanced activity in the D1R/cAMP/PKA/protein phosphatase 1 pathway and diminished calcineurin function.  Though D1R-mediated signaling is enhanced by repeated cocaine exposure, little is known whether and how the D2R is implicated in the cocaine-induced nucleus accumbens dysfunction.  Here, we performed a combined electrophysiological, biochemical, and neuroimaging study that reveals the cocaine-induced dysregulation of Ca2+ homeostasis with involvement of D2R.  Our novel findings reveal that D2R stimulation reduced Ca2+ influx preferentially via the L-type Ca2+ channels and evoked intracellular Ca2+ release, likely via inhibiting the cAMP/PKA cascade, in the nucleus accumbens medium spiny neurons of drug-free rats.  However, repeated cocaine exposure abolished the D2R effects on modulating Ca2+ homeostasis with enhanced PKA activity and led to a decrease in whole-cell Ca2+ influx.  These adaptations, which persisted for 21 days during cocaine abstinence, may contribute to the mechanism of cocaine withdrawal.Fil: Perez, Mariela Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Farmacología Experimental de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Farmacología Experimental de Córdoba; ArgentinaFil: Ford, Kerstin A.. Rosalind Franklin University of Medicine and Science; Estados UnidosFil: Goussakov, Ivan. Rosalind Franklin University of Medicine and Science; Estados UnidosFil: Stutzmann, Grace E.. Rosalind Franklin University of Medicine and Science; Estados UnidosFil: Hu, Xiu-Ti. Rush University Medical Center; Estados Unido

Optical properties of Mn-doped GaN

Molecular beam epitaxy-grown GaN with different Mn concentrations (5-23×1019 cm-3) and codoped with Si were investigated by cathodoluminescence (CL) spectroscopy and optical transmission measurements. In the GaN:Mn, an intense absorption peak at 1.414 +/- 0.002 eV was observed. This peak was attributed to an internal 5T 2→ 5E transition of the deep neutral Mn3+ state since its intensity scaled with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm -3 had three effects on the emission spectrum: (i) the donor bound exciton at 3.460 eV was reduced by more than one order of magnitude, (ii) the donor-acceptor-pair band at 3.27 eV was completely quenched and (iii) the yellow luminescence centered at 2.2 eV was the strongly decreased. The latter two effects were attributed to a reduced concentration of VGa. In the infrared spectral range, three broad, Mn-doping related CL emission bands centered at 1.01 ± 0.02 eV, 1.09 ± 0.02 eV and 1.25 ± 0.03 eV were observed. These bands might be related to deep donor complexes, which are generated as a result of the heavy Mn-doping, rather than internal transitions at the Mn atom

MPGDs in Compton imaging with liquid-xenon

The interaction of radiation with liquid xenon, inducing both scintillation and ionization signals, is of particular interest for Compton-sequences reconstruction. We report on the development and recent results of a liquid-xenon time-projection chamber, dedicated to a novel nuclear imaging technique named "3 gamma imaging". In a first prototype, the scintillation is detected by a vacuum photomultiplier tube and the charges are collected with a MICROMEGAS structure; both are fully immersed in liquid xenon. In view of the final large-area detector, and with the aim of minimizing dead-zones, we are investigating a gaseous photomultiplier for recording the UV scintillation photons. The prototype concept is presented as well as preliminary results in liquid xenon. We also present soft x-rays test results of a gaseous photomultiplier prototype made of a double Thick Gaseous Electron Multiplier (THGEM) at normal temperature and pressure conditions.Comment: presented at MPGD09, CRETE, June 2009; to be published in JINST Proceedings, PDF, 10 pages, 11 figure