X-ray diffraction studies of GaN p-i-n structures for high power electronics

We have investigated the influence of the ambient exposure and/or ICP etching on the structure and properties of GaN p-i-n structures for high power electronics. To quantify the concentration of various native and extrinsic point defects, we utilize a combination of ion beam analyses in conjunction with x-ray diffraction. The full width at half max (FWHM) of phi and omega scans were used to quantify the mosaicity and threading dislocation (TD) densities at the p-i interfaces. The lowest densities of c-type and highest densities a-type TD components are observed for the “in-situ” GaN structure, which also produces the highest interfacial donor-acceptor pair (DAP) cathodoluminescence (CL) emissions. Interestingly, elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy reveal the lowest interfacial [H] but the highest fraction of displaced Ga atoms, suggesting efficient incorporation of MgGa in the in-situ structure. On the other hand, for the ex-situ structures, minimal interfacial [H] is also observed, but the lowest interfacial NBE and DAP CL emission is apparent as well as the highest density of c-type TD components. The relationship between interfacial [H], displaced Ga, CL emission features, and c- and a-type dislocation densities will be discussed.http://deepblue.lib.umich.edu/bitstream/2027.42/169564/1/zimmerman-alex-capstone-report.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/169564/2/Zimmerman-Alex-Honors-Capstone-Poster.pd

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Structural Basis of Functional Transitions in Mammalian NMDA Receptors

Excitatory neurotransmission meditated by glutamate receptors including N-methyl-D-aspartate receptors (NMDARs) is pivotal to brain development and function. NMDARs are heterotetramers composed of GluN1 and GluN2 subunits, which bind glycine and glutamate, respectively, to activate their ion channels. Despite importance in brain physiology, the precise mechanisms by which activation and inhibition occur via subunit-specific binding of agonists and antagonists remain largely unknown. Here, we show the detailed patterns of conformational changes and inter-subunit and -domain reorientation leading to agonist-gating and subunit-dependent competitive inhibition by providing multiple structures in distinct ligand states at 4 Å or better. The structures reveal that activation and competitive inhibition by both GluN1 and GluN2 antagonists occur by controlling the tension of the linker between the ligand-binding domain and the transmembrane ion channel of the GluN2 subunit. Our results provide detailed mechanistic insights into NMDAR pharmacology, activation, and inhibition, which are fundamental to the brain physiology