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Recent documents in Physics Faculty Research & Creative Worksen-usThu, 19 Nov 2020 08:02:55 PST3600GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo during the First Half of the Third Observing Run
https://scholarsmine.mst.edu/phys_facwork/2076
https://scholarsmine.mst.edu/phys_facwork/2076Mon, 09 Nov 2020 07:50:59 PST
We report on gravitational wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near real-time through GCN Notices and Circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of ~0.8, as well as events which could plausibly originate from binary neutron stars, neutron star-black hole binaries, or binary black holes. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational wave data alone. The range of candidate events which are unambiguously identified as binary black holes (both objects ≥ 3M_{⊙}) is increased compared to GWTC-1, with total masses from ~14M_{⊙} for GW190924_021846 to ~150M_{⊙} for GW190521. For the first time, this catalog includes binary systems with asymmetric mass ratios, which had not been observed in data taken before April 2019. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in ~26 weeks of data (~1.5 per week) is consistent with GWTC-1.
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R. Abbott et al.GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙
https://scholarsmine.mst.edu/phys_facwork/2075
https://scholarsmine.mst.edu/phys_facwork/2075Wed, 07 Oct 2020 10:07:54 PDT
On May 21, 2019 at 03:02:29 UTC Advanced LIGO and Advanced Virgo observed a short duration gravitational-wave signal, GW190521, with a three-detector network signal-to-noise ratio of 14.7, and an estimated false-alarm rate of 1 in 4900 yr using a search sensitive to generic transients. If GW190521 is from a quasicircular binary inspiral, then the detected signal is consistent with the merger of two black holes with masses of 85^{+21}_{−14} M⊙ and 66^{+17}_{−18} M⊙ (90% credible intervals). We infer that the primary black hole mass lies within the gap produced by (pulsational) pair-instability supernova processes, with only a 0.32% probability of being below 65 M⊙. We calculate the mass of the remnant to be 142^{+28}_{−16} M⊙, which can be considered an intermediate mass black hole (IMBH). The luminosity distance of the source is 5.3^{+2.4}_{−2.6} Gpc, corresponding to a redshift of 0.82^{+0.28}_{−0.34}. The inferred rate of mergers similar to GW190521 is 0.13^{+0.30}_{−0.11} Gpc^{−3} yr^{−1}.
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R. Abbott et al.The Synthetic Emission Line COSMOS Catalogue: Hα and [O II] Galaxy Luminosity Functions and Counts at 0.3 < Z < 2.5
https://scholarsmine.mst.edu/phys_facwork/2074
https://scholarsmine.mst.edu/phys_facwork/2074Tue, 09 Jun 2020 12:58:38 PDT
Star-forming galaxies with strong nebular and collisional emission lines are privileged target galaxies in forthcoming cosmological large galaxy redshift surveys. We use the COSMOS2015 photometric catalogue to model galaxy spectral energy distributions and emission-line fluxes. We adopt an empirical but physically motivated model that uses information from the best-fitting spectral energy distribution of stellar continuum to each galaxy. The emission-line flux model is calibrated and validated against direct flux measurements in subsets of galaxies that have 3D-HST or zCOSMOS-Bright spectra. We take a particular care in modelling dust attenuation such that our model can explain both Hα and [O II] observed fluxes at different redshifts. We find that a simple solution to this is to introduce a redshift evolution in the dust attenuation fraction parameter, f = E_{star}(B − V)/E_{gas}(B − V), as f(z) = 0.44 + 0.2z. From this catalogue, we derive the Hα and [O II] luminosity functions up to redshifts of about 2.5 after carefully accounting for emission line flux and redshift errors. This allows us to make predictions for Hα and [O II] galaxy number counts in next-generation cosmological redshift surveys. Our modelled emission lines and spectra in the COSMOS2015 catalogue shall be useful to study the target selection for planned next-generation galaxy redshift surveys and we make them publicly available as 'EL-COSMOS' on the ASPIC data base.
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Shun Saito et al.Photo-Ionization of Polarized Lithium Atoms Out of an All-Optical Atom Trap: A Complete Experiment
https://scholarsmine.mst.edu/phys_facwork/2073
https://scholarsmine.mst.edu/phys_facwork/2073Tue, 09 Jun 2020 12:58:35 PDT
An all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2^{2}P_{3/2}(m_{ℓ} = +1) state are ionized by a 266 nm laser source, and emitted electrons and Li^{+} ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, it also enables to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement.
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F. Thini et al.A Joint Fermi-Gbm and Ligo/Virgo Analysis of Compact Binary Mergers from the First and Second Gravitational-Wave Observing Runs
https://scholarsmine.mst.edu/phys_facwork/2072
https://scholarsmine.mst.edu/phys_facwork/2072Tue, 09 Jun 2020 12:58:32 PDT
We present results from offline searches of Fermi Gamma-ray Burst Monitor (GBM) data for gamma-ray transients coincident with the compact binary coalescences observed by the gravitational-wave (GW) detectors Advanced LIGO and Advanced Virgo during their first and second observing runs. In particular, we perform follow-up for both confirmed events and low significance candidates reported in the LIGO/Virgo catalog GWTC-1. We search for temporal coincidences between these GW signals and GBM-triggered gamma-ray bursts (GRBs). We also use the GBM Untargeted and Targeted subthreshold searches to find coincident gamma-rays below the onboard triggering threshold. This work implements a refined statistical approach by incorporating GW astrophysical source probabilities and GBM visibilities of LIGO/Virgo sky localizations to search for cumulative signatures of coincident subthreshold gamma-rays. All search methods recover the short gamma-ray burst GRB 170817A occurring ∼1.7 s after the binary neutron-star merger GW170817. We also present results from a new search seeking GBM counterparts to LIGO single-interferometer triggers. This search finds a candidate joint event, but given the nature of the GBM signal and localization, as well as the high joint false alarm rate of 1.1 10-6 Hz, we do not consider it an astrophysical association. We find no additional coincidences.
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R. Hamburg et al.Quantum Interference of K Capture in Energetic G E31+(1s)-Kr Collisions
https://scholarsmine.mst.edu/phys_facwork/2071
https://scholarsmine.mst.edu/phys_facwork/2071Tue, 09 Jun 2020 12:58:28 PDT
We have measured characteristic K x rays in coincidence with the scattered particles from collisions of hydrogenlike Ge ions with Kr atoms. The ions were first accelerated to 8.6 MeV/amu, post-stripped to H-like charge state, and decelerated to around 2.5 MeV/amu. From the measurements the probabilities for K-shell to K-shell charge transfer as a function of collision impact parameters were obtained. The probabilities show an onset of oscillations which are interpreted as quantum interference between the K-shell to K-shell electron transfer amplitudes in two spatially separated coupling regions in the incoming and outgoing parts of the collision. The probabilities of K-shell vacancy distribution created by the collision are calculated within a relativistic independent electron model using the coupled-channel approach with atomlike Dirac-Fock-Sturm orbitals. A reasonable agreement between the theoretical results and the experimental data is found.
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R. Schuch et al.Optically Targeted Search for Gravitational Waves Emitted by Core-Collapse Supernovae during the First and Second Observing Runs of Advanced LIGO and Advanced Virgo
https://scholarsmine.mst.edu/phys_facwork/2070
https://scholarsmine.mst.edu/phys_facwork/2070Tue, 09 Jun 2020 12:58:25 PDT
We present the results from a search for gravitational-wave transients associated with core-collapse supernovae observed within a source distance of approximately 20 Mpc during the first and second observing runs of Advanced LIGO and Advanced Virgo. No significant gravitational-wave candidate was detected. We report the detection efficiencies as a function of the distance for waveforms derived from multidimensional numerical simulations and phenomenological extreme emission models. The sources with neutrino-driven explosions are detectable at the distances approaching 5 kpc, and for magnetorotationally driven explosions the distances are up to 54 kpc. However, waveforms for extreme emission models are detectable up to 28 Mpc. For the first time, the gravitational-wave data enabled us to exclude part of the parameter spaces of two extreme emission models with confidence up to 83%, limited by coincident data coverage. Besides, using ad hoc harmonic signals windowed with Gaussian envelopes, we constrained the gravitational-wave energy emitted during core collapse at the levels of 4.27 x 10^{-4} M_{⊙}·c^{2} and 1.28 x 10^{-1} M_{⊙}·c^{2} for emissions at 235 and 1304 Hz, respectively. These constraints are 2 orders of magnitude more stringent than previously derived in the corresponding analysis using initial LIGO, initial Virgo, and GEO 600 data.
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B. P. Abbott et al.GW190425: Observation of a Compact Binary Coalescence with Total Mass ∼ 3.4 M O
https://scholarsmine.mst.edu/phys_facwork/2069
https://scholarsmine.mst.edu/phys_facwork/2069Tue, 09 Jun 2020 12:58:21 PDT
On 2019 April 25, the LIGO Livingston detector observed a compact binary coalescence with signal-to-noise ratio 12.9. The Virgo detector was also taking data that did not contribute to detection due to a low signal-to-noise ratio, but were used for subsequent parameter estimation. The 90% credible intervals for the component masses range from to if we restrict the dimensionless component spin magnitudes to be smaller than 0.05). These mass parameters are consistent with the individual binary components being neutron stars. However, both the source-frame chirp mass and the total mass of this system are significantly larger than those of any other known binary neutron star (BNS) system. The possibility that one or both binary components of the system are black holes cannot be ruled out from gravitational-wave data. We discuss possible origins of the system based on its inconsistency with the known Galactic BNS population. Under the assumption that the signal was produced by a BNS coalescence, the local rate of neutron star mergers is updated to 250-2810.
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B. P. Abbott et al.Structure-Charge Transport Relationships in Fluoride-Doped Amorphous Semiconducting Indium Oxide: Combined Experimental and Theoretical Analysis
https://scholarsmine.mst.edu/phys_facwork/2068
https://scholarsmine.mst.edu/phys_facwork/2068Thu, 07 May 2020 11:04:44 PDT
Anion doping of transparent amorphous metal oxide (a-MO) semiconductors is virtually unexplored but offers the possibility of creating unique optoelectronic materials owing to the chemical tuning, modified crystal structures, and unusual charge-transport properties that added anions may impart. We report here the effects of fluoride (F^{-}) doping by combustion synthesis, in an archetypical metal oxide semiconductor, indium oxide (In-O). Optimized fluoride-doped In-O (F:In-O) thin films are characterized in depth by grazing incidence X-ray diffraction, X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure (EXAFS). Charge-transport properties are investigated in thin-film transistors (TFTs), revealing that increasing fluoride content (0.0 → 1.57 atom %) slightly lowers the on-current (I_{on}) and electron mobility due to scattering from loosely bound F^{-} centers but enhances important TFT performance parameters such as the I_{on}n/I_{off} ratio, subthreshold swing, and bias stress stability, yielding superior TFT switching versus undoped In-O. These results are convincingly explained by ab initio molecular dynamics simulations and density functional theory electronic structure calculations. Combined with the EXAFS data, the experimental and theoretical results show that F^{-} hinders crystallization by enhancing the local and medium-range disorder, promotes a uniform film morphology, and favors the formation of deeper, more localized trap states as compared to F^{-}-free In-O. These data also show that the local organization and electronic structure of amorphous F^{-}-doped oxide semiconductors are significantly different from those of F^{-}-doped crystalline oxide semiconductors and suggest new avenues to further modify a-MOs for enhanced optoelectronic properties.
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Aritra Sil et al.Quantum Superconductor-Metal Transitions in the Presence of Quenched Disorder
https://scholarsmine.mst.edu/phys_facwork/2067
https://scholarsmine.mst.edu/phys_facwork/2067Thu, 07 May 2020 11:04:41 PDT
InO_{x} films that are less disordered than those exhibiting direct quantum superconductor-insulator transitions feature quantum superconductor-metal transitions tuned by magnetic field. Resistance data across this superconductor-metal transition obey activated scaling, with critical exponents suggesting that the transition is governed by an infinite-randomness critical point in the universality class of the random transverse-field Ising model in two dimensions. The transition is accompanied by quantum Griffiths effects. This unusual behavior is expected for systems with quenched disorder in the presence of ohmic dissipation. Disorder leads to the formation of large rare regions which are locally ordered superconducting puddles dispersed in a metallic matrix. Their dissipative dynamics causes the activated scaling, as predicted by a renormalization group theory.
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Nicholas A. Lewellyn et al.Pressure Shifts in High-Precision Hydrogen Spectroscopy: II. Impact Approximation and Monte-Carlo Simulations
https://scholarsmine.mst.edu/phys_facwork/2066
https://scholarsmine.mst.edu/phys_facwork/2066Thu, 07 May 2020 11:04:38 PDT
We investigate collisional shifts of spectral lines involving excited hydrogenic states, where van der Waals coefficients have recently been shown to have large numerical values when expressed in atomic units. Particular emphasis is laid on the recent hydrogen 2S-4P experiment (and an ongoing 2S-6P experiment) in Garching, but numerical input data are provided for other transitions (e.g. involving S states), as well. We show that the frequency shifts can be described, to sufficient accuracy, in the impact approximation. The pressure related effects were separated into two parts, (i) related to collisions of atoms inside of the beam, and (ii) related to collisions of the atoms in the atomic beam with the residual background gas. The latter contains both atomic as well as molecular hydrogen. The dominant effect of intra-beam collisions is evaluated by a Monte-Carlo simulation, taking the geometry of the experimental apparatus into account. While, in the Garching experiment, the collisional shift is on the order of 10 Hz, and thus negligible, it can decisively depend on the experimental conditions. We present input data which can be used in order to describe the effect for other transitions of current and planned experimental interest.
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A. Matveev et al.In-Situ Measurement of Irradiation Behavior in LiNbO₃
https://scholarsmine.mst.edu/phys_facwork/2065
https://scholarsmine.mst.edu/phys_facwork/2065Thu, 07 May 2020 11:04:35 PDT
In-situ measurement of LiNbO_{3} based surface acoustic wave (SAW) crystal resonator device under irradiation was demonstrated and used to characterize the impact of radiation on physical properties of this material. The resonant frequency of the SAW device was monitored as the output power of the reactor was varied. Upon step increase of the reactor power, a gradual shift in the device's resonant frequency was observed. This frequency shift initially exhibits a linear growth and eventually reaches an equilibrium value proportional to the reactor power. The observed behavior can be attributed to two competing processes: increase of temperature due to gamma heating or accumulation of irradiation induced defects. In both cases, the response is attributed to changes in the physical properties of LiNbO_{3}, particularly the elastic constants. This demonstrated ability to measure materials properties under irradiation is attractive for development of sensors and performing materials science under irradiation.
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Gaofeng Sha et al.Detecting Baryon Acoustic Oscillations in Dark Matter from Kinematic Weak Lensing Surveys
https://scholarsmine.mst.edu/phys_facwork/2064
https://scholarsmine.mst.edu/phys_facwork/2064Thu, 07 May 2020 11:03:32 PDT
We investigate the feasibility of extracting baryon acoustic oscillations (BAO) from cosmic shear tomography. We particularly focus on the BAO scale precision that can be achieved by future spectroscopy-based, kinematic weak lensing (KWL) surveys in comparison to the traditional photometry-based weak lensing surveys. We simulate cosmic shear tomography data of such surveys with a few simple assumptions to focus on the BAO information, extract the spatial power spectrum, and constrain the recovered BAO feature. Due to the small shape noise and the shape of the lensing kernel, we find that a Dark Energy Task Force Stage IV version of such KWL survey can detect the BAO feature in dark matter by 3σ and measure the BAO scale at the precision level of 4 per cent, while it will be difficult to detect the feature in photometry-based weak lensing surveys.With a more optimistic assumption, a KWL-Stage IV could achieve a ~2 per cent BAO scale measurement with 4.9σ confidence. A built-in spectroscopic galaxy survey within such KWL survey will allow cross-correlation between galaxies and cosmic shear, which will tighten the constraint beyond the lower limit we present in this paper and therefore possibly allow a detection of the BAO scale bias between galaxies and dark matter.
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Zhejie Ding et al.Unconventional Josephson Junctions with Topological Kondo Insulator Weak Links
https://scholarsmine.mst.edu/phys_facwork/2063
https://scholarsmine.mst.edu/phys_facwork/2063Thu, 07 May 2020 11:03:29 PDT
Proximity-induced superconductivity in three-dimensional (3D) topological insulators forms a new quantum phase of matter and accommodates exotic quasiparticles such as Majorana bound states. One of the biggest drawbacks of the commonly studied 3D topological insulators is the presence of conducting bulk that obscures both surface states and low energy bound states. Introducing superconductivity in topological Kondo insulators such as SmB6, however, is promising due to their true insulating bulk at low temperatures. In this work, we develop an unconventional Josephson junction by coupling superconducting Nb leads to the surface states of a SmB6 crystal. We observe a robust critical current at low temperatures that responds to the application of an out-of-plane magnetic field with significant deviations from usual Fraunhofer patterns. The appearance of Shaphiro steps under microwave radiation gives further evidence of a Josephson effect. Moreover, we explore the effects of Kondo breakdown in our devices, such as ferromagnetism at the surface and anomalous temperature dependence of supercurrent. Particularly, the magnetic diffraction patterns show an anomalous hysteresis with the field sweep direction suggesting the coexistence of magnetism with superconductivity at the SmB6 surface. The experimental work will advance the current understanding of topologically nontrivial superconductors and emergent states associated with such unconventional superconducting phases.
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Xuecheng Ye et al.Radiative Transfer Distortions of Lyman Α Emitters: A New Fingers-Of-God Damping in the Clustering in Redshift Space
https://scholarsmine.mst.edu/phys_facwork/2062
https://scholarsmine.mst.edu/phys_facwork/2062Thu, 07 May 2020 11:03:26 PDT
Complex radiative transfer (RT) of the Lyman α photons poses a theoretical challenge to galaxy surveys that infer the large-scale structure with Lyman α emitters (LAEs). Guided by RT simulations, prior studies investigated the impact of RT on the large-scale LAE clustering, and claimed that RT induces a selection effect which results in an anisotropic distortion even in real space but in an otherwise negligible effect in redshift space. However, our previous study, which relies on a full RT code run on the Illustris simulations, shows that the anisotropic selection effect was drastically reduced with higher spatial resolution. Adopting the same simulation framework, we further study the impact of RT on the LAE clustering in redshift space. Since we measure LAE's radial position through a spectral peak of Lyman α emission, the frequency shift due to RT contaminates the redshift measurement and hence the inferred radial position in redshift space. We demonstrate that this additional RT offset suppresses the LAE clustering along the line of sight, which can be interpreted as a novel Fingers-of- God (FoG) effect. To assess the FoG effect, we develop a theoretical framework modelling the impact of the RT similar to that of the small-scale peculiar velocity which is commonly studied in the context of the redshift space distortion (RSD). Although our findings strongly encourage a more careful RSD modelling in LAE surveys, we also seek a method to mitigate the additional FoG effect due to RT by making use of other information in a Lyman α spectrum.
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Chris Byrohl et al.Pseudo-Periodic Natural Higgs Inflation
https://scholarsmine.mst.edu/phys_facwork/2061
https://scholarsmine.mst.edu/phys_facwork/2061Thu, 07 May 2020 11:03:23 PDT
Inflationary cosmology represents a well-studied framework to describe the expansion of space in the early universe, as it explains the origin of the large-scale structure of the cosmos and the isotropy of the cosmic microwave background radiation. The recent detection of the Higgs boson renewed research activities based on the assumption that the inflaton could be identified with the Higgs field. At the same time, the question whether the inflationary potential can be extended to the electroweak scale and whether it should be necessarily chosen ad hoc in order to be physically acceptable are at the center of an intense debate. Here, we propose and perform the slow-roll analysis of the so-called Massive Natural Inflation (MNI) model which has three adjustable parameters, the explicit mass term, a Fourier amplitude u, and a frequency parameter β, in addition to a constant term of the potential. This theory has the advantage to present a structure of infinite non-degenerate minima and is amenable to an easy integration of high-energy modes. We show that, using PLANCK data, one can fix, in the large β-region, the parameters of the model in a unique way. We also demonstrate that the value for the parameters chosen at the cosmological scale does not influence the results at the electroweak scale. We argue that other models can have similar properties both at cosmological and electroweak scales, but with the MNI model one can complete the theory towards low energies and easily perform the integration of modes up to the electroweak scale, producing the correct order-of-magnitude for the Higgs mass.
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I. G. Marian et al.Probability Density of the Fractional Langevin Equation with Reflecting Walls
https://scholarsmine.mst.edu/phys_facwork/2060
https://scholarsmine.mst.edu/phys_facwork/2060Thu, 07 May 2020 11:03:20 PDT
We investigate anomalous diffusion processes governed by the fractional Langevin equation and confined to a finite or semi-infinite interval by reflecting potential barriers. As the random and damping forces in the fractional Langevin equation fulfill the appropriate fluctuation-dissipation relation, the probability density on a finite interval converges for long times towards the expected uniform distribution prescribed by thermal equilibrium. In contrast, on a semi-infinite interval with a reflecting wall at the origin, the probability density shows pronounced deviations from the Gaussian behavior observed for normal diffusion. If the correlations of the random force are persistent (positive), particles accumulate at the reflecting wall while antipersistent (negative) correlations lead to a depletion of particles near the wall. We compare and contrast these results with the strong accumulation and depletion effects recently observed for nonthermal fractional Brownian motion with reflecting walls, and we discuss broader implications.
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Thomas Vojta et al.Model Comparison from LIGO-Virgo Data on GW170817's Binary Components and Consequences for the Merger Remnant
https://scholarsmine.mst.edu/phys_facwork/2059
https://scholarsmine.mst.edu/phys_facwork/2059Thu, 07 May 2020 11:03:17 PDT
GW170817 is the very first observation of gravitational waves originating from the coalescence of two compact objects in the mass range of neutron stars, accompanied by electromagnetic counterparts, and offers an opportunity to directly probe the internal structure of neutron stars. We perform Bayesian model selection on a wide range of theoretical predictions for the neutron star equation of state. For the binary neutron star hypothesis, we find that we cannot rule out the majority of theoretical models considered. In addition, the gravitational-wave data alone does not rule out the possibility that one or both objects were low-mass black holes. We discuss the possible outcomes in the case of a binary neutron star merger, finding that all scenarios from prompt collapse to long-lived or even stable remnants are possible. For long-lived remnants, we place an upper limit of 1.9 kHz on the rotation rate. If a black hole was formed any time after merger and the coalescing stars were slowly rotating, then the maximum baryonic mass of non-rotating neutron stars is at most 3.05M_{⊙}, and three equations of state considered here can be ruled out. We obtain a tighter limit of 2.67M_{⊙} for the case that the merger results in a hypermassive neutron star.
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B. P. Abbott et al.A Dynamical (e,2e) Investigation into the Ionization of the Outermost Orbitals of R-Carvone
https://scholarsmine.mst.edu/phys_facwork/2058
https://scholarsmine.mst.edu/phys_facwork/2058Thu, 07 May 2020 11:03:14 PDT
We report an experimental and theoretical investigation into the dynamics of electron-impact ionization of R-carvone. Experimental triple differential cross sections are obtained in asymmetric coplanar kinematic conditions for the ionization of the unresolved combination of the three outermost molecular orbitals (41a-39a) of R-carvone. These cross sections are compared with theoretical cross sections calculated within a molecular 3-body distorted wave (M3DW) framework employing either a proper orientation average or orbital average to account for the random orientation of the molecule probed in the experiment. Here, we observe that the overall scattering behavior observed in the experiment is fairly well reproduced within the M3DW framework when implementing the proper average over orientations. The character of the ionized orbitals also provides some qualitative explanation for the observed scattering behavior. This represents substantial progress when trying to describe the scattering dynamics observed for larger molecules under intermediate-impact energy and asymmetric energy sharing scattering conditions.
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D. B. Jones et al.Copper Cobalt Selenide as a High-Efficiency Bifunctional Electrocatalyst for overall Water Splitting: Combined Experimental and Theoretical Study
https://scholarsmine.mst.edu/phys_facwork/2057
https://scholarsmine.mst.edu/phys_facwork/2057Fri, 01 May 2020 09:16:30 PDT
Nonprecious metal-based catalysts for full water splitting are still being sought after by several groups of researchers, owing to their promising practical application in energy conversion devices. In this article, nanostructured CuCo_{2}Se_{4} comprising earth-abundant elements have been reported to exhibit superior bifunctional electrocatalytic activity for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions. The CuCo_{2}Se_{4} electrocatalyst with a spinel structure type requires low overpotential of 320 mV to reach current density of 50 mA cm^{-2} for OER and 125 mV to achieve 10 mA cm^{-2} for HER, respectively, which is lower than other reported transition metal chalcogenide electrocatalysts based on Co/Cu, and significantly lower than the well-known precious metal oxide catalysts (IrOx and RuOx). To understand the origin of high catalytic performance in CuCo_{2}Se_{4}, density functional theory (DFT) has been utilized to study the structural, electronic, and magnetic properties of bulk CuCo_{2}Se_{4} as well as slabs with (100) and (111) surface orientations with and without OH^{-} adsorption. The theoretical results show that CuCo_{2}Se_{4} is in a metallic state with a high electrical conductivity which plays a crucial role in the catalytic activity. Comparison between Co and Cu revealed that Co sites exhibit better OER catalytic activity. Importantly, a surface enhancement of the local magnetic moment on the Co atoms is found to be limited to the top layer in the (100) slab, whereas such variation of the local magnetic moment affects all layers of the (111) slab, strongly favoring OH^{-} adsorption on Co atom at the (111) surface and making the (111) surface more catalytically active. The different surface energies of (111) and (100) surfaces were also observed from DFT studies which will have a pronounced influence on the observed catalytic activity of these surfaces.
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Xi Cao et al.