Residual stress measurements in carbon steel

External dc magnetic field-induced changes in natural velocity of Rayleigh surface waves were measured in steel specimens under various stress conditions. The low field slopes of curves representing the fractional changes of natural velocity were proved to provide correct stress information in steels with different metallurgical properties. The slopes of curves under uniaxial compression, exceeding about one third of the yield stress, fell below zero in all the specimens when magnetized along the stress axis. The slopes under tension varied among different steels but remained positive in any circumstances. The stress effect was observed for both applied and residual stress. A physical interpretation of these results is given based on the stress-induced domain structure changes and the delta epsilon effect. Most importantly, it is found that the influence of detailed metallurgical properties cause only secondary effects on the obtained stress information

Near-Optimal Distributed Approximation of Minimum-Weight Connected Dominating Set

This paper presents a near-optimal distributed approximation algorithm for the minimum-weight connected dominating set (MCDS) problem. The presented algorithm finds an $O(\log n)$ approximation in $\tilde{O}(D+\sqrt{n})$ rounds, where $D$ is the network diameter and $n$ is the number of nodes. MCDS is a classical NP-hard problem and the achieved approximation factor $O(\log n)$ is known to be optimal up to a constant factor, unless P=NP. Furthermore, the $\tilde{O}(D+\sqrt{n})$ round complexity is known to be optimal modulo logarithmic factors (for any approximation), following [Das Sarma et al.---STOC'11].Comment: An extended abstract version of this result appears in the proceedings of 41st International Colloquium on Automata, Languages, and Programming (ICALP 2014

Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies

We present 12CO (J=1-0) observations of a sample of local galaxies (0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The data were obtained using the Redshift Search Receiver on the FCRAO 14 m telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our sample selection is purely based on the amount of neutral hydrogen, thereby providing a chance to study how atomic and molecular gas relate to each other in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of (1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the most gas-massive galaxies identified to date in the Local Universe. Observed trends associated with HI, H2, and stellar properties of the HI massive galaxies and the field comparison sample are analyzed in the context of theoretical models of galaxy cold gas content and evolution, and the importance of total gas content and improved recipes for handling spatially differentiated behaviors of disk and halo gas are identified as potential areas of improvement for the modeling.Comment: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRA

Resonance Patterns in a Stadium-shaped Microcavity

We investigate resonance patterns in a stadium-shaped microcavity around $n_ck R \simeq 10$, where $n_c$ is the refractive index, $k$ the vacuum wavenumber, and $R$ the radius of the circular part of the cavity. We find that the patterns of high $Q$ resonances can be classified, even though the classical dynamics of the stadium system is chaotic. The patterns of the high $Q$ resonances are consistent with the ray dynamical consideration, and appears as the stationary lasing modes with low pumping rate in the nonlinear dynamical model. All resonance patterns are presented in a finite range of $kR$.Comment: 8 pages, 9 figure

Void-mediated formation of Sn quantum dots in a Si matrix

Atomic scale analysis of Sn quantum dots (QDs) formed during the molecular beam-epitaxy (MBE) growth of Sn_xSi_(1−x) (0.05 ⩽ x ⩽ 0.1) multilayers in a Si matrix revealed a void-mediated formation mechanism. Voids below the Si surface are induced by the lattice mismatch strain between Sn_xSi_(1−x) layers and Si, taking on their equilibrium tetrakaidecahedron shape. The diffusion of Sn atoms into these voids leads to an initial rapid coarsening of quantum dots during annealing. Since this formation process is not restricted to Sn, a method to grow QDs may be developed by controlling the formation of voids and the diffusion of materials into these voids during MBE growth

Dynamic behaviors in directed networks

Motivated by the abundance of directed synaptic couplings in a real biological neuronal network, we investigate the synchronization behavior of the Hodgkin-Huxley model in a directed network. We start from the standard model of the Watts-Strogatz undirected network and then change undirected edges to directed arcs with a given probability, still preserving the connectivity of the network. A generalized clustering coefficient for directed networks is defined and used to investigate the interplay between the synchronization behavior and underlying structural properties of directed networks. We observe that the directedness of complex networks plays an important role in emerging dynamical behaviors, which is also confirmed by a numerical study of the sociological game theoretic voter model on directed networks

Magnetic field dependence of antiferromagnetic resonance in NiO

We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3 K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles

A Limit Relation between Black Hole Mass and Hβ\beta Width: Testing Super-Eddington Accretion in Active Galactic Nuclei

(abbreviated) We show that there is a limit relation between the black hole mass and the width at the half maximum of H$\beta$ for active galactic nuclei (AGNs) with super-Eddington accretion rates. When a black hole has a super-Eddington accretion rate, the empirical relation of reverberation mapping has two possible ways. First, it reduces to a relation between the black hole mass and the size of the broad line region due to the photon trapping effects inside the accretion disk. For the Kaspi et al.'s empirical reverberation relation, we get the limit relation as $M_{\rm BH}=(2.9 - 12.6)\times 10^6M_{\odot} (\upsilon_{\rm FWHM}/10^3{\rm km s^{-1}})^{6.67}$, called as the Eddington limit. Second, the Eddington limit luminosity will be relaxed if the trapped photons can escape from the magnetized super-Eddington accretion disk via the photon bubble instability, and the size of the broad line region will be enlarged according to the empirical reverberation relation, leading to a relatively narrow width of H$\beta$. We call this the Begelman limit. Super-Eddington accretions in a sample composed of 164 AGNs have been searched by this limit relation. We find there are a handful of objects locate between the Eddington and Begelman limit lines, they may be candidates of super-Eddington accretors in a hybrid structure of photon trapping and photon bubble instability. The maximum width of H$\beta$ is in the reange of $(3.0 - 3.8)\times 10^3$ km s$^{-1}$ for the maximum mass black holes with super-Eddington accretion rates among AGNs. We suggest that this limit relation is more reliable and convenient to test whether a source is super-Eddington and useful to probe the structure of the super-Eddington accretion process.Comment: 5 pages (emulateapj5.sty), 1 figure. Astronomical Journal, 125 (June Issue 2003) in pres

SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-ray Variability

We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS J$075101.42+291419.1$. It has a black hole mass of $1.6\times 10^7~\rm M_\odot$ measured from reverberation mapping (RM), and the black hole is accreting with a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May reveals a flux drop by a factor of $\sim 22$ with respect to the Swift observation in 2013 May when it showed a typical level of X-ray emission relative to its UV/optical emission. The lack of correlated UV variability results in a steep X-ray-to-optical power-law slope ($\alpha_{\rm OX}$) of -1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of 36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild UV/optical continuum and emission-line variability also suggest that the accretion rate did not change significantly. A single power-law model modified by Galactic absorption describes well the $0.3-10$ keV spectra of the X-ray observations in general. The spectral fitting reveals steep spectral shapes with $\Gamma\approx3$. We search for active galactic nuclei (AGNs) with such extreme X-ray variability in the literature and find that most of them are narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The fraction of extremely X-ray variable objects among super-Eddington accreting AGNs is estimated to be $\approx 15-24\%$. We discuss two possible scenarios, disk reflection and partial covering absorption, to explain the extreme X-ray variability of SDSS J$075101.42+291419.1$. We propose a possible origin for the partial covering absorber, which is the thick inner accretion disk and its associated outflow in AGNs with high accretion rates.Comment: 15 pages, 9 figures, accepted for publication in Ap