Cross Sections for He, Li, and Be Isotopes Produces in the a + a Reactions at 198.4 MeV

This research was sponsored by the National Science Foundation Grant NSF PHy 87-1440

Continuity and Uniqueness of Regularized Output Least Squares Optimal Estimators

AbstractThe continuity and uniqueness properties of optimal estimators with respect to data are considered for different regularizations. It is found that there is a weak stability for optimal estimators as set-valued mappings under a weak regularization. For stronger regularization results are obtained that give stability in stronger topologies and the finiteness of the set of optimal estimators. Finally, we give conditions that imply uniqueness of optimal estimators and Lipschitz continuity with respect to data

Demonstration of large ionization coefficient ratio in AlAs0.56Sb0.44 lattice matched to InP

The electron and hole avalanche multiplication characteristics have been measured in bulk AlAs0.56Sb0.44 p-i-n and n-i-p homojunction diodes, lattice matched to InP, with nominal avalanche region thicknesses of ~0.6 μm, 1.0 μm and 1.5 μm. From these and data from two much thinner devices, the bulk electron and hole impact ionization coefficients (α and β respectively), have been determined over an electric-field range from 220-1250 kV/cm for α and from 360-1250 kV/cm for β for the first time. The α/β ratio is found to vary from 1000 to 2 over this field range, making it the first report of a wide band-gap III-V semiconductor with ionization coefficient ratios similar to or larger than that observed in silicon

EGAM Induced by Energetic-electrons and Nonlinear Interactions among EGAM, BAEs and Tearing Modes in a Toroidal Plasma

In this letter, it is reported that the first experimental results are associated with the GAM induced by energetic electrons (eEGAM) in HL-2A Ohmic plasma. The energetic-electrons are generated by parallel electric fields during magnetic reconnection associated with tearing mode (TM). The eEGAM localizes in the core plasma, i.e. in the vicinity of q=2 surface, and is very different from one excited by the drift-wave turbulence in the edge plasma. The analysis indicated that the eEGAM is provided with the magnetic components, whose intensities depend on the poloidal angles, and its mode numbers are jm/nj=2/0. Further, there exist intense nonlinear interactions among eEGAM, BAEs and strong tearing modes (TMs). These new findings shed light on the underlying physics mechanism for the excitation of the low frequency (LF) Alfv\'enic and acoustic uctuations.Comment: 5 pages,4 figure

Two-Boson Exchange Physics: A Brief Review

Current status of the two-boson exchange contributions to elastic electron-proton scattering, both for parity conserving and parity-violating, is briefly reviewed. How the discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments can be understood, in large part, by the two-photon exchange corrections is discussed. We also illustrate how the measurement of the ratio between positron-proton and electron-proton scattering can be used to differentiate different models of two-photon exchange. For the parity-violating electron-proton scattering, the interest is on how the two-boson exchange (TBE), \gamma Z-exchange in particular, could affect the extraction of the long-sought strangeness form factors. Various calculations all indicate that the magnitudes of effect of TBE on the extraction of strangeness form factors is small, though can be large percentage-wise in certain kinematics.Comment: 6 pages, 5 figures, prepared for Proceedings of the fifth Asia-Pacific Conference on Few-Body Problems in Physics (APFB2011), Seoul, Korea, August 22-26, 2011, to appear in Few-Body Systems, November 201

Goos-H\"{a}nchen-like shifts for Dirac fermions in monolayer graphene barrier

We investigate the Goos-H\"{a}nchen-like shifts for Dirac fermions in transmission through a monolayer graphene barrier. The lateral shifts, as the functions of the barrier's width and the incidence angle, can be negative and positive in Klein tunneling and classical motion, respectively. Due to their relations to the transmission gap, the lateral shifts can be enhanced by the transmission resonances when the incidence angle is less than the critical angle for total reflection, while their magnitudes become only the order of Fermi wavelength when the incidence angle is larger than the critical angle. These tunable beam shifts can also be modulated by the height of potential barrier and the induced gap, which gives rise to the applications in graphene-based devices.Comment: 5 pages, 5 figure

Measuring Black Hole Spin using X-ray Reflection Spectroscopy

I review the current status of X-ray reflection (a.k.a. broad iron line) based black hole spin measurements. This is a powerful technique that allows us to measure robust black hole spins across the mass range, from the stellar-mass black holes in X-ray binaries to the supermassive black holes in active galactic nuclei. After describing the basic assumptions of this approach, I lay out the detailed methodology focusing on "best practices" that have been found necessary to obtain robust results. Reflecting my own biases, this review is slanted towards a discussion of supermassive black hole (SMBH) spin in active galactic nuclei (AGN). Pulling together all of the available XMM-Newton and Suzaku results from the literature that satisfy objective quality control criteria, it is clear that a large fraction of SMBHs are rapidly-spinning, although there are tentative hints of a more slowly spinning population at high (M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of the spins of stellar-mass black holes in X-ray binaries. In general, reflection-based and continuum-fitting based spin measures are in agreement, although there remain two objects (GROJ1655-40 and 4U1543-475) for which that is not true. I end this review by discussing the exciting frontier of relativistic reverberation, particularly the discovery of broad iron line reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk reflection, this detection of reverberation demonstrates that future large-area X-ray observatories such as LOFT will make tremendous progress in studies of strong gravity using relativistic reverberation in AGN.Comment: 19 pages. To appear in proceedings of the ISSI-Bern workshop on "The Physics of Accretion onto Black Holes" (8-12 Oct 2012). Revised version adds a missing source to Table 1 and Fig.6 (IRAS13224-3809) and corrects the referencing of the discovery of soft lags in 1H0707-495 (which were in fact first reported in Fabian et al. 2009

Valence band engineering of GaAsBi for low noise avalanche photodiodes

Avalanche Photodiodes (APDs) are key semiconductor components that amplify weak optical signals via the impact ionization process, but this process’ stochastic nature introduces ‘excess’ noise, limiting the useful signal to noise ratio (or sensitivity) that is practically achievable. The APD material’s electron and hole ionization coefficients (α and β respectively) are critical parameters in this regard, with very disparate values of α and β necessary to minimize this excess noise. Here, the analysis of thirteen complementary p-i-n/n-i-p diodes shows that alloying GaAs with ≤ 5.1 % Bi dramatically reduces β while leaving α virtually unchanged—enabling a 2 to 100-fold enhancement of the GaAs α/β ratio while extending the wavelength beyond 1.1 µm. Such a dramatic change in only β is unseen in any other dilute alloy and is attributed to the Bi-induced increase of the spin-orbit splitting energy (∆so). Valence band engineering in this way offers an attractive route to enable low noise semiconductor APDs to be developed

Measurements of the Mass and Full-Width of the ηc\eta_c Meson

In a sample of 58 million $J/\psi$ events collected with the BES II detector, the process J/$\psi\to\gamma\eta_c$ is observed in five different decay channels: $\gamma K^+K^-\pi^+\pi^-$, $\gamma\pi^+\pi^-\pi^+\pi^-$, $\gamma K^\pm K^0_S \pi^\mp$ (with $K^0_S\to\pi^+\pi^-$), $\gamma \phi\phi$ (with $\phi\to K^+K^-$) and $\gamma p\bar{p}$. From a combined fit of all five channels, we determine the mass and full-width of $\eta_c$ to be $m_{\eta_c}=2977.5\pm1.0 ({stat.})\pm1.2 ({syst.})$ MeV/$c^2$ and $\Gamma_{\eta_c} = 17.0\pm3.7 ({stat.})\pm7.4 ({syst.})$ MeV/$c^2$.Comment: 9 pages, 2 figures and 4 table. Submitted to Phys. Lett.