Multi-wavelength interferometry of evolved stars using VLTI and VLBA

We report on our project of coordinated VLTI/VLBA observations of the atmospheres and circumstellar environments of evolved stars. We illustrate in general the potential of interferometric measurements to study stellar atmospheres and envelopes, and demonstrate in particular the advantages of a coordinated multi-wavelength approach including near/mid-infrared as well as radio interferometry. We have so far made use of VLTI observations of the near- and mid-infrared stellar sizes and of concurrent VLBA observations of the SiO maser emission. To date, this project includes studies of the Mira stars S Ori and RR Aql as well as of the supergiant AH Sco. These sources all show strong silicate emission features in their mid-infrared spectra. In addition, they each have relatively strong SiO maser emission. The results from our first epochs of S Ori measurements have recently been published and the main results are reviewed here. The S Ori maser ring is found to lie at a mean distance of about 2 stellar radii, a result that is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase. We discuss the status of our more recent S Ori, RR Aql, and AH Sco observations, and present an outlook on the continuation of our project.Comment: 9 pages, to appear in the proceedings of the ESO workshop "The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation", ESO Astrophysics Symposi

Anisotropic optical response of the diamond (111)-2x1 surface

The optical properties of the 2$\times$1 reconstruction of the diamond (111) surface are investigated. The electronic structure and optical properties of the surface are studied using a microscopic tight-binding approach. We calculate the dielectric response describing the surface region and investigate the origin of the electronic transitions involving surface and bulk states. A large anisotropy in the surface dielectric response appears as a consequence of the asymmetric reconstruction on the surface plane, which gives rise to the zigzag Pandey chains. The results are presented in terms of the reflectance anisotropy and electron energy loss spectra. While our results are in good agreement with available experimental data, additional experiments are proposed in order to unambiguously determine the surface electronic structure of this interesting surface.Comment: REVTEX manuscript with 6 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.html Submitted to Phys. Rev.

I=3/2 KπK \pi Scattering in the Nonrelativisitic Quark Potential Model

We study $I=3/2$ elastic $K\pi$ scattering to Born order using nonrelativistic quark wavefunctions in a constituent-exchange model. This channel is ideal for the study of nonresonant meson-meson scattering amplitudes since s-channel resonances do not contribute significantly. Standard quark model parameters yield good agreement with the measured S- and P-wave phase shifts and with PCAC calculations of the scattering length. The P-wave phase shift is especially interesting because it is nonzero solely due to $SU(3)_f$ symmetry breaking effects, and is found to be in good agreement with experiment given conventional values for the strange and nonstrange constituent quark masses.Comment: 12 pages + 2 postscript figures, Revtex, MIT-CTP-210

Active Amplification of the Terrestrial Albedo to Mitigate Climate Change: An Exploratory Study

This study explores the potential to enhance the reflectance of solar insolation by the human settlement and grassland components of the Earth's terrestrial surface as a climate change mitigation measure. Preliminary estimates derived using a static radiative transfer model indicate that such efforts could amplify the planetary albedo enough to offset the current global annual average level of radiative forcing caused by anthropogenic greenhouse gases by as much as 30 percent or 0.76 W/m2. Terrestrial albedo amplification may thus extend, by about 25 years, the time available to advance the development and use of low-emission energy conversion technologies which ultimately remain essential to mitigate long-term climate change. However, additional study is needed to confirm the estimates reported here and to assess the economic and environmental impacts of active land-surface albedo amplification as a climate change mitigation measure.Comment: 21 pages, 3 figures. In press with Mitigation and Adaptation Strategies for Global Change, Springer, N

Metamaterial/graphene amplitude and frequency modulators for the active control of terahertz quantum cascade lasers

Hybrid metamaterial/graphene amplitude and frequency modulators have been implemented as external optoelectronic mirrors in external cavity configurations with terahertz quantum cascade lasers (QCLs). These devices' tunability is accomplished via the interplay between metamaterial resonant units, normally engineered in mm-size arrays, and graphene. The integration of these devices in external cavity QCLs offers unique emission features and realizes an unprecedented studied regime. The implementation of an external amplitude modulation allows the full switching of laser emission in single mode operation by electrostatically gating graphene. The introduction of more dispersive tunable architectures in frequency modulators yields additionally an all-electronic spectral laser bistability. © 2018 SPIE

Thermal conductivity via magnetic excitations in spin-chain materials

We discuss the recent progress and the current status of experimental investigations of spin-mediated energy transport in spin-chain and spin-ladder materials with antiferromagnetic coupling. We briefly outline the central results of theoretical studies on the subject but focus mainly on recent experimental results that were obtained on materials which may be regarded as adequate physical realizations of the idealized theoretical model systems. Some open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure

Gamma-Hadron Separation Methods for the VERITAS Array of Four Imaging Atmospheric Cherenkov Telescopes

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive gamma-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 100,000 square meter, combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic Ray initiated air showers that resemble gamma-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of sigma[63%]= 0.1 degree (0.2 degree) and sigma[68%]= 15% (22%), respectively. The main emphasis of the paper is the discussion of gamma-hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a gamma-hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.Comment: Astroparticle Physics, in press, 22 pages, 10 figure

Weak Localization Effect in Superconductors by Radiation Damage

Large reductions of the superconducting transition temperature $T_{c}$ and the accompanying loss of the thermal electrical resistivity (electron-phonon interaction) due to radiation damage have been observed for several A15 compounds, Chevrel phase and Ternary superconductors, and $\rm{NbSe_{2}}$ in the high fluence regime. We examine these behaviors based on the recent theory of weak localization effect in superconductors. We find a good fitting to the experimental data. In particular, weak localization correction to the phonon-mediated interaction is derived from the density correlation function. It is shown that weak localization has a strong influence on both the phonon-mediated interaction and the electron-phonon interaction, which leads to the universal correlation of $T_{c}$ and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information, Plesse see http://www.fen.bilkent.edu.tr/~yjki

The Free Energy of the Quantum Heisenberg Ferromagnet at Large Spin

We consider the spin-S ferromagnetic Heisenberg model in three dimensions, in the absence of an external field. Spin wave theory suggests that in a suitable temperature regime the system behaves effectively as a system of non-interacting bosons (magnons). We prove this fact at the level of the specific free energy: if $S \to \infty$ and the inverse temperature $\beta \to 0$ in such a way that $\beta S$ stays constant, we rigorously show that the free energy per unit volume converges to the one suggested by spin wave theory. The proof is based on the localization of the system in small boxes and on upper and lower bounds on the local free energy, and it also provides explicit error bounds on the remainder.Comment: 11 pages, pdfLate