Collapse of the Gd3+Gd^{3+} ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor CeFe4P12CeFe_{4}P_{12}

Experiments on the $Gd^{3+}$ Electron Spin Resonance (ESR) in the filled skutterudite $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ ($x \approx 0.001$), at temperatures where the host resistivity manifests a smooth insulator-metal crossover, provides evidence of the underlying Kondo physics associated with this system. At low temperatures (below $T \approx K$), $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ behaves as a Kondo-insulator with a relatively large hybridization gap, and the $Gd^{3+}$ ESR spectra displays a fine structure with lorentzian line shape, typical of insulating media. The electronic gap is attributed to the large hybridization present in the coherent regime of a Kondo lattice, when Ce 4f-electrons cooperate with band properties at half-filling. Mean-field calculations suggest that the electron-phonon interaction is fundamental at explaining the strong 4f-electron hybridization in this filled skutterudite. The resulting electronic structure is strongly temperature dependent, and at about $T^{*} \approx 160 K$ the system undergoes an insulator-to-metal transition induced by the withdrawal of 4f-electrons from the Fermi volume, the system becoming metallic and non-magnetic. The $Gd^{3+}$ ESR fine structure coalesces into a single dysonian resonance, as in metals. Still, our simulations suggest that exchange-narrowing via the usual Korringa mechanism, alone, is not capable of describing the thermal behavior of the ESR spectra in the entire temperature region ($4.2$ - $300$ K). We propose that temperature activated fluctuating-valence of the Ce ions is the missing ingredient that, added to the usual exchange-narrowing mechanism, fully describes this unique temperature dependence of the $Gd^{3+}$ ESR fine structure observed in $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$.Comment: 19 pages, 6 figure

Recent results on self-dual configurations on the torus

We review the recent progress on our understanding of self-dual SU(N) Yang-Mills configurations on the torus.Comment: Latex 3 pages, 1 figure. Contribution to the Lat99 Proceeding

An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies

Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening SiIII-selected absorbers and their relation to the circumgalactic medium (CGM) of galaxies at low redshift (z<=0.1), based on the analysis of UV absorption spectra along 303 extragalactic lines of sight obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). Along a total redshift path of Dz=24 we identify 69 intervening SiIII systems that all show associated absorption from other low and high ions. We derive a bias-corrected number density of dN/dz(SiIII)=2.5 for absorbers with column densities log N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section of the CGM around the local galaxy population and find excellent agreement between the model predictions and the observations. We further compare redshifts and positions of the absorbers with that of ~64,000 galaxies using archival galaxy-survey data. For the majority of the absorbers we identify possible host galaxies within 300 km/s of the absorbers and derive impact parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII absorbers and galaxies are highly correlated. Our study indicates that the majority of SiIII-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 per cent. From a detailed ionization model we estimate that diffuse gas in the CGM around galaxies, as traced by SiIII, contains substantially more baryonic mass than their neutral interstellar medium.Comment: 32 pages, 17 figures; final version accepted for publication in A&

Thermally activated exchange narrowing of the Gd3+ ESR fine structure in a single crystal of Ce1-xGdxFe4P12 (x = 0.001) skutterudite

We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As the temperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine and hyperfine structures coalesce into a broad inhomogeneous single resonance. At T = 200 K the line narrows and as T increases further, the resonance becomes homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest that the origin of these features may be associated to a subtle interdependence of thermally activated mechanisms that combine: i) an increase with T of the density of activated conduction-carriers across the T-dependent semiconducting pseudogap; ii) the Gd3+ Korringa relaxation process due to an exchange interaction, J_{fd}S.s, between the Gd3+ localized magnetic moments and the thermally activated conduction-carriers and; iii) a relatively weak confining potential of the rare-earth ions inside the oversized (Fe2P3)4 cage, which allows the rare-earths to become rattler Einstein oscillators above T = 148 K. We argue that the rattling of the Gd3+ ions, via a motional narrowing mechanism, also contributes to the coalescence of the ESR fine and hyperfine structure.Comment: 7 pages, 9 figures, accepted for publication in Phys Rev

What Produced the Ultraluminous Supernova Remnant in NGC 6946?

The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this remnant and its progenitor, we have obtained high-dispersion optical echelle spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and resolve these lines into a narrow (FWHM ~20--40 km/s) component from un-shocked material and a broad (FWHM ~250 km/s) component from shocked material. Both narrow and broad components have unusually high [N II]/H-alpha ratios, ~1. Using the echelle observation, archival HST images, and archival ROSAT X-ray observations, we conclude that the SNR was produced by a normal supernova, whose progenitor was a massive star, either a WN star or a luminous blue variable. The high luminosity of the remnant is caused by the supernova ejecta expanding into a dense, nitrogen-rich circumstellar nebula created by the progenitor.Comment: 20 pages, 5 figures. To be published in The Astronomical Journal, March 200