Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon

We use accurate first principles methods to study the network dynamics of hydrogenated amorphous silicon, including the motion of hydrogen. In addition to studies of atomic dynamics in the electronic ground state, we also adopt a simple procedure to track the H dynamics in light-excited states. Consistent with recent experiments and computer simulations, we find that dihydride structures are formed for dynamics in the light-excited states, and we give explicit examples of pathways to these states. Our simulations appear to be consistent with aspects of the Staebler-Wronski effect, such as the light-induced creation of well separated dangling bonds.Comment: 9 pages, 8 figures, submitted to PR

Reducing the Effects of Unequal Number of Games on Rankings

Ranking is an important mathematical process in a variety of contexts such as information retrieval, sports and business. Sports ranking methods can be applied both in and beyond the context of athletics. In both settings, once the concept of a game has been defined, teams (or individuals) accumulate wins, losses, and ties, which are then factored into the ranking computation. Many settings involve an unequal number of games between competitors. This paper demonstrates how to adapt two sports rankings methods, the Colley and Massey ranking methods, to settings where an unequal number of games are played between the teams. In such settings, the standard derivations of the methods can produce nonsensical rankings. This paper introduces the idea of including a super-user into the rankings and considers the effect of this fictitious player on the ratings. We apply such techniques to rank batters and pitchers in Major League baseball, professional tennis players, and participants in a free online social game. The ideas introduced in this paper can further the scope that such methods are applied and the depth of insight they offer

The large scale distribution of warm ionized gas around nearby radio galaxies with jet-cloud interactions

Deep, narrow-band Halpha observations taken with the TAURUS Tunable Filter (TTF) on the 4.2m WHT telescope are presented for two nearby radio galaxies with strong jet-cloud interactions. Although the brightest emission line components are closely aligned with the radio jets --- providing nearby examples of the ``alignment effect'' most commonly observed in high redshift (z > 0.5) radio galaxies --- lower surface brightness emission line structures are detected at large distances (10's of kpc) from the radio jet axis. These latter structures cannot be reconciled with anisotropic illumination of the ISM by obscured quasar-like sources, since parts of the structures lay outside any plausible quasar ionization cones. Rather, the distribution of the emission lines around the fringes of the extended radio lobes suggests that the gas is ionized either by direct interaction with the radio components, or by the diffuse photoionizing radiation fields produced in the shocks generated in such interactions. These observations serve to emphasise that the ionizing effects of the radio components can extend far from the radio jet axes, and that deep emission line imaging observations are required to reveal the true distribution of warm gas in the host galaxies. We expect future deep imaging observations to reveal similar structures perpendicular to the radio axes in the high-z radio galaxies.Comment: 18 pages, 4 figures, to be published in MNRA

Electronic Excitations and Insulator-Metal Transition in Poly(3-hexylthiophene) Organic Field-Effect Transistors

We carry out a comprehensive theoretical and experimental study of charge injection in Poly(3-hexylthiophene) (P3HT) to determine the most likely scenario for metal-insulator transition in this system. We calculate the optical absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first-- or a second--order metal--insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin film field-effect transistors (FET). Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal--insulator transition.Comment: 9 pages, 4 figures. Phys. Rev. B, in pres

PKS1932-46: a radio source in an interacting group?

We present the results of a multiwavelength study of the z=0.23 radio source PKS1932-46. VIMOS IFU spectroscopy is used to study the morphology, kinematics and ionisation state of the EELR surrounding this source, and also a companion galaxy at a similar redshift. Near- and far-IR imaging observations obtained using the NTT and SPITZER are used to analyse the underlying galaxy morphologies and the nature of the AGN. The host galaxy is identified as an ~M* elliptical. Combining Spitzer mid-IR with X-ray, optical and near-IR imaging observations of this source, we conclude that its AGN is underluminous for a radio source of this type, despite its status as a BLRG. However, given its relatively large [OIII] luminosity it is likely that the AGN was substantially more luminous in the recent past (<10^4 years ago). The EELR is remarkably extensive and complex, reminiscent of the systems observed around sources at higher redshifts/radio powers, and the gas is predominantly ionised by a mixture of AGN photoionisation and emission from young stars. We confirm the presence of a series of star-forming knots extending N-S from the host galaxy, with more prodigious star formation occuring in the merging companion galaxy to the northeast, which has sufficient luminosity at mid- to far-IR wavelengths to be classified as a LIRG. The most plausible explanation of our observations is that PKS1932-46 is a member of an interacting galaxy group, and that the impressive EELR is populated by star-forming, tidal debris. We suggest that the AGN itself may currently be fuelled by material associated either with the current interaction, or with a previous merger event. Surprisingly, it is the companion object, rather than the radio source host galaxy, which is undergoing the bulk of the star formation activity within the group.Comment: 15 pages, 14 figures (compressed for astro-ph, 1 colour). Accepted for publication in MNRAS. Abstract abridge

SMASH 1 : A VERY FAINT GLOBULAR CLUSTER DISRUPTING in the OUTER REACHES of the LMC?

We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the Magellanic Stellar History (SMASH), SMASH 1 is a compact (rh=9.1-3.4+5.9pc) and very low luminosity (Mv=-1.0±0.9,Lv=102.3±0.4L⊙ ) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of ∼18.8, i.e., a distance of . Situated at 11.°3 from the LMC in projection, its three-dimensional distance from the Cloud is 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least . Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only ≲19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (ϵ=0.62-0.21+0.17) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.Peer reviewe

Compton Echoes from Gamma-ray Bursts

Recent observations of gamma-ray bursts (GRBs) have provided growing evidence for collimated outflows and emission, and strengthened the connection between GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard photon pulse will propagate in the pre-burst, dense environment. Circumstellar material will Compton scatter the prompt GRB radiation and give rise to a reflection echo. We calculate luminosities, spectra, and light curves of such Compton echoes in a variety of emission geometries and ambient gas distributions, and show that the delayed hard X-ray flash from a pulse propagating into a red supergiant wind could be detectable by Swift out to z~0.2. Independently of the gamma-ray spectrum of the prompt burst, reflection echoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^2 because of Compton downscattering. At fixed burst energy per steradian, the luminosity of the reflected echo is proportional to the beaming solid angle, Omega_b, of the prompt pulse, while the number of bright echoes detectable in the sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it is smaller in the case of more collimated jets. The lack of an X-ray echo at one month delay from the explosion poses severe constraints on the possible existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed in GRB990123 is fainter than the optical echo expected in a dense red supergiant environment from a isotropic prompt optical flash. Significant MeV delayed emission may be produced through the bulk Compton (or Compton drag) effect resulting from the interaction of the decelerating fireball with the scattered X-ray radiation.Comment: LaTeX, 18 pages, 4 figures, revised version accepted for publication in the Ap

A study to define meteorological uses and performance requirements for the Synchronous Earth Observatory Satellite

The potential meteorological uses of the Synchronous Earth Observatory Satellite (SEOS) were studied for detecting and predicting hazards to life, property, or the quality of the environment. Mesoscale meteorological phenonmena, and the observations requirements for SEOS are discussed along with the sensor parameters