Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies

The narrow emission line spectra of active galactic nuclei are not accurately described by simple photoionization models of single clouds. Recent Hubble Space Telescope images of Seyfert 2 galaxies show that these objects are rich with ionization cones, knots, filaments, and strands of ionized gas. Here we extend to the narrow line region the ``locally optimally emitting cloud'' (LOC) model, in which the observed spectra are predominantly determined by powerful selection effects. We present a large grid of photoionization models covering a wide range of physical conditions and show the optimal conditions for producing many of the strongest emission lines. We show that the integrated narrow line spectrum can be predicted by an integration of an ensemble of clouds, and we present these results in the form of diagnostic line ratio diagrams making comparisons with observations. We also predict key diagnostic line ratios as a function of distance from the ionizing source, and compare these to observations. The predicted radial dependence of the [O III]/[O II] ratio may be matched to the observed one in NGC4151, if the narrow line clouds see a more intense continuum than we see. The LOC scenario when coupled with a simple Keplerian gravitational velocity field will quite naturally predict the observed line width versus critical density relationship. The influence of dust within the ionized portion of the clouds is discussed and we show that the more neutral gas is likely to be dusty, although a high ionization dust-free region is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap

Measured and computed stresses in three castellated beams

"Reprinted from AISC Journal, January, 1966.

Wigner solids of wide quantum wells near Landau filling ν=1\nu=1

Microwave spectroscopy within the Landau filling ($\nu$) range of the integer quantum Hall effect (IQHE) has revealed pinning mode resonances signifying Wigner solids (WSs) composed of quasi-particles or -holes. We study pinning modes of WSs in wide quantum wells (WQWs) for $0.8\le\nu\le1.2$, varying the density, $n$, and tilting the sample by angle $\theta$ in the magnetic field. Three distinct WS phases are accessed. One phase, S1, is phenomenologically the same as the WS observed in the IQHEs of narrow QWs. The second phase, S2, exists at $\nu$ further from $\nu=1$ than S1, and requires a sufficiently large $n$ or $\theta$, implying S2 is stabilized by the Zeeman energy. The melting temperatures of S1 and S2, estimated from the disappearance of the pinning mode, show different behavior vs $\nu$. At the largest $n$ or $\theta$, S2 disappears and the third phase, S1A, replaces S1, also exhibiting a pinning mode. This occurs as the WQW $\nu=1$ IQHE becomes a two-component, Halperin-Laughlin \pone state. We interpret S1A as a WS of the excitations of \pone, which has not been previously observed

Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer

We report on quantum interference between a pair of counterpropagating quantum Hall edge states that are separated by a high quality tunnel barrier. Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling between coupled Luttinger liquids that creates bound electronic states between pairs of tunnel centers that act like interference slits. We place a lower bound in the range of 20-40 $\mu$m for the phase coherence length and directly confirm the extended phase coherence of quantum Hall edge states.Comment: 4 pages, 3 figures, 1 tabl

Observation of a One-Dimensional Spin-Orbit Gap in a Quantum Wire

Understanding the flow of spins in magnetic layered structures has enabled an increase in data storage density in hard drives over the past decade of more than two orders of magnitude1. Following this remarkable success, the field of 'spintronics' or spin-based electronics is moving beyond effects based on local spin polarisation and is turning its attention to spin-orbit interaction (SOI) effects, which hold promise for the production, detection and manipulation of spin currents, allowing coherent transmission of information within a device. While SOI-induced spin transport effects have been observed in two- and three-dimensional samples, these have been subtle and elusive, often detected only indirectly in electrical transport or else with more sophisticated techniques. Here we present the first observation of a predicted 'spin-orbit gap' in a one-dimensional sample, where counter-propagating spins, constituting a spin current, are accompanied by a clear signal in the easily-measured linear conductance of the system.Comment: 10 pages, 5 figures, supplementary informatio