Light Collimation and Focussing by a Thin Flat Metallic Slab

We present experimental and theoretical work showing that a flat metallic slab can collimate and focus light impinging on the slab from a punctual source. The effect is optimised when the radiation is around the bulk, not at the surface, plasma frequency. And the smaller the imaginary part of the permittivity is, the better the collimation. Experiments for Ag in the visible as well as calculations are presented. We also discuss the interesting case of the Aluminium whose imaginary part of the permittivity is very small at the plasma frequency in UV radiation. Generalization to other materials and radiations are also discussed.Comment: 6 pages, 3 figures. To be published on Optics Lette

Dynamical windows for real-time evolution with matrix product states

We propose the use of a dynamical window to investigate the real-time evolution of quantum many-body systems in a one-dimensional lattice. In a recent paper [H. Phien et al, arxiv:????.????], we introduced infinite boundary conditions (IBC) in order to investigate real-time evolution of an infinite system under a local perturbation. This was accomplished by restricting the update of the tensors in the matrix product state to a finite window, with left and right boundaries held at fixed positions. Here we consider instead the use of a dynamical window, namely a window where the positions of left and right boundaries are allowed to change in time. In this way, all simulation efforts can be devoted to the space-time region of interest, which leads to a remarkable reduction in computational costs. For illustrative purposes, we consider two applications in the context of the spin-1 antiferromagnetic Heisenberg model in an infinite spin chain: one is an expanding window, with boundaries that are adjusted to capture the expansion in time of a local perturbation of the system; the other is a moving window of fixed size, where the position of the window follows the front of a propagating wave

A New High Resolution CO Map of the inner 2.'5 of M51 I. Streaming Motions and Spiral Structure

[Abridged] The Owens Valley mm-Array has been used to map the CO 1--0 emission in the inner 2'.5 of the grand design spiral galaxy M51 at 2''-3'' resolution. The molecular spiral arms are revealed with unprecedented clarity: supermassive cloud complexes, Giant Molecular Associations, are for the first time resolved both along and perpendicular to the arms. Major complexes occur symmetrically opposite each other in the two major arms. Streaming motions can be studied in detail along the major and minor axes of M51. The streaming velocities are very large, 60-150 km/s. For the first time, sufficient resolution to resolve the structure in the molecular streaming motions is obtained. Our data support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the SW arm, only one shows reduced shear. We find an expansion in the NE molecular arm at 25'' radius SE of the center. This broadening occurs right after the end of the NE arm at the Inner Lindblad Resonance. Bifurcations in the molecular spiral arm structure, at a radius of 73'', may be evidence of a secondary compression of the gas caused by the 4/1 ultraharmonic resonance. Inside the radius of the ILR, we detect narrow (~ 5'') molecular spiral arms possibly related to the K-band arms found in the same region. We find evidence of non-circular motions in the inner 20'' which are consistent with gas on elliptical orbits in a bar.Comment: 29 pages, 15 figures, uses latex macros for ApJ; accepted for publication in Ap

Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts

We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs). We find that the echo emission can provide an alternative way of understanding X-ray shallow decays and jet breaks. In particular, a shallow decay followed by a "normal" decay and a further rapid decay of X-ray afterglows can be together explained as being due to the echo from prompt X-ray emission scattered by dust grains in a massive wind bubble around a GRB progenitor. We also introduce an extra temporal break in the X-ray echo emission. By fitting the afterglow light curves, we can measure the locations of the massive wind bubbles, which will bring us closer to finding the mass loss rate, wind velocity, and the age of the progenitors prior to the GRB explosions.Comment: 25 pages, 3 figures, 2 tables. Accepted for publication in Ap

Bosonic versus fermionic pairs of topological spin defects in monolayered high-T_c superconductors

The energy associated with bosonic and fermionic pairs of topological spin defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated within a resonating valence bond scenario, as described by a t-t'-J-like model Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional screening of the electrostatic repulsion within the bosonic pairs. For parameters appropriate for monolayered high-T_c superconductors, both fermionic and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping such that the energy of the bosonic pairs goes below twice the energy of two fermionic pairs at their Fermi level. This finding could be related to the onset of high-T_c superconductivity.Comment: 10 pages, 6 figures. To be published in Phys. Rev.

Health of American Indian and Alaska Native Elders in California

Analyzes the health risks, health status, and utilization of and access to health services among American Indians/Alaska Natives age 55 and older in California, as well as trends in demographics and underlying factors including economic security

Analysis of curtailment at The Geysers geothermal Field, California

Geothermal energy has traditionally been viewed as a baseload energy source, but the rapid growth of intermittent renewable energy has led to a need for more flexibility in power generation to avoid mandatory curtailment imposed by grid operators. This study of curtailment at The Geysers provides insights into the magnitude, duration, frequency, temporal and spatial distribution, and potential causes of curtailment events between 2013 and 2018. Annual levels of curtailment range during this period from 9 to 47 GW h, representing 0.15 to 0.81 % of the net generation. Most curtailments occurred at the power plants connected to a lower capacity transmission line and may result from transmission constriction. There is a clear link between negative pricing and economic curtailment, especially when solar production is higher. Economic curtailment events tend to be only a few hours and vary in magnitude up to almost 300 MW, whereas transmission-related curtailment events can be up to several weeks in duration. It is likely that curtailment of geothermal power will be an increasing concern, and could be mitigated by flexible generation strategies and increases in energy storage. It is critical to know the nature of curtailment events so that flexible generation options can be assessed properly