1,842 research outputs found
The new HiVIS spectropolarimeter and spectropolarimetric calibration of the AEOS telescope
We designed, built, and calibrated a new spectropolarimeter for the HiVIS
spectrograph (R 12000-49000) on the AEOS telescope. We also did a polarization
calibration of the telescope and instrument. We will introduce the design and
use of the spectropolarimeter as well as a new data reduction package we have
developed, then discuss the polarization calibration of the spectropolarimeter
and the AEOS telescope. We used observations of unpolarized standard stars at
many pointings to measure the telescope induced polarization and compare it
with a Zemax model. The telescope induces polarization of 1-6% with a strong
variation with wavelength and pointing, consistent with the altitude and
azimuth variation expected. We then used scattered sunlight as a linearly
polarized source to measure the telescopes spectropolarimetric response to
linearly polarized light. We then made an all-sky map of the telescope's
polarization response to calibrate future spectropolarimetry.Comment: PASP 118, June 200
Atomic Scale Memory at a Silicon Surface
The limits of pushing storage density to the atomic scale are explored with a
memory that stores a bit by the presence or absence of one silicon atom. These
atoms are positioned at lattice sites along self-assembled tracks with a pitch
of 5 atom rows. The writing process involves removal of Si atoms with the tip
of a scanning tunneling microscope. The memory can be reformatted by controlled
deposition of silicon. The constraints on speed and reliability are compared
with data storage in magnetic hard disks and DNA.Comment: 13 pages, 5 figures, accepted by Nanotechnolog
Mars and the early Sun
Global mean temperatures near 273 K on early Mars are difficult to explain in the context of standards solar evolution models. Even assuming maximum CO2 greenhouse warming, the required flux is approximately 15 percent too low. Here we consider two astrophysical models that could increase the flux by this amount. The first model is a nonstandard solar model in which the early Sun had a mass somewhat greater than today's mass (1.02-1.06 solar mass). The second model is based on a standard evolutionary solar model, but the ecliptic flux is increased due to focusing by an (expected) heavily spotted early Sun
Structure and Stability of Si(114)-(2x1)
We describe a recently discovered stable planar surface of silicon, Si(114).
This high-index surface, oriented 19.5 degrees away from (001) toward (111),
undergoes a 2x1 reconstruction. We propose a complete model for the
reconstructed surface based on scanning tunneling microscopy images and
first-principles total-energy calculations. The structure and stability of
Si(114)-(2x1) arises from a balance between surface dangling bond reduction and
surface stress relief, and provides a key to understanding the morphology of a
family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures
is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published
in Phys. Rev. Let
Engineering electron and hole tunneling with asymmetric InAs quantum dot molecules
Most self-assembled quantum dot molecules are intrinsically asymmetric with
inequivalent dots resulting from imperfect control of crystal growth. We have
grown vertically-aligned pairs of InAs/GaAs quantum dots by molecular beam
epitaxy, introducing intentional asymmetry that limits the influence of
intrinsic growth fluctuations and allows selective tunneling of electrons or
holes. We present a systemic investigation of tunneling energies over a wide
range of interdot barrier thickness. The concepts discussed here provide an
important tool for the systematic design and characterization of more
complicated quantum dot nanostructures.Comment: 9 pages, 4 figures. To be published in Applied Physics Letters.
version 2: Typographical corrections and reference format for journa
First principles theory of inelastic currents in a scanning tunneling microscope
A first principles theory of inelastic tunneling between a model probe tip
and an atom adsorbed on a surface is presented, extending the elastic tunneling
theory of Tersoff and Hamann. The inelastic current is proportional to the
change in the local density of states at the center of the tip due to the
addition of the adsorbate. We use the theory to investigate the vibrational
heating of an adsorbate below an STM tip. We calculate the desorption rate of H
from Si(100)-H(21) as function of the sample bias and tunnel current,
and find excellent agreement with recent experimental data.Comment: 5 pages, RevTeX, epsf file
Asymptotic Structure of Symmetry Reduced General Relativity
Gravitational waves with a space-translation Killing field are considered. In
this case, the 4-dimensional Einstein vacuum equations are equivalent to the
3-dimensional Einstein equations with certain matter sources. This interplay
between 4- and 3- dimensional general relativity can be exploited effectively
to analyze issues pertaining to 4 dimensions in terms of the 3-dimensional
structures. An example is provided by the asymptotic structure at null
infinity: While these space-times fail to be asymptotically flat in 4
dimensions, they can admit a regular completion at null infinity in 3
dimensions. This completion is used to analyze the asymptotic symmetries,
introduce the analog of the 4-dimensional Bondi energy-momentum and write down
a flux formula.
The analysis is also of interest from a purely 3-dimensional perspective
because it pertains to a diffeomorphism invariant 3-dimensional field theory
with {\it local} degrees of freedom, i.e., to a midi-superspace. Furthermore,
due to certain peculiarities of 3 dimensions, the description of null infinity
does have a number of features that are quite surprising because they do not
arise in the Bondi-Penrose description in 4 dimensions.Comment: 39 Pages, REVTEX, CGPG-96/5-
Search for polarization in Ξ0 hyperons
Inclusive hyperon production by 400 GeV protons at Fermilab has shown that the hyperons are produced with significant polarization. However no polarization has been seen for Λ’s produced at these energies. In this paper we present the results of a searcch for Ξ0 polarization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87405/2/126_1.pd
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