677 research outputs found
Amine-Linked Single Molecule Circuits: Systematic Trends Across Molecular Families
A comprehensive review is presented of single molecule junction conductance
measurements across families of molecules measured while breaking a gold point
contact in a solution of molecules with amine end groups. A theoretical
framework unifies the picture for the amine-gold link bonding and the tunnel
coupling through the junction using Density Functional Theory based
calculations. The reproducible electrical characteristics and utility for many
molecules is shown to result from the selective binding between the gold
electrodes and amine link groups through a donor-acceptor bond to
undercoordinated gold atoms. While the bond energy is modest, the maximum force
sustained by the junction is comparable to, but less than, that required to
break gold point contacts. The calculated tunnel coupling provides conductance
trends for all 41 molecule measurements presented here, as well as insight into
the variability of conductance due to the conformational changes within
molecules with torsional degrees of freedom. The calculated trends agree to
within a factor of two of the measured values for conductance ranging from 10-7
G0 to 10-2 G0, where G0 is the quantum of conductance (2e2/h).Comment: Invited paper for forthcoming special issue of Journal of Physics:
Condensed Matte
Single-Molecule Circuits with Well-Defined Molecular Conductance
We measure the conductance of amine-terminated molecules by breaking Au point
contacts in a molecular solution at room temperature. We find that the
variability of the observed conductance for the diamine molecule-Au junctions
is much less than the variability for diisonitrile and dithiol-Au junctions.
This narrow distribution enables unambiguous conductance measurements of single
molecules. For an alkane diamine series with 2-8 carbon atoms in the
hydrocarbon chain, our results show a systematic trend in the conductance from
which we extract a tunneling decay constant of 0.91 +/- 0.03 per methylene
group. We hypothesize that the diamine link binds preferentially to
undercoordinated Au atoms in the junction. This is supported by density
functional theory-based calculations that show the amine binding to a gold
adatom with sufficient angular flexibility for easy junction formation but
well-defined electronic coupling of the N lone pair to the Au. Therefore, the
amine linkage leads to well-defined conductance measurements of a single
molecule junction in a statistical study
9286 Stars: An Agglomeration of Stellar Polarization Catalogs
This is a revision. The revisions are minor. The new version of the catalog
should be used in preference to the old. The most serious error in the older
version was that was incorrect, being sometimes far too large,
for Reiz and Franco entries; the correct values are all zero for that
reference.
We present an agglomeration of stellar polarization catalogs with results for
9286 stars. We have endeavored to eliminate errors, provide accurate
(arcsecond) positions, sensibly weight multiple observations of the same star,
and provide reasonable distances. This catalog is included as an ASCII file
(catalog.txt) in the source of this submission.Comment: The most serious error in the older version was that
was incorrect, being sometimes far too large, for Reiz and Franco entries;
the correct values are all zero for that reference. 11 pages, no figures.
Accepted for Astronomical Journal. Catalog also available as an ASCII file by
anonymous FTP from ftp://vermi.berkeley.edu/pub/polcat/p14.ou
Modeling of the Super-Eddington Phase for Classical Novae: Five IUE Novae
We present a light curve model for the super-Eddington luminosity phase of
five classical novae observed with IUE. Optical and UV light curves are
calculated based on the optically thick wind theory with a reduced effective
opacity for a porous atmosphere. Fitting a model light curve with the UV 1455
\AA light curve, we determine the white dwarf mass and distance to be (1.3
M_sun, 4.4 kpc) for V693 CrA, (1.05 M_sun, 1.8 kpc) for V1974 Cyg, (0.95 M_sun,
4.1 kpc) for V1668 Cyg, (1.0 M_sun, 2.1 kpc) for V351 Pup, and (1.0 M_sun, 4.3
kpc) for OS And.Comment: 9 pages including 8 figures, to appear in the Astrophysical Journa
Shifts in Global Security Policies: Why They Matter for the South
The global security order has been evolving since 1989, led initially by the USA to expand its post?1945 order in Europe to the rest of the world but propelled as well by competition and debates within that post?Second World War alliance, as collective victors in the Cold War, about how to define a new international order. This article identifies three US policies that began this restructuring; their parallel redefinitions of security, and the tensions provoked by this agenda and its consequences, both within the ‘North’, replacing the ‘West’, between North and ‘South’, replacing the‘East’and the resulting multiple opportunities for alternative political coalitions, North against South, between North and South, and within the South, that have yet to play themselves out fully. The resulting fluidity has not yet stabilised into a new international security order
Dynamics of Line-Driven Winds from Disks in Cataclysmic Variables. I. Solution Topology and Wind Geometry
We analyze the dynamics of 2-D stationary, line-driven winds from accretion
disks in cataclysmic variable stars. The driving force is that of line
radiation pressure, in the formalism developed by Castor, Abbott & Klein for O
stars. Our main assumption is that wind helical streamlines lie on straight
cones. We find that the Euler equation for the disk wind has two eigenvalues,
the mass loss rate and the flow tilt angle with the disk. Both are calculated
self-consistently. The wind is characterized by two distinct regions, an outer
wind launched beyond four white dwarf radii from the rotation axis, and an
inner wind launched within this radius. The inner wind is very steep, up to 80
degrees with the disk plane, while the outer wind has a typical tilt of 60
degrees. In both cases the ray dispersion is small. We, therefore, confirm the
bi-conical geometry of disk winds as suggested by observations and kinematical
modeling. The wind collimation angle appears to be robust and depends only on
the disk temperature stratification. The flow critical points lie high above
the disk for the inner wind, but close to the disk photosphere for the outer
wind. Comparison with existing kinematical and dynamical models is provided.
Mass loss rates from the disk as well as wind velocity laws are discussed in a
subsequent paper.Comment: 21 pages, 10 Postscript figures; available also from
http://www.pa.uky.edu/~shlosman/publ.html. Astrophysical Journal, submitte
Hydrogen bonding of nitroxide spin labels in membrane proteins
On the basis of experiments at 275 GHz, we reconsider the dependence of the
continuous-wave EPR spectra of nitroxide spin-labeled protein sites in
sensory- and bacteriorhodopsin on the micro-environment. The high magnetic
field provides the resolution necessary to disentangle the effects of hydrogen
bonding and polarity. In the gxx region of the 275 GHz EPR spectrum, bands are
resolved that derive from spin-label populations carrying no, one or two
hydrogen bonds. The gxx value of each population varies hardly from site to
site, significantly less than deduced previously from studies at lower
microwave frequencies. The fractions of the populations vary strongly, which
provides a consistent description of the variation of the average gxx and the
average nitrogen-hyperfine interaction Azz from site to site. These variations
reflect the difference in the proticity of the micro-environment, and
differences in polarity contribute marginally. Concomitant W-band ELDOR-
detected NMR experiments on the corresponding nitroxide in perdeuterated water
resolve population-specific nitrogen-hyperfine bands, which underlies the
interpretation for the proteins
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