8,727 research outputs found
Preferential attachment during the evolution of a potential energy landscape
It has previously been shown that the network of connected minima on a
potential energy landscape is scale-free, and that this reflects a power-law
distribution for the areas of the basins of attraction surrounding the minima.
Here, we set out to understand more about the physical origins of these
puzzling properties by examining how the potential energy landscape of a
13-atom cluster evolves with the range of the potential. In particular, on
decreasing the range of the potential the number of stationary points increases
and thus the landscape becomes rougher and the network gets larger. Thus, we
are able to follow the evolution of the potential energy landscape from one
with just a single minimum to a complex landscape with many minima and a
scale-free pattern of connections. We find that during this growth process, new
edges in the network of connected minima preferentially attach to more
highly-connected minima, thus leading to the scale-free character. Furthermore,
minima that appear when the range of the potential is shorter and the network
is larger have smaller basins of attraction. As there are many of these smaller
basins because the network grows exponentially, the observed growth process
thus also gives rise to a power-law distribution for the hyperareas of the
basins.Comment: 10 pages, 10 figure
Paramagnon dispersion in -FeSe observed by Fe -edge resonant inelastic x-ray scattering
We report an Fe -edge resonant inelastic x-ray scattering (RIXS) study of
the unusual superconductor -FeSe. The high energy resolution of this
RIXS experiment (55meV FWHM) made it possible to resolve
low-energy excitations of the Fe manifold. These include a broad peak
which shows dispersive trends between 100-200meV along the and
directions of the one-Fe square reciprocal lattice, and which can
be attributed to paramagnon excitations. The multi-band valence state of FeSe
is among the most metallic in which such excitations have been discerned by
soft x-ray RIXS
Remote participation during glycosylation reactions of galactose building blocks: Direct evidence from cryogenic vibrational spectroscopy
The stereoselective formation of 1,2âcisâglycosidic bonds is challenging. However, 1,2âcisâselectivity can be induced by remote participation of C4 or C6 ester groups. Reactions involving remote participation are believed to proceed via a key ionic intermediate, the glycosyl cation. Although mechanistic pathways were postulated many years ago, the structure of the reaction intermediates remained elusive owing to their shortâlived nature. Herein, we unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory. Acetyl groups at C4 ensure αâselective galactosylations by forming a covalent bond to the anomeric carbon in dioxoleniumâtype ions. Unexpectedly, also benzyl ether protecting groups can engage in remote participation and promote the stereoselective formation of 1,2âcisâglycosidic bonds
CP violation in gauge theories
We define the CP transformation properties of scalars, fermions and vectors
in a gauge theory and show that only three types of interactions can lead to CP
violation: scalar interactions, fermion-scalar interactions and
associated with the strong CP problem and which involve only the gauge fields.
For technicolor theories this implies the absence of CP violation within
perturbation theory.Comment: 5 pages, 1 figure, revtex and epsf require
Two-neutron separation energies, binding energies and phase transitions in the interacting boson model
In the framework of the interacting boson model the three transitional
regions (rotational-vibrational, rotational--unstable and,
vibrational--unstable transitions) are reanalyzed. A new kind of plot
is presented for studying phase transitions in finite systems such as atomic
nuclei. The importance of analyzing binding energies and not only energy
spectra and electromagnetic transitions, describing transitional regions is
emphasized. We finally discuss a number of realistic examples.Comment: 34 pages, TeX (ReVTeX). 12 ps figures. 3 tables. Submitted to Nucl.
Phys.
LSST optical beam simulator
We describe a camera beam simulator for the LSST which is capable of
illuminating a 60mm field at f/1.2 with realistic astronomical scenes, enabling
studies of CCD astrometric and photometric performance. The goal is to fully
simulate LSST observing, in order to characterize charge transport and other
features in the thick fully depleted CCDs and to probe low level systematics
under realistic conditions. The automated system simulates the centrally
obscured LSST beam and sky scenes, including the spectral shape of the night
sky. The doubly telecentric design uses a nearly unit magnification design
consisting of a spherical mirror, three BK7 lenses, and one beam-splitter
window. To achieve the relatively large field the beam-splitter window is used
twice. The motivation for this LSST beam test facility was driven by the need
to fully characterize a new generation of thick fully-depleted CCDs, and assess
their suitability for the broad range of science which is planned for LSST. Due
to the fast beam illumination and the thick silicon design [each pixel is 10
microns wide and over 100 microns deep] at long wavelengths there can be
effects of photon transport and charge transport in the high purity silicon.
The focal surface covers a field more than sufficient for a 40x40 mm LSST CCD.
Delivered optical quality meets design goals, with 50% energy within a 5 micron
circle. The tests of CCD performance are briefly described.Comment: 9 pages, 9 figure
Phantom Black Holes and Sigma Models
We construct static multicenter solutions of phantom Einstein-Maxwell-dilaton
theory from null geodesics of the target space, leading to regular black holes
without spatial symmetry for certain discrete values of the dilaton coupling
constant. We also discuss the three-dimensional gravitating sigma models
obtained by reduction of phantom Einstein-Maxwell, phantom Kaluza-Klein and
phantom Einstein-Maxwell-dilaton-axion theories. In each case, we generate by
group transformations phantom charged black hole solutions from a neutral seed.Comment: 19 page
A constant dark matter halo surface density in galaxies
We confirm and extend the recent finding that the central surface density
r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core
radius and central density, is nearly constant and independent of galaxy
luminosity. Based on the co-added rotation curves of about 1000 spiral
galaxies, mass models of individual dwarf irregular and spiral galaxies of late
and early types with high-quality rotation curves and, galaxy-galaxy weak
lensing signals from a sample of spiral and elliptical galaxies, we find that
log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the
observed kinematics of Local Group dwarf spheroidal galaxies are consistent
with this value. Our results are obtained for galactic systems spanning over 14
magnitudes, belonging to different Hubble Types, and whose mass profiles have
been determined by several independent methods. In the same objects, the
approximate constancy of rho_0*r_0 is in sharp contrast to the systematical
variations, by several orders of magnitude, of galaxy properties, including
rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure
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