3,364 research outputs found
Constant Size Molecular Descriptors For Use With Machine Learning
A set of molecular descriptors whose length is independent of molecular size
is developed for machine learning models that target thermodynamic and
electronic properties of molecules. These features are evaluated by monitoring
performance of kernel ridge regression models on well-studied data sets of
small organic molecules. The features include connectivity counts, which
require only the bonding pattern of the molecule, and encoded distances, which
summarize distances between both bonded and non-bonded atoms and so require the
full molecular geometry. In addition to having constant size, these features
summarize information regarding the local environment of atoms and bonds, such
that models can take advantage of similarities resulting from the presence of
similar chemical fragments across molecules. Combining these two types of
features leads to models whose performance is comparable to or better than the
current state of the art. The features introduced here have the advantage of
leading to models that may be trained on smaller molecules and then used
successfully on larger molecules.Comment: 18 pages, 5 figure
Role of domain walls in the abnormal photovoltaic effect in BiFeO3
Recently, the anomalous photovoltaic (PV) effect in BiFeO3 (BFO) thin
films, which resulted in open circuit voltages (V-oc) considerably
larger than the band gap of the material, has generated a revival of the
entire field of photoferroelectrics. Here, via temperature-dependent PV
studies, we prove that the bulk photovoltaic (BPV) effect, which has
been studied in the past for many non-centrosymmetric materials, is at
the origin of the anomalous PV effect in BFO films. Moreover, we show
that irrespective of the measurement geometry, V-oc as high as 50V can
be achieved by controlling the conductivity of domain walls (DW). We
also show that photoconductivity of the DW is markedly higher than in
the bulk of BFO
Size of Fireballs Created in High Energy Lead-Lead Collisions as Inferred from Coulomb Distortions of Pion Spectra
We compute the Coulomb effects produced by an expanding, highly charged
fireball on the momentum distribution of pions. We compare our results to data
on Au+Au at 11.6 A GeV from E866 at the BNL AGS and to data on Pb+Pb at 158 A
GeV from NA44 at the CERN SPS. We conclude that the distortion of the spectra
at low transverse momentum and mid-rapidity can be explained in both
experiments by the effect of the large amount of participating charge in the
central rapidity region. By adjusting the fireball expansion velocity to match
the average transverse momentum of protons, we find a best fit when the
fireball radius is about 10 fm, as determined by the moment when the pions
undergo their last scattering. This value is common to both the AGS and CERN
experiments.Comment: Enlarged discussion, new references added, includes new analysis of
pi-/pi+ at AGS energies. 12 pages 5 figures, uses LaTex and epsfi
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Low Energy Analyzing Powers in Pion-Proton Elastic Scattering
Analyzing powers of pion-proton elastic scattering have been measured at PSI
with the Low Energy Pion Spectrometer LEPS as well as a novel polarized
scintillator target. Angular distributions between 40 and 120 deg (c.m.) were
taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic
energy for pi+ p scattering, and at 67.3 and 87.2 MeV for pi- p scattering.
These new measurements constitute a substantial extension of the polarization
data base at low energies. Predictions from phase shift analyses are compared
with the experimental results, and deviations are observed at low energies.Comment: 15 pages, 4 figure
Adaptive Optimization of Wave Functions for Fermion Lattice Models
We present a simulation algorithm for Hamiltonian fermion lattice models. A
guiding trial wave function is adaptively optimized during Monte Carlo
evolution. We apply the method to the two dimensional Gross-Neveu model and
analyze systematc errors in the study of ground state properties. We show that
accurate measurements can be achieved by a proper extrapolation in the
algorithm free parameters.Comment: 4 pages, 6 figures (Encapsulated PostScript
Confronting particle emission scenarios with strangeness data
We show that a hadron gas model with continuous particle emission instead of
freeze out may solve some of the problems (high values of the freeze out
density and specific net charge) that one encounters in the latter case when
studying strange particle ratios such as those by WA85. This underlines the
necessity to understand better particle emission in hydrodynamics to be able to
analyze data. It also re-opens the possibility of a quark-hadron transition
occuring with phase equilibrium instead of explosively.Comment: 13 pages, 2 figures, to appear in Phys. Rev. Let
A quark model framework for the study of nuclear medium effects
A quark-model framework for studying nuclear medium effects on nucleon
resonances is described and applied here to pion photoproduction on the
deuteron, which is the simplest composite nucleon system and serves as a first
test case. Pion photoproduction on nuclei is discussed within a chiral
constituent quark model in which the quark degrees of freedom are explicitly
introduced through an effective chiral Lagrangian for the
quark-pseudoscalar-meson coupling. The advantage of this model is that a
complete set of nucleon resonances can be systematically included with a
limited number of parameters. Also, the systematic description of the nucleon
and its resonances at quark level allows us to self-consistently relate the
nuclear medium's influence on the baryon properties to the intrinsic dynamic
aspects of the baryons. As the simplest composite nucleus, the deuteron
represents the first application of this effective theory for meson
photoproduction on light nuclei. The influence of the medium on the transition
operators for a free nucleon is investigated in the Delta resonance region. No
evidence is found for a change of the Delta properties in the pion
photoproduction reaction on the deuteron since the nuclear medium here involves
just one other nucleon and the low binding energy implies low nuclear density.
However, we show that the reaction mechanism is in principle sensitive to
changes of Delta properties that would be produced by the denser nuclear medium
of heavier nuclei through the modification of the quark model parameters.Comment: Revtex, 8 pages, 4 figure
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