3,460 research outputs found
Vector Fields with the Oriented Shadowing Property
We give a description of the \Cone-interior (\Int^1(\OrientSh)) of the
set of smooth vector fields on a smooth closed manifold that have the oriented
shadowing property. A special class \Bb of vector fields that are not
structurally stable is introduced. It is shown that the set
\Int^1(\OrientSh\setminus\Bb) coincides with the set of structurally stable
vector fields. An example of a field of the class \Bb belonging to
\Int^1(\OrientSh) is given. Bibliography: 18 titles.Comment: 42 page
Dramatic Changes in the Electronic Structure Upon Transition to the Collapsed Tetragonal Phase in CaFe2As2
We use angle-resolved photoemission spectroscopy (ARPES) and density
functional theory (DFT) calculations to study the electronic structure of
CaFeAs in previously unexplored collapsed tetragonal (CT) phase. This
unusual phase of the iron arsenic high temperature superconductors was hard to
measure as it exists only under pressure. By inducing internal strain, via the
post growth, thermal treatment of the single crystals, we were able to
stabilize the CT phase at ambient-pressure. We find significant differences in
the Fermi surface topology and band dispersion data from the more common
orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent
with electronic structure calculations. The top of the hole bands sinks below
the Fermi level, which destroys the nesting present in parent phases. The
absence of nesting in this phase along with apparent loss of Fe magnetic
moment, are now clearly experimentally correlated with the lack of
superconductivity in this phase.Comment: 5 pages, 4 figures, accepted in PRB(RC
Commensurate antiferromagnetic ordering in Ba(Fe{1-x}Co{x})2As2 determined by x-ray resonant magnetic scattering at the Fe K-edge
We describe x-ray resonant magnetic diffraction measurements at the Fe K-edge
of both the parent BaFe2As2 and superconducting Ba(Fe0.953Co0.047)2As2
compounds. From these high-resolution measurements we conclude that the
magnetic structure is commensurate for both compositions. The energy spectrum
of the resonant scattering is in reasonable agreement with theoretical
calculations using the full-potential linear augmented plane wave method with a
local density functional.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev. B Rapid
Com
Givental graphs and inversion symmetry
Inversion symmetry is a very non-trivial discrete symmetry of Frobenius
manifolds. It was obtained by Dubrovin from one of the elementary Schlesinger
transformations of a special ODE associated to a Frobenius manifold. In this
paper, we review the Givental group action on Frobenius manifolds in terms of
Feynman graphs and obtain an interpretation of the inversion symmetry in terms
of the action of the Givental group. We also consider the implication of this
interpretation of the inversion symmetry for the Schlesinger transformations
and for the Hamiltonians of the associated principle hierarchy.Comment: 26 pages; revised according to the referees' remark
Modeling Amphiphilic Solutes in a Jagla Solvent
Methanol is an amphiphilic solute whose aqueous solutions exhibit distinctive
physical properties. The volume change upon mixing, for example, is negative
across the entire composition range, indicating strong association. We explore
the corresponding behavior of a Jagla solvent, which has been previously shown
to exhibit many of the anomalous properties of water. We consider two models of
an amphiphilic solute: (i) a "dimer" model, which consists of one hydrophobic
hard sphere linked to a Jagla particle with a permanent bond, and (ii) a
"monomer" model, which is a limiting case of the dimer, formed by
concentrically overlapping a hard sphere and a Jagla particle. Using discrete
molecular dynamics, we calculate the thermodynamic properties of the resulting
solutions. We systematically vary the set of parameters of the dimer and
monomer models and find that one can readily reproduce the experimental
behavior of the excess volume of the methanolwater system as a function of
methanol volume fraction. We compare the pressure and temperature dependence of
the excess volume and the excess enthalpy of both models with experimental data
on methanol-water solutions and find qualitative agreement in most cases. We
also investigate the solute effect on the temperature of maximum density and
find that the effect of concentration isorders of magnitude stronger than
measured experimentally
Pressure-induced collapsed-tetragonal phase in SrCo2As2
We present high-energy x-ray diffraction data under applied pressures up to p
= 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical
resistance measurements up to p = 5.9 GPa, on SrCo2As2. Our x-ray diffraction
data demonstrate that there is a first-order transition between the tetragonal
(T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6
GPa at T = 7 K. The pressure for the onset of the cT phase and the range of
coexistence between the T and cT phases appears to be nearly temperature
independent. The compressibility along the a-axis is the same for the T and cT
phases whereas, along the c-axis, the cT phase is significantly stiffer, which
may be due to the formation of an As-As bond in the cT phase. Our resistivity
measurements found no evidence of superconductivity in SrCo2As2 for p <= 5.9
GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a
pressure-induced phase transition for p >= 5.5 GPa. Single-crystal neutron
diffraction measurements performed up to 1.1 GPa in the T phase found no
evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K.
Spin-polarized total-energy calculations demonstrate that the cT phase is the
stable phase at high pressure with a c/a ratio of 2.54. Furthermore, these
calculations indicate that the cT phase of SrCo2As2 should manifest either
A-type antiferromagnetic or ferromagnetic order.Comment: 6 pages, 5 figure
Long-lived states in synchronized traffic flow. Empirical prompt and dynamical trap model
The present paper proposes a novel interpretation of the widely scattered
states (called synchronized traffic) stimulated by Kerner's hypotheses about
the existence of a multitude of metastable states in the fundamental diagram.
Using single vehicle data collected at the German highway A1, temporal velocity
patterns have been analyzed to show a collection of certain fragments with
approximately constant velocities and sharp jumps between them. The particular
velocity values in these fragments vary in a wide range. In contrast, the flow
rate is more or less constant because its fluctuations are mainly due to the
discreteness of traffic flow.
Subsequently, we develop a model for synchronized traffic that can explain
these characteristics. Following previous work (I.A.Lubashevsky, R.Mahnke,
Phys. Rev. E v. 62, p. 6082, 2000) the vehicle flow is specified by car
density, mean velocity, and additional order parameters and that are
due to the many-particle effects of the vehicle interaction. The parameter
describes the multilane correlations in the vehicle motion. Together with the
car density it determines directly the mean velocity. The parameter , in
contrast, controls the evolution of only. The model assumes that
fluctuates randomly around the value corresponding to the car configuration
optimal for lane changing. When it deviates from this value the lane change is
depressed for all cars forming a local cluster. Since exactly the overtaking
manoeuvres of these cars cause the order parameter to vary, the evolution
of the car arrangement becomes frozen for a certain time. In other words, the
evolution equations form certain dynamical traps responsible for the long-time
correlations in the synchronized mode.Comment: 16 pages, 10 figures, RevTeX
Effects of transition metal substitutions on the incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering
The spin fluctuation spectra from nonsuperconducting Cu-substituted, and
superconducting Co-substituted, BaFe2As2 are compared quantitatively by
inelastic neutron scattering measurements and are found to be indis-
tinguishable. Whereas diffraction studies show the appearance of incommensurate
spin-density wave order in Co and Ni substituted samples, the magnetic phase
diagram for Cu substitution does not display incommensu- rate order,
demonstrating that simple electron counting based on rigid-band concepts is
invalid. These results, supported by theoretical calculations, suggest that
substitutional impurity effects in the Fe plane play a signifi- cant role in
controlling magnetism and the appearance of superconductivity, with Cu
distinguished by enhanced impurity scattering and split-band behavior.Comment: 5 pages, 5 figures, Major change in the manuscrip
Metallic surface electronic state in half-Heusler compounds RPtBi (R = Lu, Dy, Gd)
Rare-earth platinum bismuth (RPtBi) has been recently proposed to be a
potential topological insulator. In this paper we present measurements of the
metallic surface electronic structure in three members of this family, using
angle resolved photoemission spectroscopy (ARPES). Our data shows clear
spin-orbit splitting of the surface bands and the Kramers' degeneracy of spins
at the Gamma and M points, which is nicely reproduced with our full-potential
augmented plane wave calculation for a surface electronic state. No direct
indication of topologically non-trivial behavior is detected, except for a weak
Fermi crossing detected in close vicinity to the Gamma point, making the total
number of Fermi crossings odd. In the surface band calculation, however, this
crossing is explained by another Kramers' pair where the two splitting bands
are very close to each other. The classification of this family of materials as
topological insulators remains an open question.Comment: 6 pages, 5 figure
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