31,294 research outputs found
Optical absorption preceding resonant double photoionization of aromatic hydrocarbons hydrocarbons
We analyze resonances in the double photoionization of a variety of aromatic
hydrocarbons. The resonances reflect the breakup of quasi-bound electron pairs.
The basic premise of this paper is that there is a direct connection between
the quasi-bound pairs and resonant peaks in the optical absorption that are
associated with doubly occupied sites on the perimeter and inside the perimeter
of the molecule. The optical absorption leading to the high-energy resonance
(approximately 40 eV), calculated from a many-site one-dimensional Hubbard
model, has a peak at U, the electrostatic interaction energy for two electrons
with antiparallel spins on the same carbon atom. In the model, there are also
two satellites whose separation from the main resonance is approximately +/-10
eV suggesting that unresolved satellite structure may be contributing to the
linewidth of the resonant peak. The low energy resonances (approximately 10 eV)
involve carbon atoms located inside the perimeter, a configuration present only
in pyrene and coronene (among the hydrocarbons studied). In the case of pyrene,
which has two carbon atoms inside the perimeter, we employ a two-site Hubbard
model to characterize the absorption leading to the quasi-bound state. A brief
analysis of the double photoionization resonance of the heterocyclic inorganic
molecule 1,3,5-triazine presented. We also discuss recent results for the
double photoionization of the cyclic inorganic molecule tribromoborazine and
the organic molecules furan, pyrrole, selenophene, and thiophene where the 2+
ion concentration varies linearly with the difference between the photon energy
and the threshold energy. A theory for the linear behavior is outlined
Coulomb Pairing and Double Photoionization in Aromatic Hydrocarbons
Recently reported anomalies in the double-photonionization spectra of the
aromatic molecules partially deuterated benzene, naphthalene, anthracene,
pentacene, azulene, phenanthrene, pyrene and coronene are attributed to
Coulomb-pair resonances of pi electrons. The properties of the resonance in
benzene are investigated in detail. The linear behavior in the 2+/1+ ion ratio
above the resonance is attributed to a two-electron transition associated with
excitation from the ground state to a two-electron continuum. A similar
explanation accounts for the linear behavior seen in the pentagonal rings
pyrrole, furan, selenophene and thiophene which do not display resonance peaks.Comment: 6 pages. arXiv admin note: substantial text overlap with
arXiv:1312.049
Comments on Coulomb pairing in aromatic hydrocarbons
Recently reported anomalies in the double-photonionization spectra of
aromatic molecules such as benzene, naphthalene, anthracene and coronene are
attributed to Coulomb-pair resonances of pi electrons.Comment: 5 page
Linear temperature dependence of electron spin resonance linewidths in La0.7Ca0.3MnO3 and YBaMn2O6
We analyze recent electron spin resonance (ESR) experiments in La0.7Ca0.3MnO3
and YBaMn2O6 focusing on the behavior of the linewidth at high temperatures
where it is a linear function of the temperature. Noting that the g-factors of
the resonances are characteristic of the Mn4+ ion in a cubic environment, we
make the assumption that the linewidth involves the static susceptibility of
the Mn4+ spins which we analyze in the molecular field approximation. We
conclude that the linear dependence on temperature is associated with the
susceptibility having a Curie or Curie-Weiss form while the
temperature-dependent relaxation mechanism has a microscopic rate proportional
to the temperature. In La0.7Ca0.3MnO3, the Mn4+ susceptibility has the
ferromagnetic Curie-Weiss form, and the static contribution to the linewidth
arising from distortions of the oxygen octahedra is absent due to motional
narrowing brought on by the rapid hopping of the eg polarons. In YBaMn2O6
either of two scenarios is possible. The Mn4+ susceptibility above 520 K is
Curie-like and the static term is present, or the susceptibility has the
antiferromagnetic Curie-Weiss form and the static term is absent due to
motional narrowing. It is concluded that the Curie model, with offsetting
double exchange and and superexchange Curie-Weiss parameters, is the more
likely scenario. It is suggested that the linear-T variation of the linewidth
in both materials arises from either a Korringa-like mechanism involving
interactions with mobile carriers or from a spin-phonon process coming from
interactions between the Mn4+ ions and the lattice vibrations
Euclid Asteroseismology and Kuiper Belt Objects
Euclid, which is primarily a dark-energy/cosmology mission, may have a
microlensing component, consisting of perhaps four dedicated one-month
campaigns aimed at the Galactic bulge. We show that such a program would yield
excellent auxilliary science, including asteroseimology detections for about
100,000 giant stars, and detection of about 1000 Kuiper Belt Objects (KBOs),
down to 2--2.5 mag below the observed break in the KBO luminosity function at I
~26. For the 400 KBOs below the break, Euclid will measure accurate orbits,
with fractional period errors <~ 2.5%.Comment: 8 pages, 4 figures, submitted to JKA
Dynamics of an Ensemble of Noisy Bistable Elements with Global Time-Delayed Coupling
The dynamics of an ensemble of bistable elements with global time-delayed
coupling under the influence of noise is studied analytically and numerically.
Depending on the noise level the system undergoes ordering transitions and
demonstrates multi-stability. That is, for a strong enough positive feedback it
exhibits a non-zero stationary mean field and a variety of stable oscillatory
mean field states are accessible for positive and negative feedback. The
regularity of the oscillatory states is maximal for a certain noise level,
i.e., the system demonstrates coherence resonance. While away from the
transition points the system dynamics is well described by a Gaussian
approximation, near the bifurcation points a description in terms of a
dichotomous theory is more adequate.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. Let
Anomalous Fermi arcs in a periodically driven Weyl system
Three dimensional Weyl semimetals exhibit open Fermi arcs on their sample
surfaces connecting the projection of bulk Weyl points of opposite chirality.
The canonical interpretation of these surfaces states is in terms of chiral
edge modes of a layer quantum Hall effect: The two-dimensional momentum-space
planes perpendicular to the momentum connecting the two Weyl points are
characterized by a non-zero Chern number. It might be interesting to note, that
in analogy to the known two-dimensional Floquet anomalous chiral edge states,
one can realize open Fermi arcs in the absence of Chern numbers in periodically
driven system. Here, we present a way to construct such anomalous Fermi arcs in
a concrete model
Bose condensation in flat bands
We derive effective Hamiltonians for lattice bosons with strong geometrical
frustration of the kinetic energy by projecting the interactions on the flat
lowest Bloch band. Specifically, we consider the Bose Hubbard model on the one
dimensional sawtooth lattice and the two dimensional kagome lattice. Starting
from a strictly local interaction the projection gives rise to effective
long-range terms stabilizing a supersolid phase at densities above nu_c=1/9 of
the kagome lattice. In the sawtooth lattice on the other hand we show that the
solid order, which exists at the magic filling nu_c=1/4, is unstable to further
doping. The universal low-energy properties at filling 1/4+delta nu are
described by the well known commensurate-incommensurate transition. We support
the analytic results by detailed numerical calculations using the Density
Matrix Renormalization Group and exact diagonalization. Finally, we discuss
possible realizations of the models using ultracold atoms as well as frustrated
quantum magnets in high magnetic fields. We compute the momentum distribution
and the noise correlations, that can be extracted from time of flight
experiments or neutron scattering, and point to signatures of the unique
supersolid phase of the kagome lattice.Comment: 18 pages, 13 figure
Observation of phononic helical edge states in a mechanical 'topological insulator'
A topological insulator is characterized by a dichotomy between the interior
and the edge of a finite system: While the bulk has a non-zero energy gap, the
edges are forced to sustain excitations traversing these gaps. Originally
proposed for electrons governed by quantum mechanics, it has remained an
important open question if the same physics can be observed for systems obeying
Newton's equations of motion. Here, we report on measurements that characterize
the collective behavior of mechanical oscillators exhibiting the phenomenology
of the quantum spin hall effect. The phononic edge modes are shown to be
helical and we demonstrate their topological protection via the stability
against imperfections. Our results open the door to the design of topological
acoustic meta-materials that can capitalize on the stability of the surfaces
phonons as reliable wave guides.Comment: 6 pages, 4 figure
Universal Dephasing of Many-Body Rabi Oscillations of Atoms in One-Dimensional Traps
We study a quantum quench in a system of two coupled one-dimensional tubes of
interacting atoms. After the quench the system is out of equilibrium and
oscillates between the tubes with a frequency determined by microscopic
parameters. Despite the high energy at which the system is prepared we find an
emergent long time scale responsible for the dephasing of the oscillations and
a transition at which this time scale diverges. We show that the universal
properties of the dephasing and the transition arise from an infrared
orthogonality catastrophe. Furthermore, we show how this universal behavior is
realized in a realistic model of fermions with attractive interactions.Comment: 4 pages, 2 figure
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