3,286 research outputs found
Mediation of Long Range Charge Transfer by Kondo Bound States
We present a theory of non-equilibrium long range charge transfer between
donor and acceptor centers in a model polymer mediated by magnetic exciton
(Kondo) bound states. Our model produces electron tunneling lengths easily
exceeding 10, as observed recently in DNA and organic charge transfer
systems. This long ranged tunneling is effective for weak to intermediate
donor-bridge coupling, and is enhanced both by weak to intermediate strength
Coulomb hole-electron attraction (through the orthogonality catastrophe) and by
coupling to local vibrational modes.Comment: Revised content (broadened scope, vibrations added), submitted to
Phys Rev Lett, added autho
Measurement of the 187Re({\alpha},n)190Ir reaction cross section at sub-Coulomb energies using the Cologne Clover Counting Setup
Uncertainties in adopted models of particle+nucleus optical-model potentials
directly influence the accuracy in the theoretical predictions of reaction
rates as they are needed for reaction-network calculations in, for instance,
{\gamma}-process nucleosynthesis. The improvement of the {\alpha}+nucleus
optical-model potential is hampered by the lack of experimental data at
astrophysically relevant energies especially for heavier nuclei. Measuring the
Re187({\alpha},n)Ir190 reaction cross section at sub-Coulomb energies extends
the scarce experimental data available in this mass region and helps
understanding the energy dependence of the imaginary part of the
{\alpha}+nucleus optical-model potential at low energies. Applying the
activation method, after the irradiation of natural rhenium targets with
{\alpha}-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the
reaction cross section were determined via {\gamma}-ray spectroscopy by using
the Cologne Clover Counting Setup and the method of {\gamma}{\gamma}
coincidences. Cross-section values at five energies close to the
astrophysically relevant energy region were measured. Statistical model
calculations revealed discrepancies between the experimental values and
predictions based on widely used {\alpha}+nucleus optical-model potentials.
However, an excellent reproduction of the measured cross-section values could
be achieved from calculations based on the so-called Sauerwein-Rauscher
{\alpha}+nucleus optical-model potential. The results obtained indicate that
the energy dependence of the imaginary part of the {\alpha}+nucleus
optical-model potential can be described by an exponential decrease. Successful
reproductions of measured cross sections at low energies for {\alpha}-induced
reactions in the mass range 141{\leq}A{\leq}187 confirm the global character of
the Sauerwein-Rauscher potential
Possible experimental signature of octupole correlations in the 0 states of the actinides
= 0 states have been investigated in the actinide nucleus
Pu up to an excitation energy of 3 MeV with a high-resolution (p,t)
experiment at = 24 MeV. To test the recently proposed = 0
double-octupole structure, the phenomenological approach of the
spdf-interacting boson model has been chosen. In addition, the total 0
strength distribution and the strength fragmentation have been compared
to the model predictions as well as to the previously studied (p,t) reactions
in the actinides. The results suggest that the structure of the 0 states
in the actinides might be more complex than the usually discussed pairing
isomers. Instead, the octupole degree of freedom might contribute
significantly. The signature of two close-lying 0 states below the
2-quasiparticle energy is presented as a possible manifestation of strong
octupole correlations in the structure of the 0 states in the actinides.Comment: 6 pages, 5 figures, published in Phys. Rev. C 88, 041303(R) (2013
Increased accuracy of ligand sensing by receptor internalization
Many types of cells can sense external ligand concentrations with
cell-surface receptors at extremely high accuracy. Interestingly, ligand-bound
receptors are often internalized, a process also known as receptor-mediated
endocytosis. While internalization is involved in a vast number of important
functions for the life of a cell, it was recently also suggested to increase
the accuracy of sensing ligand as the overcounting of the same ligand molecules
is reduced. Here we show, by extending simple ligand-receptor models to
out-of-equilibrium thermodynamics, that internalization increases the accuracy
with which cells can measure ligand concentrations in the external environment.
Comparison with experimental rates of real receptors demonstrates that our
model has indeed biological significance.Comment: 9 pages, 4 figures, accepted for publication in Physical Review
Strong electron correlations in cobalt valence tautomers
We have examined cobalt based valence tautomer molecules such as
Co(SQ)(phen) using density functional theory (DFT) and variational
configuration interaction (VCI) approaches based upon a model Hamiltonian. Our
DFT results extend earlier work by finding a reduced total energy gap (order
0.6 eV) between high temperature and low temperature states when we fully relax
the coordinates (relative to experimental ones). Futhermore we demonstrate that
the charge transfer picture based upon formal valence arguments succeeds
qualitatively while failing quantitatively due to strong covalency between the
Co 3 orbitals and ligand orbitals. With the VCI approach, we argue that
the high temperature, high spin phase is strongly mixed valent, with about 30 %
admixture of Co(III) into the predominantly Co(II) ground state. We confirm
this mixed valence through a fit to the XANES spectra. Moreover, the strong
electron correlations of the mixed valent phase provide an energy lowering of
about 0.2-0.3 eV of the high temperature phase relative to the low temperature
one. Finally, we use the domain model to account for the extraordinarily large
entropy and enthalpy values associated with the transition.Comment: 10 pages, 4 figures, submitted to J. Chem. Phy
Aviram-Ratner rectifying mechanism for DNA base pair sequencing through graphene nanogaps
We demonstrate that biological molecules such as Watson-Crick DNA base pairs
can behave as biological Aviram-Ratner electrical rectifiers because of the
spatial separation and weak hydrogen bonding between the nucleobases. We have
performed a parallel computational implementation of the ab-initio
non-equilibrium Green's function (NEGF) theory to determine the electrical
response of graphene---base-pair---graphene junctions. The results show an
asymmetric (rectifying) current-voltage response for the Cytosine-Guanine base
pair adsorbed on a graphene nanogap. In sharp contrast we find a symmetric
response for the Thymine-Adenine case. We propose applying the asymmetry of the
current-voltage response as a sensing criterion to the technological challenge
of rapid DNA sequencing via graphene nanogaps
Using Synchronic and Diachronic Relations for Summarizing Multiple Documents Describing Evolving Events
In this paper we present a fresh look at the problem of summarizing evolving
events from multiple sources. After a discussion concerning the nature of
evolving events we introduce a distinction between linearly and non-linearly
evolving events. We present then a general methodology for the automatic
creation of summaries from evolving events. At its heart lie the notions of
Synchronic and Diachronic cross-document Relations (SDRs), whose aim is the
identification of similarities and differences between sources, from a
synchronical and diachronical perspective. SDRs do not connect documents or
textual elements found therein, but structures one might call messages.
Applying this methodology will yield a set of messages and relations, SDRs,
connecting them, that is a graph which we call grid. We will show how such a
grid can be considered as the starting point of a Natural Language Generation
System. The methodology is evaluated in two case-studies, one for linearly
evolving events (descriptions of football matches) and another one for
non-linearly evolving events (terrorist incidents involving hostages). In both
cases we evaluate the results produced by our computational systems.Comment: 45 pages, 6 figures. To appear in the Journal of Intelligent
Information System
Comparing Microwave and Classical Synthesis of Oxymethylene Dimethyl Ethers
Polyoxymethylene dimethyl ethers (OME n ) are considered as substituents or additives for fossil diesel fuel. Efficiency of the synthesis is crucial for the development of industrial scale production plants. Therefore, the design of suitable catalysts and the efficient heating play important roles in OME fuel synthesis. In this work, microwave‐assisted synthesis (MAS) is carried out and compared to a classical approach using standard thermal heating. Different polymeric materials, e.g., Amerlyst15, are utilized as catalysts, and screened for the catalytic synthesis of OME. Within this approach, the kinetics of the reaction are analyzed in detail
Probing many-body dynamics on a 51-atom quantum simulator
Controllable, coherent many-body systems can provide insights into the
fundamental properties of quantum matter, enable the realization of new quantum
phases and could ultimately lead to computational systems that outperform
existing computers based on classical approaches. Here we demonstrate a method
for creating controlled many-body quantum matter that combines
deterministically prepared, reconfigurable arrays of individually trapped cold
atoms with strong, coherent interactions enabled by excitation to Rydberg
states. We realize a programmable Ising-type quantum spin model with tunable
interactions and system sizes of up to 51 qubits. Within this model, we observe
phase transitions into spatially ordered states that break various discrete
symmetries, verify the high-fidelity preparation of these states and
investigate the dynamics across the phase transition in large arrays of atoms.
In particular, we observe robust manybody dynamics corresponding to persistent
oscillations of the order after a rapid quantum quench that results from a
sudden transition across the phase boundary. Our method provides a way of
exploring many-body phenomena on a programmable quantum simulator and could
enable realizations of new quantum algorithms.Comment: 17 pages, 13 figure
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