555 research outputs found
Effect of interchain coupling on conducting polymer luminescence: excimers in derivatives of poly(phenylene vinylene)
Optical excitation of a chain in a polymer film may result in formation of an
excimer, a superposition of on-chain excitons and charge-transfer excitons on
the originally excited chain and a neighboring chain. The excimer emission is
red-shifted compared to that of an on-chain exciton by an amount depending on
the interchain coupling . Setting up the excimer wavefunction and
calculating the red shift, we determine average values, referred to a
monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy -phenylene
cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4
p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the
effect of interchain distance on the emission.Comment: 10 pages, RevTeX, 1 PS figure, replaced version of cond-mat/9707095,
accepted for publication in Phys. Rev. B, Rapid Communicatio
Integrating biodiversity and ecosystem services into the economic analysis of agricultural systems
Based on existing literature, this paper outlines the relationships between biodiversity, ecosystem services and food security. It additionally reviews tools which can be used to integrate biodiversity and ecosystem services into analytic frameworks to allow a better understanding of the trade-offs between different agricultural systems and their ecosystem service provisioning, as well as the resulting impact on productivity and human well-being
Electron-electron interaction effects on optical excitations in semiconducting single-walled carbon nanotubes
We report correlated-electron calculations of optically excited states in ten
semiconducting single-walled carbon nanotubes with a wide range of diameters.
Optical excitation occurs to excitons whose binding energies decrease with the
increasing nanotube diameter, and are smaller than the binding energy of an
isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of
the second optical exciton polarized along the nanotube axis to that of the
lowest exciton is smaller than the value predicted within single-particle
theory. The experimentally observed weak photoluminescence is an intrinsic
feature of semiconducting nanotubes, and is consequence of dipole-forbidden
excitons occurring below the optical exciton.Comment: 5 pages, 3 figures, To appear in PR
Condensation of oligonucleotides assembled into nicked and gapped duplexes: potential structures for oligonucleotide delivery
The condensation of nucleic acids into well-defined particles is an integral part of several approaches to artificial cellular delivery. Improvements in the efficiency of nucleic acid delivery in vivo are important for the development of DNA- and RNA-based therapeutics. Presently, most efforts to improve the condensation and delivery of nucleic acids have focused on the synthesis of novel condensing agents. However, short oligonucleotides are not as easy to condense into well-defined particles as gene-length DNA polymers and present particular challenges for discrete particle formation. We describe a novel strategy for improving the condensation and packaging of oligonucleotides that is based on the self-organization of half-sliding complementary oligonucleotides into long duplexes (ca. 2 kb). These non-covalent assemblies possess single-stranded nicks or single-stranded gaps at regular intervals along the duplex backbones. The condensation behavior of nicked- and gapped-DNA duplexes was investigated using several cationic condensing agents. Transmission electron microscopy and light-scattering studies reveal that these DNA duplexes condense much more readily than short duplex oligonucleotides (i.e. 21 bp), and more easily than a 3 kb plasmid DNA. The polymeric condensing agents, poly-l-lysine and polyethylenimine, form condensates with nicked- and gapped-DNA that are significantly smaller than condensates formed by the 3 kb plasmid DNA. These results demonstrate the ability for DNA structure and topology to alter nucleic acid condensation and suggest the potential for the use of this form of DNA in the design of vectors for oligonucleotide and gene delivery. The results presented here also provide new insights into the role of DNA flexibility in condensate formation
Polarons with a twist
We consider a polaron model where molecular \emph{rotations} are important.
Here, the usual hopping between neighboring sites is affected directly by the
electron-phonon interaction via a {\em twist-dependent} hopping amplitude. This
model may be of relevance for electronic transport in complex molecules and
polymers with torsional degrees of freedom, such as DNA, as well as in
molecular electronics experiments where molecular twist motion is significant.
We use a tight-binding representation and find that very different polaronic
properties are already exhibited by a two-site model -- these are due to the
nonlinearity of the restoring force of the twist excitations, and of the
electron-phonon interaction in the model. In the adiabatic regime, where
electrons move in a {\em low}-frequency field of twisting-phonons, the
effective splitting of the energy levels increases with coupling strength. The
bandwidth in a long chain shows a power-law suppression with coupling, unlike
the typical exponential dependence due to linear phonons.Comment: revtex4 source and one eps figur
Cooperative dynamics in charge-ordered state of alpha-(BEDT-TTF)2I3
Electric-field-dependent pulse measurements are reported in the
charge-ordered state of alpha-(BEDT-TTF)2I3. At low electric fields up to about
50 V/cm only negligible deviations from Ohmic behavior can be identified with
no threshold field. At larger electric fields and up to about 100 V/cm a
reproducible negative differential resistance is observed with a significant
change in shape of the measured resistivity in time. These changes critically
depend whether constant voltage or constant current is applied to the single
crystal. At high enough electric fields the resistance displays a dramatic drop
down to metallic values and relaxes subsequently in a single-exponential manner
to its low-field steady-state value. We argue that such an electric-field
induced negative differential resistance and switching to transient states are
fingerprints of cooperative domain-wall dynamics inherent to two-dimensional
bond-charge density wave with ferroelectric-like nature.Comment: 6 pages, 4 figures, resubmitted to Phys. Rev.
Quantum inelastic conductance through molecular wires
We calculate non-perturbatively the inelastic effects on the conductance
through a conjugated molecular wire-metal heterojunction, including realistic
electron-phonon coupling. We show that at sub-band-gap energies the current is
dominated by quantum coherent transport of virtual polarons through the
molecule. In this regime, the tunneling current is strongly increased relative
to the case of elastic scattering. It is essential to describe the full quantum
coherence of the polaron formation and transport in order to obtain correct
physics. Our results are generally applicable to one-dimensional atomic or
molecular wires.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Fluctuation-Facilitated Charge Migration along DNA
We propose a model Hamiltonian for charge transfer along the DNA double helix
with temperature driven fluctuations in the base pair positions acting as the
rate limiting factor for charge transfer between neighboring base pairs. We
compare the predictions of the model with the recent work of J.K. Barton and
A.H. Zewail (Proc.Natl.Acad.Sci.USA, {\bf 96}, 6014 (1999)) on the unusual
two-stage charge transfer of DNA.Comment: 4 pages, 2 figure
Quality of Life in Chronic Pancreatitis is Determined by Constant Pain, Disability/Unemployment, Current Smoking, and Associated Co-Morbidities
OBJECTIVES: Chronic pancreatitis (CP) has a profound independent effect on quality of life (QOL). Our aim was to identify factors that impact the QOL in CP patients. METHODS: We used data on 1,024 CP patients enrolled in the three NAPS2 studies. Information on demographics, risk factors, co-morbidities, disease phenotype, and treatments was obtained from responses to structured questionnaires. Physical and mental component summary (PCS and MCS, respectively) scores generated using responses to the Short Form-12 (SF-12) survey were used to assess QOL at enrollment. Multivariable linear regression models determined independent predictors of QOL. RESULTS: Mean PCS and MCS scores were 36.7+/-11.7 and 42.4+/-12.2, respectively. Significant (P \u3c 0.05) negative impact on PCS scores in multivariable analyses was noted owing to constant mild-moderate pain with episodes of severe pain or constant severe pain (10 points), constant mild-moderate pain (5.2), pain-related disability/unemployment (5.1), current smoking (2.9 points), and medical co-morbidities. Significant (P \u3c 0.05) negative impact on MCS scores was related to constant pain irrespective of severity (6.8-6.9 points), current smoking (3.9 points), and pain-related disability/unemployment (2.4 points). In women, disability/unemployment resulted in an additional 3.7 point reduction in MCS score. Final multivariable models explained 27% and 18% of the variance in PCS and MCS scores, respectively. Etiology, disease duration, pancreatic morphology, diabetes, exocrine insufficiency, and prior endotherapy/pancreatic surgery had no significant independent effect on QOL. CONCLUSIONS: Constant pain, pain-related disability/unemployment, current smoking, and concurrent co-morbidities significantly affect the QOL in CP. Further research is needed to identify factors impacting QOL not explained by our analyses
Thermoelectric power of nondegenerate Kane semiconductors under the conditions of mutual electron-phonon drag in a high electric field
The thermoelectric power of nondegenerate Kane semiconductors with due regard
for the electron and phonon heating, and their thermal and mutual drags is
investigated. The electron spectrum is taken in the Kane two-band form. It is
shown that the nonparabolicity of electron spectrum significantly influences
the magnitude of the thermoelectric power and leads to a change of its sign and
dependence on the heating electric field. The field dependence of the
thermoelectric power is determined analytically under various drag conditions.Comment: 25 pages, RevTex formatted, 3 table
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