617 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
Integrated Optics: a Report on the 2nd OSA Topical Meeting
This report surveys the papers presented at the 2nd OSA Topical Meeting on Integrated Optics, which was held 21–24 January 1974 in New Orleans, La
Suicide Prevention for Older Adults in Residential Communities: Implications for Policy and Practice
Examining an often under-appreciated area, Carol Podgorski and colleagues discuss the suicide risk and opportunities for suicide prevention in seniors' residential communities
Mott-Peierls Transition in the extended Peierls-Hubbard model
The one-dimensional extended Peierls-Hubbard model is studied at several band
fillings using the density matrix renormalization group method. Results show
that the ground state evolves from a Mott-Peierls insulator with a correlation
gap at half-filling to a soliton lattice with a small band gap away from
half-filling. It is also confirmed that the ground state of the Peierls-Hubbard
model undergoes a transition to a metallic state at finite doping. These
results show that electronic correlations effects should be taken into account
in theoretical studies of doped polyacetylene. They also show that a
Mott-Peierls theory could explain the insulator-metal transition observed in
this material.Comment: 4 pages with 3 embedded eps figure
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
Theory of Electric Field-Induced Photoluminescence Quenching in Disordered Molecular Solids
The dynamics of excitons in disordered molecular solids is studied
theoretically, taking into account migration between different sites,
recombination, and dissociation into free charge carriers in the presence of an
electric field. The theory is applied to interpret the results of electric
field-induced photoluminescence (PL) quenching experiments on molecularly doped
polymers by Deussen et al. [Chem. Phys. 207, 147 (1996)]. Using an
intermolecular dissociation mechanism, the dependence of the PL quenching on
the electric field strength and the dopant concentration, and the time
evolution of the transient PL quenching can be well described. The results
constitute additional proof of the distinct exciton dissociation mechanisms in
conjugated polymer blends and molecularly doped polymers.Comment: 4 pages RevTeX, 3 Postscript figure
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
Soft Tissue Attenuation Patterns Associated with Upright Acquisition SPECT Myocardial Perfusion Imaging: A Descriptive Study
Abstract: Purpose: Soft-tissue attenuation patterns in SPECT-myocardial perfusion imaging (MPI) of supine acquisition systems are well recognized. Their prevalence and interaction with body-habitus and gender are ill-defined, which we sought to describe in this study. Methods: In a cross-sectional study, we described the prevalence of soft-tissue attenuation patterns in normal SPECT-MPI studies acquired with a supine patient-position SPECT system. Results: In 263 normal, clinically-indicated, supine-acquisition SPECT-MPIs the attenuation patterns observed were: anterior (35.4%), inferior (41.8%) and lateral (13.3%). Anterior attenuation was more prevalent among women (50.7 % vs. 15.7%, P<0.001) and was associated with chest circumference among men. Conversely, inferior attenuation was more prevalent among men (78.3 % vs. 13.5%, P<0.001) and was not affected by body-habitus. Lateral attenuation was more common among women (19.6 % vs. 5.2%, p=0.001) and was associated with obesity (p=0.015). Conclusions: Soft-tissue attenuation artifacts are common in supine-acquisition SPECT-MPI. The recognition of their prevalence and association with body-habitus and gender is critical for the accurate interpretation of SPECT-MPI
Investigation of conduction band structure, electron scattering mechanisms and phase transitions in indium selenide by means of transport measurements under pressure
In this work we report on Hall effect, resistivity and thermopower
measurements in n-type indium selenide at room temperature under either
hydrostatic and quasi-hydrostatic pressure. Up to 40 kbar (= 4 GPa), the
decrease of carrier concentration as the pressure increases is explained
through the existence of a subsidiary minimum in the conduction band. This
minimum shifts towards lower energies under pressure, with a pressure
coefficient of about -105 meV/GPa, and its related impurity level traps
electrons as it reaches the band gap and approaches the Fermi level. The
pressure value at which the electron trapping starts is shown to depend on the
electron concentration at ambient pressure and the dimensionality of the
electron gas. At low pressures the electron mobility increases under pressure
for both 3D and 2D electrons, the increase rate being higher for 2D electrons,
which is shown to be coherent with previous scattering mechanisms models. The
phase transition from the semiconductor layered phase to the metallic sodium
cloride phase is observed as a drop in resistivity around 105 kbar, but above
40 kbar a sharp nonreversible increase of the carrier concentration is
observed, which is attributed to the formation of donor defects as precursors
of the phase transition.Comment: 18 pages, Latex, 10 postscript figure
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