243 research outputs found
Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA
The electronic polarization energies, P = (P+) + (P-), of a PTCDA
(perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline
thin film on a metallic substrate are computed and compared with measurements
of the PTCDA transport gap on gold and silver. Both experiments and theory show
that P is 500 meV larger in a PTCDA monolayer than in 50 A films. Electronic
polarization in systems with surfaces and interfaces are obtained
self-consistently in terms of charge redistribution within molecules.Comment: 5 pages, 4 postscript figures embedde
Adsorption of Fibrinogen on Thin Oriented Poly(Tetrafluoroethylene) (PTFE) Fibres Studied by Scanning Force Microscopy
We have investigated fibrinogen adsorption on ordered poly(tetrafluoroethylene), PTFE, fibres deposited on hydrophilic and hydrophobic silicon substrates. Fibrinogen molecules appear to adsorb with their long axis perpendicular to the fibre direction for PTFE fibres having widths of less than 100 nm. On these thin fibres, fibrinogen apparently forms close packed bands or clusters, consisting of small integer numbers of molecules arranged parallel to each other. On broader (\u3e 100 nm) PTFE fibres, the fibrinogen forms two dimensional networks. The orientation of the molecules in these networks is random in the central flat part of the fibres but perpendicular to the fibre direction at the fibre edges. As a tentative explanation, we propose that the observed orientation may be linked to the radius of curvature of the fibre surface
Optical excitations of Peierls-Mott insulators with bond disorder
The density-matrix renormalization group (DMRG) is employed to calculate
optical properties of the half-filled Hubbard model with nearest-neighbor
interactions. In order to model the optical excitations of oligoenes, a Peierls
dimerization is included whose strength for the single bonds may fluctuate.
Systems with up to 100 electrons are investigated, their wave functions are
analyzed, and relevant length-scales for the low-lying optical excitations are
identified. The presented approach provides a concise picture for the size
dependence of the optical absorption in oligoenes.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Dynamics of Quasi-ordered Structure in a Regio-regulated pi-Conjugated Polymer:Poly(4-methylthiazole-2,5-diyl)
Dynamics of regio-regulated Poly(4-methylthiazole-2,5-diyl) [HH-P4MeTz] was
inves tigated by solid-state 1H, 2D, 13C NMR spectroscopies, and differential
scanning calorimetry(DSC) measurements. DSC, 2D quadrupolar echo NMR, 13C
cross-polarization and magic-angle spinning(CPMAS) NMR, and 2D spin-echo(2DSE)
CPMAS NMR spectroscopy suggest existence of a quasi-ordered phase in which
backbone twists take place with weakened pi-stackings. Two-dimensional exchange
2D NMR(2DEX) detected slow dynamics with a rate of an order of 10^2Hz for the
CD_3 group in d_3-HH-P4MeTz at 288K. The frequency dependence of proton
longitudinal relaxation rate at 288K shows a omega^-1/2 dependence, which is
due to the one-dimensional diffusion-like motion of backbone conformational
modulation waves. The diffusion rate was estimated as 3+/-2 GHz, which was
approximately 10^7 times larger than that estimated by 2DEX NMR measurements.
These results suggest that there exists anomalous dispersion of modulation
waves in HH-P4MeTz. The one-dimensional group velocity of the wave packet is
responsible for the behavior of proton longitudinal relaxation time. On the
other hand, the 2DEX NMR is sensitive to phase velocity of the nutation of
methyl groups that is associated with backbone twists. From proton T_1 and T_2
measurements, the activation energy was estimated as 2.9 and 3.4 kcal/mol,
respectively. These were in agreement with 3.0 kcal/mol determined by
Moller-Plesset(MP2) molecular orbital(MO) calculation. We also performed
chemical shielding calculation of the methyl-carbon in order to understand
chemical shift tensor behavior, leading to the fact that a quasi-ordered phase
coexist with the crystalline phase.Comment: 14 pages, 11 figures, to appear in Phys.Rev.
Excitons in one-dimensional Mott insulators
We employ dynamical density-matrix renormalization group (DDMRG) and
field-theory methods to determine the frequency-dependent optical conductivity
in one-dimensional extended, half-filled Hubbard models. The field-theory
approach is applicable to the regime of `small' Mott gaps which is the most
difficult to access by DDMRG. For very large Mott gaps the DDMRG recovers
analytical results obtained previously by means of strong-coupling techniques.
We focus on exciton formation at energies below the onset of the absorption
continuum. As a consequence of spin-charge separation, these Mott-Hubbard
excitons are bound states of spinless, charged excitations (`holon-antiholon'
pairs). We also determine exciton binding energies and sizes. In contrast to
simple band insulators, we observe that excitons exist in the Mott-insulating
phase only for a sufficiently strong intersite Coulomb repulsion. Furthermore,
our results show that the exciton binding energy and size are not related in a
simple way to the strength of the Coulomb interaction.Comment: 15 pages, 6 eps figures, corrected typos in labels of figures 4,5,
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