63 research outputs found
Correlated theory of triplet photoinduced absorption in phenylene-vinylene chains
In this paper we present results of large-scale correlated calculations of
triplet photoinduced absorption (PA) spectrum of oligomers of
poly-(para)phenylenevinylene (PPV) containing up to five phenyl rings. In
particular, the high-energy features in the triplet PA spectrum of oligo-PPVs
are the focus of this study, which, so far, have not been investigated
theoretically, or experimentally. The calculations were performed using the
Pariser-Parr-Pople (PPP) model Hamiltonian, and many-body effects were taken
into account by means of multi-reference singles-doubles configuration
interaction procedure (MRSDCI), without neglecting any molecular orbitals. The
computed triplet PA spectrum of oligo-PPVs exhibits rich structure consisting
of alternating peaks of high and low intensities. The predicted higher energy
features of the triplet spectrum can be tested in future experiments.
Additionally, theoretical estimates of exciton binding energy are also
presented.Comment: To appear in Phys. Rev.
Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory
There is currently a great need for solid state lasers that emit in the
infrared, as this is the operating wavelength regime for applications in
telecommunications. Existing --conjugated polymers all emit in the visible
or ultraviolet, and whether or not --conjugated polymers that emit in the
infrared can be designed is an interesting challenge. On the one hand, the
excited state ordering in trans-polyacetylene, the --conjugated polymer
with relatively small optical gap, is not conducive to light emission because
of electron-electron interaction effects. On the other hand, excited state
ordering opposite to that in trans-polyacetylene is usually obtained by
chemical modification that increases the effective bond-alternation, which in
turn increases the optical gap. We develop a theory of electron correlation
effects in a model -conjugated polymer that is obtained by replacing the
hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and
show that the effective on-site correlation in this system is smaller than the
bare correlation in the unsubstituted system. An optical gap in the infrared as
well as excited state ordering conducive to light emission is thereby predicted
upon similar structural modifications.Comment: 15 pages, 15 figures, 1 tabl
A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)
The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.Comment: LaTeX, 19 pages, 7 eps figures included using epsf. To appear in
Physical Review B, 199
The low-lying excitations of polydiacetylene
The Pariser-Parr-Pople Hamiltonian is used to calculate and identify the
nature of the low-lying vertical transition energies of polydiacetylene. The
model is solved using the density matrix renormalisation group method for a
fixed acetylenic geometry for chains of up to 102 atoms. The non-linear optical
properties of polydiacetylene are considered, which are determined by the
third-order susceptibility. The experimental 1Bu data of Giesa and Schultz are
used as the geometric model for the calculation. For short chains, the
calculated E(1Bu) agrees with the experimental value, within solvation effects
(ca. 0.3 eV). The charge gap is used to characterise bound and unbound states.
The nBu is above the charge gap and hence a continuum state; the 1Bu, 2Ag and
mAg are not and hence are bound excitons. For large chain lengths, the nBu
tends towards the charge gap as expected, strongly suggesting that the nBu is
the conduction band edge. The conduction band edge for PDA is agreed in the
literature to be ca. 3.0 eV. Accounting for the strong polarisation effects of
the medium and polaron formation gives our calculated E(nBu) ca. 3.6 eV, with
an exciton binding energy of ca. 1.0 eV. The 2Ag state is found to be above the
1Bu, which does not agree with relaxed transition experimental data. However,
this could be resolved by including explicit lattice relaxation in the Pariser-
Parr-Pople-Peierls model. Particle-hole separation data further suggest that
the 1Bu, 2Ag and mAg are bound excitons, and that the nBu is an unbound
exciton.Comment: LaTeX, 23 pages, 4 postscript tables and 8 postscript figure
Magnetic critical behavior of two-dimensional random-bond Potts ferromagnets in confined geometries
We present a numerical study of 2D random-bond Potts ferromagnets. The model
is studied both below and above the critical value which discriminates
between second and first-order transitions in the pure system. Two geometries
are considered, namely cylinders and square-shaped systems, and the critical
behavior is investigated through conformal invariance techniques which were
recently shown to be valid, even in the randomness-induced second-order phase
transition regime Q>4. In the cylinder geometry, connectivity transfer matrix
calculations provide a simple test to find the range of disorder amplitudes
which is characteristic of the disordered fixed point. The scaling dimensions
then follow from the exponential decay of correlations along the strip. Monte
Carlo simulations of spin systems on the other hand are generally performed on
systems of rectangular shape on the square lattice, but the data are then
perturbed by strong surface effects. The conformal mapping of a semi-infinite
system inside a square enables us to take into account boundary effects
explicitly and leads to an accurate determination of the scaling dimensions.
The techniques are applied to different values of Q in the range 3-64.Comment: LaTeX2e file with Revtex, revised versio
Theory of excited state absorptions in phenylene-based -conjugated polymers
Within a rigid-band correlated electron model for oligomers of
poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that
there exist two fundamentally different classes of two-photon A states in
these systems to which photoinduced absorption (PA) can occur. At relatively
lower energies there occur A states which are superpositions of one
electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations,
that are both comprised of the highest delocalized valence band and the lowest
delocalized conduction band states only. The dominant PA is to one specific
member of this class of states (the mA). In addition to the above class of
A states, PA can also occur to a higher energy kA state whose 2e--2h
component is {\em different} and has significant contributions from excitations
involving both delocalized and localized bands. Our calculated scaled energies
of the mA and the kA agree reasonably well to the experimentally
observed low and high energy PAs in PPV. The calculated relative intensities of
the two PAs are also in qualitative agreement with experiment. In the case of
ladder-type PPP and its oligomers, we predict from our theoretical work a new
intense PA at an energy considerably lower than the region where PA have been
observed currently. Based on earlier work that showed that efficient
charge--carrier generation occurs upon excitation to odd--parity states that
involve both delocalized and localized bands, we speculate that it is the
characteristic electronic nature of the kA that leads to charge generation
subsequent to excitation to this state, as found experimentally.Comment: Revtex4 style, 2 figures inserted in the text, three tables, 10 page
Comparative study of deuterium retention and vacancy content of self-ion irradiated tungsten
Self-ion irradiation of pure tungsten with 2 MeV W ions provides a way of simulating microstructures generated by neutron irradiation in tungsten components of a fusion reactor. Transmission electron microscopy (TEM) has been used to characterize defects formed in tungsten samples by ion irradiation. It was found that tungsten irradiated to 0.85 dpa at relatively low temperatures develops a characteristic microstructure dominated by dislocation loops and black dots. The density and size distribution of these defects were estimated. Some of the samples exposed to self-ion irradiation were then implanted with deuterium. Thermal Desorption Spectrometry (TDS) analysis was performed to estimate the deuterium inventory as a function of irradiation damage and deuterium release as a function of temperature. Increase of inventory with increasing irradiation dose followed by slight decrease above 0.1 dpa was found. Application of Positron Annihilation Spectroscopy (PAS) to self-irradiated but not deuterium implanted samples enabled an assessment of the density of irradiation defects as a function of exposure to highenergy ions. The PAS results show that the density of defects saturates at doses in the interval from 0.085 to 0.425 displacements per atom (dpa). These results are discussed in the context of recent theoretical simulations exhibiting the saturation of defect microstructure in the high irradiation exposure limit. The saturation of damage found in PAS agrees with the simulation data described in the paper. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )Peer reviewe
Quantitative estimates of unique continuation for parabolic equations, determination of unknown time-varying boundaries and optimal stability estimates
In this paper we will review the main results concerning the issue of
stability for the determination unknown boundary portion of a thermic
conducting body from Cauchy data for parabolic equations. We give detailed and
selfcontained proofs. We prove that such problems are severely ill-posed in the
sense that under a priori regularity assumptions on the unknown boundaries, up
to any finite order of differentiability, the continuous dependence of unknown
boundary from the measured data is, at best, of logarithmic type
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