958 research outputs found
Symmetry crossover and excitation thresholds at the neutral-ionic transition of the modified Hubbard model
Exact ground states, charge densities and excitation energies are found using
valence bond methods for N-site modified Hubbard models with uniform spacing.
At the neutral-ionic transition (NIT), the ground state has a symmetry
crossover in 4n, 4n+2 rings with periodic and antiperiodic boundary conditions,
respectively. The modified Hubbard model has a continuous NIT between a
diamagnetic band insulator on the paired side and a paramagnetic Mott insulator
on the covalent side. The singlet-triplet (ST), singlet-singlet (SS) and charge
gaps for finite N indicate that the ST and SS gaps close at the NIT with
increasing U and that the charge gap vanishes only there. Finite-N excitations
constrain all singularities to about 0.1t of the symmetry crossover. The NIT is
interpreted as a localized ground state (gs) with finite gaps on the paired
side and an extended gs with vanishing ST and SS gaps on the covalent side. The
charge gap and charge stiffness indicate a metallic gs at the transition that,
however, is unconditionally unstable to dimerization.Comment: 12 pages, including 8 figure
Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions
The dipole P(F) of systems with periodic boundary conditions (PBC) in a
static electric field F is applied to one-dimensional Peierls-Hubbard models
for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for
finite systems of N = 14 and 16 sites that are almost converged to infinite
chains in deformable lattices subject to a Peierls transition. The electronic
polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid
stacks with alternating transfer integrals t(1 +/- \delta) diverges at the
neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in
dimerized chains. The Peierls or dimerization mode couples to charge
fluctuations along the stack and results in large vibrational contributions,
\alpha_{vib}, that are related to \partial P/\partial \delta and that peak
sharply at the Peierls transition. The extension of P(F) to correlated
electronic states yields the dielectric response \kappa of models with
neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with
parameters used previously for variable ionicity \rho and vibrational spectra
of CT salts. The calculated \kappa accounts for the dielectric response of CT
salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs
(chloranil). The role of lattice stiffness appears clearly in models: soft
systems have a Peierls instability at small \rho and continuous crossover to
large \rho, while stiff stacks such as TTF-CA have a first-order transition
with discontinuous \rho that is both a neutral-ionic and Peierls transition.
The transitions are associated with tuning the electronic ground state of
insulators via temperature or pressure in experiments, or via model parameters
in calculations.Comment: 10 pages, 9 figures; J.Chem.Phys., in pres
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
Density matrix renormalization group algorithm for Bethe lattices of spin 1/2 or 1 sites with Heisenberg antiferromagnetic exchange
An efficient density matrix renormalization group (DMRG) algorithm is
presented for the Bethe lattice with connectivity and antiferromagnetic
exchange between nearest neighbor spins or 1 sites in successive
generations . The algorithm is accurate for sites. The ground states
are magnetic with spin , staggered magnetization that persists
for large and short-range spin correlation functions that decrease
exponentially. A finite energy gap to leads to a magnetization
plateau in the extended lattice. Closely similar DMRG results for = 1/2 and
1 are interpreted in terms of an analytical three-site model.Comment: 7 Pages and 8 figure
Dynamic response studies on aggregation and breakage dynamics of colloidal dispersions in stirred tanks
Aggregation and breakage of aggregates of fully destabilized polystyrene latex particles in turbulent flow was studied experimentally in both batch and continuous stirred tanks using small-angle static light scattering. It was found that the steady-state values of the root-mean-square radius of gyration are fully reversible upon changes of stirring speed as well as solid volume fraction. Steady-state values of the root-mean-square radius of gyration were decreasing with decreasing solid volume fraction as well as with increasing stirring speed. Moreover, it was found that the steady-state structure and shape of the aggregates is not influenced by the applied stirring speed
Accounting for both electron--lattice and electron--electron coupling in conjugated polymers: minimum total energy calculations on the Hubbard--Peierls hamiltonian
Minimum total energy calculations, which account for both electron--lattice
and electron--electron interactions in conjugated polymers are performed for
chains with up to eight carbon atoms. These calculations are motivated in part
by recent experimental results on the spectroscopy of polyenes and conjugated
polymers and shed light on the longstanding question of the relative importance
of electron--lattice vs. electron--electron interactions in determining the
properties of these systems.Comment: 6 pages, Plain TeX, FRL-PSD-93GR
Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes
Single-walled carbon nanotubes are strongly correlated systems with large
Coulomb repulsion between two electrons occupying the same orbital.
Within a molecular Hamiltonian appropriate for correlated -electron
systems, we show that optical excitations polarized parallel to the nanotube
axes in the so-called metallic single-walled carbon nanotubes are to excitons.
Our calculated absolute exciton energies in twelve different metallic
single-walled carbon nanotubes, with diameters in the range 0.8 - 1.4 nm, are
in nearly quantitative agreement with experimental results. We have also
calculated the absorption spectrum for the (21,21) single-walled carbon
nanotube in the E region. Our calculated spectrum gives an excellent fit
to the experimental absorption spectrum. In all cases our calculated exciton
binding energies are only slightly smaller than those of semiconducting
nanotubes with comparable diameters, in contradiction to results obtained
within the {\it ab initio} approach, which predicts much smaller binding
energies. We ascribe this difference to the difficulty of determining the
behavior of systems with strong on-site Coulomb interactions within theories
based on the density functional approach. As in the semiconducting nanotubes we
predict in the metallic nanotubes a two-photon exciton above the lowest
longitudinally polarized exciton that can be detected by ultrafast pump-probe
spectroscopy. We also predict a subgap absorption polarized perpendicular to
the nanotube axes below the lowest longitudinal exciton, blueshifted from the
exact midgap by electron-electron interactions
Charge fluctuations and electron-phonon coupling in organic charge-transfer salts with neutral-ionic and Peierls transitions
The first-order transition of the charge-transfer complex TTF-CA
(tetrathiafulvalene-chloranil) is both a neutral-ionic and a Peierls
transition. In related organic charge transfer complexes, cooling at ambient
pressure increases the ionicity in strikingly different ways, and is
sometimes accompanied by a dielectric peak, that we relate to lattice
stiffness, to structural and energetic disorder, and to the softening of the
Peierls mode in the far-IR. The position operator for systems with periodic
boundary conditions makes possible a systematic treatment of electron-phonon
interactions in extended donor-acceptor stacks in terms of correlated
Peierls-Hubbard models. The IR intensity of the Peierls mode peaks at the
Peierls transition at small in soft lattices, where the dielectric
constant also has a large peak. In dimerized stacks, the IR intensity of
totally symmetric, Raman active, molecular vibrations is related to charge
fluctuations that modulate site energies. Combination bands of molecular and
Peierls modes are identified in regular TTF-CA stacks above Tc. Energetic
disorder can suppress the Peierls transition and rationalize a continuous
crossover from small to large . The TTF-CA scenario of a neutral-regular
to ionic-dimerized transition must be broadened considerably in view of charge
transfer salts that dimerize on the neutral side, that become ionic without a
structural change, or that show vibrational evidence for dimerization at
constant .Comment: 26 pages including figure
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
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