942 research outputs found
Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering
Diffuse X-ray data for mixed stack organic charge-transfer crystals
approaching the neutral-ionic phase transition can be quantitatively explained
as due to the softening of the optical phonon branch. The interpretation is
fully consistent with vibrational spectra, and underlines the importance of
electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure
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
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
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
Magnetic susceptibility of alkali-TCNQ salts and extended Hubbard models with bond order and charge density wave phases
The molar spin susceptibilities of Na-TCNQ, K-TCNQ and Rb-TCNQ(II)
are fit quantitatively to 450 K in terms of half-filled bands of three
one-dimensional Hubbard models with extended interactions using exact results
for finite systems. All three models have bond order wave (BOW) and charge
density wave (CDW) phases with boundary for nearest-neighbor
interaction and on-site repulsion . At high , all three salts have
regular stacks of anion radicals. The fits place Na and
K in the CDW phase and Rb(II) in the BOW phase with . The Na and
K salts have dimerized stacks at while Rb(II) has regular stacks at
100K. The analysis extends to dimerized stacks and to dimerization
fluctuations in Rb(II). The three models yield consistent values of ,
and transfer integrals for closely related stacks. Model
parameters based on are smaller than those from optical data that in
turn are considerably reduced by electronic polarization from quantum chemical
calculation of , and on adjacent ions. The
analysis shows that fully relaxed states have reduced model parameters compared
to optical or vibration spectra of dimerized or regular stacks.Comment: 9 pages and 5 figure
Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition
We present exact diagonalization results on a modified Peierls-Hubbard model
for the neutral-ionic phase transition. The ground state potential energy
surface and the infrared intensity of the Peierls mode point to a strong,
non-linear electron-phonon coupling, with effects that are dominated by the
proximity to the electronic instability rather than by electronic correlations.
The huge infrared intensity of the Peierls mode at the ferroelectric
transition is related to the temperature dependence of the dielectric constant
of mixed-stack organic crystals.Comment: 4 pages, 4 figure
Implementing the GBT Data Transmission Protocol in FPGAs
International audienceThe GBT chip is a radiation tolerant ASIC that can be used to implement bidirectional multipurpose 4.8 Gb/s optical links for high-energy physics experiments. It will be proposed to the LHC experiments for combined transmission of physics data, trigger, timing, fast and slow control and monitoring. Although radiation hardness is required on detectors, it is not necessary for the electronics located in the counting rooms. Therefore, a study is being made to implement these GBT links on FPGAs. This paper will describe the GBT protocol implementation, the configuration of the transceivers on Altera Stratix II GX and Xilinx Virtex 4, the optimization of resource for multi-transceivers, the first data transmission tests and the source code availabilit
Spin-Peierls Dimerization of a s=1/2 Heisenberg Antiferromagnet on a Square Lattice
Dimerization of a spin-half Heisenberg antiferromagnet on a square lattice is
investigated for several possible dimerized configurations, some of which are
shown to have lower ground state energies than the others. In particular, the
lattice deformations resulting in alternate stronger and weaker couplings along
both the principal axes of a square lattice are shown to result in a larger
gain in magnetic energy. In addition, a `columnar' configuration is shown to
have a lower ground state energy and a faster increase in the energy gap
parameter than a `staggered' configuration. The inclusion of unexpanded
exchange coupling leads to a power law behaviour for the magnetic energy gain
and energy gap, which is qualitatively different from that reported earlier.
Instead of increasing as , the two quantities depend on
as This is true both in the near critical
regime as well as in the far regime . It is suggested that the unexpanded exchange coupling is as much a source
of the logarithmic dependence as a correction due to the contribution of
umklapp processes. Staggered magnetization is shown to follow the same -dependence in all the configurations in the small -regime, while for
, it follows the power law .Comment: 12 pages, 7 Postscript figures, RevTex forma
- …