146 research outputs found
Exact numerical diagonalization of one-dimensional interacting electrons nonadiabatically coupled to phonons
We study the role of non-adiabatic Holstein electron-phonon coupling on the
neutral-ionic phase transition of charge transfer crystals which can be tuned
from continuous to discontinuous, using exact numerical diagonalization. The
variation of electronic properties through the transition is smoothed by
nonadiabaticity. Lattice properties are strongly affected, and we observe both
squeezing and antisqueezing, depending on details of the adiabatic potentials,
and identify the quantum uncertainty of the phonons as the most sensitive
measure of nonadiabaticity. The adiabatic limit is regular for a continuous
transition but turns out completely inadequate near a discontinuous transition.
The relevance of coherent state approaches is assessed critically.Comment: latex manuscript (7 pages), 3 eps figures; revised version, better
discussion, one figure replaced; to be published in Europhys. Let
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
Reference Force Field and CDW Amplitude of Mixed-Valence Halogen-Bridged Pt Complexes
The spectroscopic effects of electron-phonon coupling in mixed-valence
chlorine-bridged Pt chains complexes are investigated through a parallel
infrared and Raman study of three compounds with decreasing Pt-Pt distance
along the chain. The e-ph interaction is analyzed in terms of the
Herzberg-Teller coupling scheme. We take into account the quadratic term and
define a precise reference state. The force field relevant to this state is
constructed, whereas the electronic structure is analyzed in terms of a simple
phenomenological model, singling out a trimeric unit along the chain. In this
way we are able to account for all the available optical data of the three
compounds, and to estimate the relevant microscopic parameters, such as the
e-ph coupling constants and the CDW amplitude.Comment: 10 pages, compressed postscript, 6 Tables and 5 Figures also in a
compressed ps.Z file. Revision is in the submission format only (postscript
instead of tex
Automating Agential Reasoning: Proof-Calculi and Syntactic Decidability for STIT Logics
This work provides proof-search algorithms and automated counter-model extraction for a class of STIT logics. With this, we answer an open problem concerning syntactic decision procedures and cut-free calculi for STIT logics. A new class of cut-free complete labelled sequent calculi G3LdmL^m_n, for multi-agent STIT with at most n-many choices, is introduced. We refine the calculi G3LdmL^m_n through the use of propagation rules and demonstrate the admissibility of their structural rules, resulting in auxiliary calculi Ldm^m_nL. In the single-agent case, we show that the refined calculi Ldm^m_nL derive theorems within a restricted class of (forestlike) sequents, allowing us to provide proof-search algorithms that decide single-agent STIT logics. We prove that the proof-search algorithms are correct and terminate
Neutral-ionic phase transition : a thorough ab-initio study of TTF-CA
The prototype compound for the neutral-ionic phase transition, namely TTF-CA,
is theoretically investigated by first-principles density functional theory
calculations. The study is based on three neutron diffraction structures
collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)).
By means of a topological analysis of the total charge densities, we provide a
very precise picture of intra and inter-chain interactions. Moreover, our
calculations reveal that the thermal lattice contraction reduces the indirect
band gap of this organic semi-conductor in the neutral phase, and nearly closes
it in the vicinity of the transition temperature. A possible mechanism of the
neutral-ionic phase transition is discussed. The charge transfer from TTF to CA
is also derived by using three different technics.Comment: 11 pages, 9 figures, 7 table
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
Effect of Benzoic Acids on Barite and Calcite Precipitation
The effect of various benzoic acids on the precipitation of barite (BaSO4) and calcite (CaCO3) was investigated. The acids varied in the number of carboxylate groups, from dibenzoic acids (phthalic, isophthalic, and terephthalic) through to the hexabenzoic acid (mellitic acid). It was found that the stereochemistry of the dibenzoic acids was important, as was the pH of the solution (trimesic acid was used as a test case and showed that greatest inhibition was achieved with all carboxylate groups deprotonated). Interestingly, for both the calcite and barite systems, mellitic acid was found to be both a potent inhibitor and a significant crystal growth modifier. In the case of barite, the presence of mellitic acid produced nanoparticles that agglomerated. The nanoparticles were found to be 20 nm in size from X-ray diffraction (XRD) line width analysis and 20-50 nm from transmission electron microscopy (TEM). Humic acid was also tested and found to form bundled fibers of barium sulfate
Dynamics of surface magnetization on a nanosecond time scale
The dynamics of surface magnetization is measured with ns time resolution by spin-polarimetry of the total photoemission yield excited by synchrotron radiation pulses. The surface response is compared to the bulk magnetization dynamics as obtained by induction measurements. The surface and the bulk show distinct magnetization dynamics indicating weak coupling during the reversal process in the (Formula presented) time domain. Ultrathin layers of Fe as well as three-layer Fe/Cu/Fe exchange coupled structures were grown on top of an amorphous soft-ferromagnetic substrate (Vitrovac) and showed different reversal dynamics
Structure and dynamics of pentacene on SiO2: From monolayer to bulk structure
We have used confocal micro Raman spectroscopy, atomic force microscopy (AFM), and x-ray diffraction (XRD) to investigate pentacene films obtained by vacuum deposition on SiO2 substrates. These methods allow us to follow the evolution of lattice structure, vibrational dynamics, and crystal morphology during the growth from monolayer, to TF, and, finally, to bulk crystal. The Raman measurements, supported by the AFM and XRD data, indicate that the film morphology depends on the deposition rate. High deposition rates yield two-dimensional nucleation and quasi-layer-by-layer growth of the T-F form only. Low rates yield three-dimensional nucleation and growth, with phase mixing occurring in sufficiently thick films, where the T-F form is accompanied by the "high-temperature" bulk phase. Our general findings are consistent with those of previous work. However, the Raman measurements, supported by lattice dynamics calculations, provide additional insight into the nature of the TFs, showing that their characteristic spectra originate from a loss of dynamical correlation between adjacent layers
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