168 research outputs found
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
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
Phonons and structures of tetracene polymorphs at low temperature and high pressure
Crystals of tetracene have been studied by means of lattice phonon Raman
spectroscopy as a function of temperature and pressure. Two different phases
(polymorphs I and II) have been obtained, depending on sample preparation and
history. Polymorph I is the most frequently grown phase, stable at ambient
conditions. A pressure induced phase transition, observed above 1 GPa, leads to
polymorph II, which is also obtained at temperatures below 140 K. Polymorph II
can also be maintained at ambient conditions.
We have calculated the crystallographic structures and phonon frequencies as
a function of temperature, starting from the configurations of the energy
minima found by exploring the potential energy surface of crystalline
tetracene. The spectra calculated for the first and second deepest minima match
satisfactorily those measured for polymorphs I and II, respectively. All
published x-ray structures, once assigned to the appropriate polymorph, are
also reproduced.Comment: 8 pages, 5 figures, RevTeX4, update after referees report
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 benzalkonium chloride on dentin bond strength and endogenous enzymatic activity
Objective: This in vitro study evaluated at baseline (T0) and over time (T12 months), the effect of a multi-mode universal adhesive compared with two experimental formulations blended with different concentrations of benzalkonium chloride (BAC), on bond strength and endogenous enzymatic activity. Methods and materials: Specimens were assigned to the following groups according to the adhesive protocol: G1) All-Bond Universal (ABU) self-etch (SE); G2) ABU + 0.5% BAC SE; G3) ABU + 1% methacrylate BAC SE; G4) ABU etch-and-rinse (E&R); G5) ABU + 0.5% BAC E&R; G6) ABU + 1% methacrylate BAC E&R. Gelatin zymography was performed on dentin powder obtained from eight human third molars. Endogenous enzymatic activity within the hybrid layer was examined using in situ zymography after 24 h (T0) or 1-year storage in artificial saliva (T12). Forty intact molars were prepared for microtensile bond strength test at T0 and T12. Results were statistically analyzed with three-way ANOVA (\u3b1 = 0.05). Results: Gelatin zymography assay and in situ zymography quantification analyses indicated that all the BAC-containing formulations decreased matrix metalloproteinase expression. However, in situ zymography showed a general trend of enzymatic activity increase after aging. Microtensile bond-strength testing showed decrease in bond strength over time in all the tested groups; performances of the 1% methacrylate BAC experimental groups were worse than the control. Conclusions: BAC-containing adhesives reduce endogenous enzymatic activity both immediately and over time. However, independently from the adhesive employed, increase in the gelatinolytic activity over time and decrease in bond strength was found (especially in the BAC + 1% methacrylate groups), probably due to impaired polymerization properties. Clinical significance: Adhesives containing protease inhibitors are practical and efficient tools in clinical practice for enhancement of the longevity of dental restorations. However, extensive investigation of the mechanical and adhesive properties of the material is necessary prior to their clinical use
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
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
Pressure Tuning of the Charge Density Wave in the Halogen-Bridged Transition-Metal (MX) Solid
We report the pressure dependence up to 95 kbar of Raman active stretching
modes in the quasi-one-dimensional MX chain solid . The data
indicate that a predicted pressure-induced insulator-to-metal transition does
not occur, but are consistent with the solid undergoing either a
three-dimensional structural distortion, or a transition from a charge-density
wave to another broken-symmetry ground state. We show that such a transition
cacan be well-modeled within a Peierls-Hubbard Hamiltonian. 1993 PACS:
71.30.+h, 71.45.Lr, 75.30.Fv, 78.30.-j, 81.40.VwComment: 4 pages, ReVTeX 3.0, figures available from the authors on request
(Gary Kanner, [email protected]), to be published in Phys Rev B Rapid
Commun, REVISION: minor typos corrected, LA-UR-94-246
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