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

Terahertz Raman scattering as a probe for electron-phonon coupling, disorder and correlation length in molecular materials

Terahertz (or low-frequency) Raman spectroscopy has been shown to be a quite useful tool to infer important information on some key properties of molecular materials, like polymorphism, phase purity and phase transitions. Based on some of our recent studies, we present promising new directions and possible development of the technique for the characterization of electron-lattice phonon coupling, disorder and correlation length in systems of low-dimensionality. The relative strength of electron-lattice phonon coupling can be extracted from the intensities of the Raman bands in the pre-resonance Raman regime, as exemplified in the charge-transfer (CT) crystal N,N-dimethylphenazine-tetracyanoquinodimethane (M2P-TCNQ). Disorder is instead reflected in the Raman bandwidth, which we analyze with polarized light for systems of reduced dimensionality. The sample system studied for the one-dimensional case is the tetramethylbenzidine-tetrafluoroTCNQ CT crystal. As an example of a quasi two-dimensional (2D) system we address pentacene, the classical case of a monomolecular material widely studied for its application in organic electronics. Here the discussion is mostly related to the dispersion of the phonon branches, eventually leading to peculiar spectral profiles depending on the 2D or 3D regime of the films grown under different deposition conditions

The Dimer Model for k-phase Organic Superconductors

We prove that the upper electronic bands of k-phase BEDT-TTF salts are adequately modeled by an half-filled tight-binding lattice with one site per cell. The band parameters are derived from recent ab-initio calculations, getting a very simple but extremely accurate one-electron picture. This picture allows us to solve the BCS gap equation adopting a real-space pairing potential. Comparison of the calculated superconducting properties with the experimental data points to isotropic s_0-pairing. Residual many-body or phonon-mediated interactions offer a plausible explanation of the large variety of physical properties observed in k-phase BEDT-TTF salts.Comment: 8 pages, 6 PostScript figures, uses RevTe

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 $\rho$ 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 $P$ 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 $\rho < 1/2$ 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 $\rho$. 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 $\rho < 1$.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

Electron-phonon coupling in crystalline Pentacene films

The electron-phonon(e-p) interaction in Pentacene (Pn) films grown on Bi(001) was investigated using photoemission spectroscopy. The spectra reveal thermal broadening from which we determine an e-p mass enhancement factor of lambda = 0.36 +/- 0.05 and an effective Einstein energy of omega_E = 11 +/- 4 meV. From omega_E it is inferred that dominant contributions to the e-p effects observed in ARPES come from intermolecular vibrations. Based on the experimental data for lambda we extract an effective Peierls coupling value of g_eff = 0.55. The e-p coupling narrows the HOMO band width by 15 +/- 8% between 75K and 300K.Comment: 19 pages, 5 figures and supplementary informatio

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

BEDT-TTF organic superconductors: the entangled role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency.Comment: 4 pages, including 4 figures. Published version, with Ref. 17 corrected after publicatio

Direct evidence of overdamped Peierls-coupled modes in TTF-CA temperature-induced phase transition

In this paper we elucidate the optical response resulting from the interplay of charge distribution (ionicity) and Peierls instability (dimerization) in the neutral-ionic, ferroelectric phase transition of tetrathiafulvalene-chloranil (TTF-CA), a mixed-stack quasi-one-dimensional charge-transfer crystal. We present far-infrared reflectivity measurements down to 5 cm-1 as a function of temperature above the phase transition (300 - 82 K). The coupling between electrons and lattice phonons in the pre-transitional regime is analyzed on the basis of phonon eigenvectors and polarizability calculations of the one-dimensional Peierls-Hubbard model. We find a multi-phonon Peierls coupling, but on approaching the transition the spectral weight and the coupling shift progressively towards the phonons at lower frequencies, resulting in a soft-mode behavior only for the lowest frequency phonon near the transition temperature. Moreover, in the proximity of the phase transition, the lowest-frequency phonon becomes overdamped, due to anharmonicity induced by its coupling to electrons. The implications of these findings for the neutral-ionic transition mechanism is shortly discussed.Comment: 11 pages, 13 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