Giovanni Visentini

Lattice dynamics and electron-phonon coupling in the β − ( BEDT − TTF ) 2 I 3 organic superconductor

The crystal structure and lattice phonons of (BEDT-TTF)_2I_3 superconducting \beta-phase are computed and analyzed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Whereas the crystal structure and its temperature and pressure dependence are properly reproduced within a rigid molecule approximation, this has to be removed to account for the specific heat data. Such a mixing between lattice and low-frequency intramolecular vibrations also yields good agreement with the observed Raman and infrared frequencies. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The hopping integrals are evaluated by the extended Hueckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function, which compares well with that experimentally obtained from point-contact spectroscopy. The corresponding dimensionless coupling constant \lambda is found to be around 0.4 .

Lattice dynamics and electron-phonon coupling in \beta-(BEDT-TTF)_2I_3 organic superconductor

The crystal structure and lattice phonons of (BEDT-TTF)_2I_3 superconducting \beta-phase are computed and analyzed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Whereas the crystal structure and its temperature and pressure dependence are properly reproduced within a rigid molecule approximation, this has to be removed to account for the specific heat data. Such a mixing between lattice and low-frequency intramolecular vibrations also yields good agreement with the observed Raman and infrared frequencies. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The hopping integrals are evaluated by the extended Hueckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function, which compares well with that experimentally obtained from point-contact spectroscopy. The corresponding dimensionless coupling constant \lambda is found to be around 0.4 .

Lattice phonons in neutral BEDT-TTF crystal

Abstract The crystallographic structure and lattice phonon frequencies of neutral bis(ethylendithio)tetrathiafulvalene (BEDT-TTF) as a function of temperature have been computed within a quasi-harmonic lattice dynamics framework. The results are compared with the known crystal structure and with the phonon frequencies obtained from polarized Raman spectra on single crystals.

Structure and phonons of α-(ET)2+I3− crystals

Abstract A potential model for the (ET)2+I3− salts is developed. The model is tested with excellent results by computing, within a quasi-harmonic lattice dynamics framework, the crystallographic structure and the phonon frequencies of α-(ET)2+I3− as a function of temperature.

Spectroscopic characterization of (BEDT-TTF)2[Pt(S2C4O2)2| charge density wave ground state

We report a detailed spectroscopic characterization of the charge transfer (CT) complex between bis(ethylendithio)tetrathiafulvalene (BEDT-TTF) and bis(dithiosquarate)platinate(II), (BEDT-TTF) 2 [Pt(S 2 C 4 O 2 ) 2 ]. In this complex BEDT-TTF dimers are separated by the [Pt(S2C4O2)2] 2– anions, forming a quasi-one-dimensional stack. We show that the stack structure is stabilized by CT between BEDT-TTF dimers and between BEDT-TTF and [Pt(S 2 C 4 O 2 ) 2 ]. Two CT transitions, polarized along the stack axis, are identified in the optical spectra. Furthermore, the average charge on the molecules is not a multiple integer, and the amplitude of the intrinsic ground state charge density wave (CDW) can be represented as: …[Pt(S 2 C 4 O 2 ) 2 ] 1.8– BEDT-TTF 0.9+ BEDT-TTF 0.9+ [Pt(S 2 C 4 O 2 ) 2 ] 1.8– … The optical spectra polarized along the (BEDT-TTF) 2 [Pt(S 2 C 4 O 2 ) 2 ] stack are well reproduced by a semiempirical model which takes into account the coupling of BEDT-TTF molecular vibrations with the CT electrons (e-mv coupling). Under pressure, (BEDT-TTF) 2 [Pt(S 2 C 4 O 2 ) 2 ] undergoes two distinct phase transitions in the ranges 1-1.6 and 2.2-2.5 GPa, the first involving a stack distortion.

Electron–phonon coupling in BEDT-TTF (ET) superconductors

Abstract By a combined experimental and computational approach we evaluate the strength of electron–phonon coupling in organic superconductors based on the bis-ethylendithio-tetrathiafulvalene (BEDT-TTF or ET) molecule. We consider electron coupling to both intra - and inter -molecular phonons. In the case of β-(ET) 2 I 3 superconductor, the strength of the two types of coupling turns out to be of the same order of magnitude.

CT interactions and e-mv coupling in BEDT-TTF (ET) salts and complexes

Accurate values of ET+ electron-molecular vibration (e-mv) coupling contants have been obtained through polarized microreflectance spectra of (ET)2[Mo6O19], The information gained allow us to fully analyze the spectra of a different CT complex, (ET)2[Pt(S2C4O2)2]where ET+ dimers are separated by the Pt(II)-dithiosquarate ion. In this case, a partial CT is observed between ET dimers and the anion.

Lattice dynamics and e-ph coupling in BEDT-TTF superconductors

Abstract The crystal structures and phonon frequencies of α- and β- phases of (BEDT-TTF)2I3 salts are computed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Experimental specific heat data of the β- phase can be reproduced only by introducing also low frequency intra-molecular vibrations. The strength of the electron-phonon (e-ph) coupling is evaluated.