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

Transition Metal Cluster Compounds: Luminescence and Design of (nano)Composites

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Incoherent Interplane Conductivity of kappa-(BEDT-TTF)2Cu[N(CN)2]Br

The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the DC resistivity. We set an upper limit to the interplane transfer integral of tb = 0.1 meV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the phonon lines in the conductivity to the asymmetric modes of the ET molecule.Comment: 7 pages with embedded figures, submitted to PR

RAMAN STUDY OF DOPED POLYACETYLENES

Nous présentons les résultats de diffusion Raman obtenus sur des échantillons de polyacétylènes dopés soit avec des accepteurs d'électrons (I2, Br2, SbF5, ClO4-, FeCl3), soit avec des donneurs d'électrons (Li par exemple). Nous montrons que l'isomérisation cis-trans est toujours induite, quel que soit le dopage effectué. Le dopage s'accompagne d'un raccourcissement des séquences trans (par rapport à un trans-(CH)x non dopé) si l'on admet le modèle des distributions de longueurs de chaînes. En plus, une bande Raman apparaît à environ 1600 cm-1. Cette bande ne dépend pas du dopant utilisé.We present Raman scattering results obtained on doped polyacetylene films, using a variety of dopants including electron acceptors (I2, Br2, SbF5, ClO4-, FeCl3) and electron donors (Li for example). We show that a cis-trans isomerization is always induced upon doping. Compared to an undoped trans-(CH)x, the doping process leads to a shortening of the trans sequences, if the model of the chain length distributions is valid. For all doped samples, an additional band is observed close to the C=C stretch Raman band at about 1600 cm-1

Oxygen vibrations in the series Bi2\mathsf{_{2}}Sr2\mathsf{_{2}}Can−1{_{n-1}}Cun{_{n}}O4+2 n+y{_{4+2\,n+y}}

We present a discussion of the oxygen vibrations in the Bi$_{2}$Sr$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{4+2\,n+y}$ high $T_{\rm c}$ superconductors with the aim of interpreting Raman spectra in the case of the non-symmorphic Amaa structure. Group theory shows that the oxygen atoms belonging to the central CuO$_{2}$ plane generate a Raman activity for the $n=1,3$ phases. Consequently, we propose a novel assignment for the lines of weak intensity at 297, 316 and 333 cm$^{-1}$. It is shown that the two components of the 460 cm$^{-1}$ band may be consistent with the Amma structure. Spectra recorded in crossed polarization exhibit weak lines which could be assigned to B $_{1\rm g}$ modes expected for the three phases.Nous présentons une discussion sur les vibrations des atomes d'oxygène dans la série des supraconducteurs Bi$_{2}$Sr$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{4+2\,n+y}$ dans le but d'interpréter les spectres Raman. L'analyse des modes normaux de vibration de la structure Amaa pour les phases $n=1$ ou 3 montre que les atomes d'oxygène du plan CuO$_{2}$ contenant les centres d'inversion donnent lieu à une activité Raman. En conséquence, nous proposons une nouvelle attribution pour les raies de faible intensité à 297, 316 et 333 cm$^{-1}$. Nous montrons que le dédoublement de la bande à 460 cm$^{-1}$ pourrait être dû à la structure Amaa. Les spectres enregistrés en polarization croisée montrent de faibles bandes qui peuvent être attribuées aux modes B $_{1\rm g}$ attendus pour les trois phases

Transient photoluminescence from highly disordered silica-rich natural phases with and without nanostructures

International audienceThe ultrafast transient photoluminescence of impact silica-rich glasses and nanostructured opals is investigated at the (sub)nanosecond time scale. Spectral and temporal data were acquired with a high-resolution streak camera under high-density energy laser excitation at 4.17 eV (297 nm). All samples reveal blue photoluminescence relaxing in less than 50 ns. Relaxation decays and luminescence energy vary strongly from opals to impact glasses. Opals are found to relax in less than 16 ns with a maximum emission at 2.6–2.8 eV (443–477 nm) while the high-silica glasses exhibit a much longer luminescence in the 50-ns time window, which is spectrally blueshifted towards 3.0–3.5 eV (354–413 nm). Results are interpreted in terms of the presence of nonbridging oxygen atoms, network modifiers, and nanostructures which produce emission from self-trapped excitons and from excitons recombining at surface defects. The short-lived emissions of opals are characteristic of intrinsic surface photoluminescence quenched after about 10 ns via nonradiative decay channels with an annihilation component, and involve recombination luminescence of self-trapped excitons

Phonon self-energy effects in κ − ( BEDT − TTF ) 2 Cu [ N ( CN ) 2 ] Br

International audienceWe report on the observation of phonon self-energy effects in the vicinity of Tc in the organic superconductor κ−(BEDT−TTF)2Cu[N(CN)2]Br, and their interpretation in the framework of strong electron-phonon coupling. The data qualify anomalous temperature dependence of the low-frequency phonons around and below Tc, and are consistent with an isotropic gap 2Δ0 close to 2.8 meV. Significant frequency shifts are recorded which allow us to evaluate the electron-phonon coupling λ for energetically weak phonons. The subsequent calculated values agree well with a superconducting transition at Tc∼11.6 K

Mapping emissive channels of quantum dots: Influence of size and environment on energy transfer in the time domain

In the quantum confinement regime, the time-resolved photoluminescence mapping of CdSe nanocrystals reveals unusual size-dependent characteristics in solute or solid-state environments. The former case typifies long-lived intradot exciton recombinations, while in the latter energy-dependent Kohlrausch relaxations quench luminescence via interdot energy transfer. The observed energy transfer rate increases for larger size (1.5 X 10(8) s(-1) for d = 2.5 nm and 6.4 X 10(8) s(-1) for d = 10 nm). (C) 2010 American Institute of Physics

Spectroscopy and DFT studies of uranyl carbonate, rutherfordine, UO 2 CO 3 : a model for uranium transport, carbon dioxide sequestration, and seawater species

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Vibrational analysis of trans-polyacetylene : (CH)x and (CD)x

We have calculated the normal modes « in the plane » of a trans-polyacetylene chain (CH)x, using a model with twelve force constants. When x is infinite, the Born cyclic condition allows us to determine both the dispersion and the density of states curves by using Fourier's dynamical, Hermitian matrix technique. It is shown that the peaks of the density of states correspond well to the normal mode frequencies of a chain of finite length (starting from x = 20) observable from IR and Raman scattering.Nous avons effectué le calcul des modes normaux « dans le plan » d'une chaîne de trans-polyacétylène (CH)x à l'aide d'un modèle à douze constantes de force. Lorsque x est infini, la condition cyclique de Born permet, à l'aide de la matrice dynamique de Fourier, de déterminer les courbes de dispersion et la courbe de densité d'états. Nous montrons que les fréquences correspondant aux pics de densité d'états coincident sensiblement avec celles des modes normaux d'une chaîne de longueur finie (à partir de x = 20) observables par effet Raman ou IR