The transient response of Ba(Fe1−xCox)2As2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range, and a slower one, corresponding to lattice thermalization and lasting ∼ = 6ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice vibration modes, with a second moment of the Eliashberg function λ ω 2 ∼ = 64 meV 2 . The simultaneous excitation of a strong fully symmetric A1g optical phonon corroborates this conclusion and makes it possible to deduce the value of λ ∼ = 0.12. This small value for the electron-phonon coupling confirms that a phonon mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides. PACS numbers: 74.70.Xa; 78.47.J-; 74.25.KcThe discovery of high temperature superconductivity in iron pnictide compounds in 2008(Refs. 1 and 2 ) has raised a lot of questions about the nature of this phenomenon. One of these questions concerns the role of electron-phonon (e-ph) coupling, which is at the heart of conventional Bardeen-Cooper-Shrieffer (BCS) superconductivity 3 .For unconventional superconductors, electron-lattice interaction mechanisms have been extensively studied in cuprates; these materials present some similarities with pnictides, such as the bidimensionality of the crystallographic structure with Cu-O planes instead of Fe-As ones and the presence of a magnetic phase in the underdoped part of the phase diagram 4,5 . While the electron pairing mechanism is still controversial in superconducting cuprates, some elements are today accepted: first, their high critical temperatures are not compatible with a BCS scheme; second, the e-ph interaction is anisotropic, as theoretically predicted 6 and verified by means of time-resolved experiments 7-11 ; indeed, the e-ph coupling constant is strongly mode-selective, and ranges from λ ∼ = 0.13 to λ ∼ = 0.55. This selectivity is linked to the marked bidimensional layered structure of cuprates, leading to a preferential coupling between electrons coming from a specific k-direction of the Fermi surface and one particular phonon mode 6,8 .Much less information is available on e-ph coupling in pnictides. Theoretical works on the 1111 family (LaFeAsO 1−x F x ), employing Density Functionnal perturbation theory, predicted an isotropic coupling, equally distributed for the whole phonon population, and too small to be responsible for superconductivity through a BCS type mechanism 5,12 . On the other hand, a very strong coupling (λ ∼ = 1) between electrons and the A 1g mode (consisting of a breathing movement of As atoms) was predicted by a model in which the electronic polarization of As atoms involve e-ph interaction 13 . For 122 compounds (doped AF e 2 As 2 , A=Ba, Sr, Ca) spin f...

The so-called "polar catastrophe," a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO 3 and SrTiO 3 . We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO 3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO 3 layer.

We report the observation of a coherent lattice oscillation in a pnictide superconductor. A coherent fully symmetric optical phonon was detected in Ba͑Fe 1−x Co x ͒ 2 As 2 ͑x = 0.06 and x = 0.08͒ using time-resolved pumpprobe reflectivity with 40 fs time resolution. The analysis of the phonon parameters for various excitation fluences reveals no evident difference below and above the critical temperature, suggesting that the A 1g mode is not involved in the superconducting phase transition.

The transient reflectivity response of phase-change Ge 2 Sb 2 Te 5 films to intense femtosecond laser pulses is studied by ultrafast coherent phonon spectroscopy. The three different phases ͑amorphous, fcc-, and hcp-crystalline͒, as well as laser-crystallized films, are investigated, featuring different photoexcited carrier and coherent optical phonon dynamics. At least two main phonon frequencies are identified for each phase/material and their evolution for increasing pump fluences is investigated for the fcc-crystalline phase and the laser-crystallized material, revealing strong differences. We find evidence that a considerable fraction of amorphous phase remains in the laser-crystallized material, which features a different phonon frequency, not related to other phases. These results are important for emerging strategies aimed at driving ultrafast phase transitions via coherent phonon excitation for applications in data storage.

The microsolvation of verbenone (C10H14O)·(H2O)n (n = 1, 2, 3) was investigated in a supersonic expansion using a cavity-based Fourier transform microwave spectrometer, in the 2.8–14 GHz frequency range and by computational chemistry.

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We present a detailed study of the optical second harmonic (SH) generated in samples of centrosymmetric muscovite mica. Samples with thicknesses ranging from 30 to 300 μm have been investigated in a transmission normal-incidence geometry. We found a strong dependence of the polarization-dependent SH signal on the sample thickness. In particular, in some of the thickest samples, the SH signal is strongly enhanced. This signal amplification is not monotonically increased with thickness, but it varies erratically. These findings show that under the present experimental conditions, quadrupolar bulk second harmonic generation in mica becomes dominant on the SH generated from the surfaces. The large variability of the SH signal with the variation in thickness is then ascribed to partial optical phase-matching effects, controlled by the mica birefringence. In order to corroborate this hypothesis, a detailed theoretical model accounting for the nonlocal response and anisotropy of a generic birefringent crystal has been developed. The predictions of our model are found to be in excellent agreement with the experimental data.

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