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The onset of electron-phonon interaction in the superconducting state is unusual in term of conventional electron-phonon interaction where one would expect that the phonon contribution is weakly dependent on the temperature [19], and increase at high T. Indeed, based on this naive expectation, this type of unconventional T dependence has been often used to rule out phonons. Here, however, we see clearly that this reasoning is not justified. Moreover, this type of unconventional enhancement of the electron phonon interaction below a characteristic temperature scale is actually expected for other systems such as spin-Peierls systems or charge density wave (CDW) systems. Thus, our results put an important constraint on the nature of the electron phonon interaction in these systems. 

To emphasize the importance of structural studies at LT or HP, let us take the example of TTF-TCNQ. This prototype charge density wave (CDW) system undergoes at ambient pressure a succession of three structural and electronic phase transitions, from a high-temperature metallic phase down to a low-temperature insulator phase. There has been a considerable debate about the mechanism of these transitions, and many distortional modes have been proposed to account for the physical properties of this material (e.g., rigid molecule displacement as translations [73,74] or librations [75,76] or even internal deformations of the molecules [77,78]. Indeed, an experi-... 

Figure 16.2 Comparison of raw data and normalized reduced total scattering struc ture function F Q) = Q[S(Q) 1]. The sample is a charge density wave organic and the data are X ray data from CHESS synchrotron source. The raw data are shown in the top panel, on expanded scales in the insets. The high Q data in the region 20 < g < 45 is smooth and featureless. However, after normalizing and dividing by the square of the atomic form factor, important diffuse scattering is evident in this region of the diffraction pattern (lower panel). ...

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