The structural phase transitions of the pyrochlore superconductor Cd 2 Re 2 O 7 are investigated by Raman light scattering. The cubic to tetragonal transition at 200 K is characterized by the appearance of a broad mode originating from oxygen ion motion in agreement with recent theoretical predictions ͓I. A. Sergienko et al., Phys. Rev. Lett. 92, 065501 ͑2004͔͒. The appearance of well-defined low-frequency modes at the 120-K transition indicates that it is driven by ordering of the Cd ions within the channel voids of the distorted pyrochlore.
We present Raman measurements on the iron-pnictide superconductors CeFeAsO 1−x F x and NdFeAsO 1−x F x . Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interactions, we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B 1g mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum, at least at small wave numbers, is well represented by phononphonon interactions without any significant electronic contribution. Even at zero temperature there is a nonnegligent effect of interactions on the phonon energy, which for the Fe B 1g mode corresponds to 6 cm −1 or 3% of the total energy of the mode. We also estimate the anharmonic expansion from Fe ͑56→ 54͒ isotope substitution to ⌬a Ϸ 5.1ϫ 10 −4 Å and ⌬d Fe-As Ϸ 2.5ϫ 10 −4 Å and the shift of harmonic zero-point fluctuations of bond lengths ͗⌬x 2 ͘ Շ 3 ϫ 10 −5 Å 2 , giving a total relative average decrease in electronic hopping integrals of ͉␦t͉ / t Շ 2.0ϫ 10 −4 . For a nonphonon-mediated weak-coupling superconductor this gives an isotope exponent ␣ ϳ 10 −2 . The results pose a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe isotope effect.
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