Phonon thermal conductivity in doped La2CuO4\rm\bf La_2CuO_4: Relevant scattering mechanisms

Results of in-plane and out-of-plane thermal conductivity measurements on $\rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ ($0\leq x\leq0.2$) single crystals are presented. The most characteristic features of the temperature dependence are a pronounced phonon peak at low temperatures and a steplike anomaly at $T_{LT}$, i.e., at the transition to the low temperature tetragonal phase (LTT-phase), which gradually decrease with increasing Sr-content. Comparison of these findings with the thermal conductivity of $\rm La_{2-x}Sr_xCuO_4$ and $\rm La_2NiO_4$ clearly reveals that in $\rm La_{2-x}Sr_xCuO_4$ the most effective mechanism for phonon scattering is impurity-scattering (dopants), as well as scattering by soft phonons that are associated with the lattice instability in the low temperature orthorhombic phase (LTO-phase). There is no evidence that stripe correlations play a major role in suppressing the phonon peak in the thermal conductivity of $\rm La_{2-x}Sr_xCuO_4$.Comment: 7 pages, 4 figure

LIGHT SCATTERING FROM CHARGE-DENSITY WAVES

On présente de nouvelles expériences de diffusion de lumière par des excitations d'ondes de densité de charge dans des matériaux à une et deux dimensions. En particulier, on discute la dépendance en température des modes en relation avec la transition Peierls.New light scattering experiments on CDW excitations in quasi-one- and two-dimensional materials are reported. In particular, the temperature dependence of the modes is discussed with respect to the Peierls transition

Raman scattering investigation of ferroelastic Sb5O7I crystals

Sb5O7I undergoes a displacive phase transition at 481 K where the symmetry is changed fromC 6h 2 toC 2h 5 . In the low temperature monoclinic phase the crystal is ferroelastic. The polarized Raman spectra of Sb5O7I have been measured at various temperatures below and above the phase transition. The frequencies and symmetries of most of the theoretically expected Raman active phonons in the ferroelastic phase have been determined. The observation of a soft mode in the ferroelastic phase which disappears above the phase transition together with the fact that the unit cell of the ferroelastic phase is twice as large as that of the paraelastic structure permits the conclusion that the phase transition results from a phonon instability at the Brillouin zone boundaryM-point of the hexagonal phase. The temperature dependent splittings and intensity changes of several Raman lines are discussed with respect to the ferroelastic property of the crystal and the phase transition