Phonon anomaly at 100-150 K in La2-xSrxCuO4

Far infra-red spectroscopy of La2-xSrxCuO4, 0.03 less than or equal to x less than or equal to 0.24, compounds show weak phonon softening at 100-150 K in underdoped samples and, at a slightly lower temperature, in 0.15 and 0.17 Sr LSCO. The maximum phonon renormalisation is 0.1% which is much weaker than 1% as observed in YBCO compounds. Assuming that the phonon renormalisations is LSCO and in YBCO have the same origin we conclude that (a) the correlation of phonon anomalies and local lattice distortions with the opening of a spin gap should be reconsidered, and (b) if magnetic excitations do drive the above phenomena then a spin gap should also exist in LSCO.