The problem of phonons and their influence on superconductivity in high temperature cuprate superconductors is a long standing one. However, advances in neutron scattering, especially at the Spallation Neutron Source, make it timely to revisit this problem. Neutron techniques are also sensitive to spin fluctuations and crystal electric field (CEF) levels. Understanding any of these degrees of freedom individually involves some quantitative understanding of all of them as there is very significant overlap in Q and energy, and their relative intensities are comparable. Neutron spectroscopic measurements have been carried out on a family of 214 cuprate superconductors of the form La(2−x−y)Nd(y)Sr(x)CuO4 with hole doping x = 1/8 and the isoelectronic Nd doping such that y varies from y = 0 to y = 0.4. This system shows superconducting TCs that vary by a factor of 9 (TC∼ 3 K for y = 0.4 to TC∼ 28 K for y = 0) even though the hole doping does not change. Our measurements of the phonon dispersions using time-of-flight neutron spectrometers shows both common features and interesting variations across this series. In particular, there is a low-lying optic phonon near 19 meV which is strong in the y = 0 member (LCSO) but absent in the y = 0.4 member.
Gaulin et al. (Wed,) studied this question.