Neutron scattering study of soft optical phonons in La2-xSrxCuO4-y

The soft optical phonons associated with the tetragonal-to-orthorhombic structural phase transition in La2-xSrxCuO4-y have been investigated using inelastic-neutron-scattering techniques. Single crystals with Sr concentration of x=0, 0.08, and 0.14 were studied. Both branches of the doubly degenerate X-point phonon which go soft at the tetragonal-to-orthorhombic transition have been observed in the orthorhombic phase. In all three samples one branch hardens, while the other remains relatively soft with behavior indicative of a further incipient structural phase transition. We identify the hard and soft branches of the orthorhombic phase as zone center and zone-boundary phonons, respectively. The zone-center mode has Ag symmetry and is, consequently, Raman active. The relation of these measurements to other structural properties and to the superconductivity in La2-xSrxCuO4-y is discussed.