New oxygen lattice modes in the metallic region of La 2-x Sr x CuO 4

Inelastic neutron scattering measurements are presented that show a profound change in the spectrum of lattice vibrations when doping across the metal–insulator transition in La2-xSrxCuO4. In the metallic state, a new band of vibrations appears near 70 meV. These vibrations can be understood as the mixing of oxygen bond-stretching modes with doped holes in the CuO2 plane.