Surface lattice dynamics of layered transition metal oxides: Sr₂RuO₄ and La_0.5Sr_1.5MnO₄

Momentum resolved inelastic electron scattering has been used to study the surface lattice dynamics of two layered isostructural perovskite crystals (Sr₂RuO₄ and La_0.5Sr_1.5MnO₄), two materials with different physical properties. There were three surface optical phonons observed with the energies at 30.0, 50.0, and 75.0 meV with a shoulder appearing at 67.5 meV for Sr₂RuO₄ and at 26.0, 56.5, and 78.5 meV for La_0.5Sr_1.5MnO₄. In the K₂NiF₄-type structure, these phonons are associated with the La/Sr vibrational (external), Mn-O-Mn (Ru-O-Ru) bending, and MnO₆ (RuO₆) stretching modes, respectively. The observed surface phonons, especially for breathing and stretching modes, are at significantly higher energies than the corresponding modes in the bulk, which are directly associated with the relative reduction of out-of-plane Mn-O/Ru-O bond lengths at the surfaces. The line shapes of the quasielastic scattering peaks reflects the surface conductivity of Sr₂RuO₄ and the insulating behavior of La_0.5Sr_1.5MnO₄. For Sr₂RuO₄, the large line width of the external mode is attributed to nonequivalent Sr vibrational sites due to the surface reconstruction. For La_0.5Sr_1.5MnO₄, the broad character of both the bending and the stretching modes results from surface electronic inhomogeneities. In contrast to the bulk, the absence of phonon restructuring in the observed temperature range suggests that the long-range charge/orbital ordering transition occurring in the bulk may not take place on the surface, probably correlated with stiffening of observed optical phonons at the surface.