Raman light scattering study and microstructural analysis of epitaxial films of the electron-doped superconductor La 2 -x Ce x CuO 4

We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La2 -x Ce x CuO4 close to optimal doping (x ~ 0.08, T c = 29 K and x ~ 0.1, T c = 27 K). The main focus of this work is a detailed characterization and microstructural analysis of the films. Based on micro-Raman spectroscopy in combination with X-ray diffraction, energy-dispersive X-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu2O. In the slightly underdoped film (x ~ 0.08), both the Cu2O modes and others that can be assigned to the La2 -x Ce x CuO4 matrix show pronounced softening and narrowing upon cooling below T ~ T c . Based on control measurements on commercial Cu2O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La2 -x Ce x CuO4/Cu2O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La2 -x Ce x CuO4 film (x ~ 0.1) did not reveal comparable phonon anomalies.