We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La_{ 2-x}Ce_{ x}CuO_{ 4} 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 Cu_{ 2}O. In the slightly underdoped film (x ~ 0.08), both the Cu_{ 2}O modes and others that can be assigned to the La_{ 2-x}Ce_{ x}CuO_{ 4} matrix show pronounced softening and narrowing upon cooling below T ~ T_c. Based on control measurements on commercial Cu_{ 2}O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La_{ 2-x}Ce_{ x}CuO_{ 4}/Cu_{ 2}O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La_{ 2-x}Ce_{ x}CuO_{ 4} film (x ~ 0.1) did not reveal comparable phonon anomalies.
Categories: cond-mat.supr-con