Electron sampling depth and saturation effects in perovskite films investigated by soft x-ray absorption spectroscopy

Knowledge of the electron sampling depth is important for quantitative analysis of x-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here, we study absorption saturation in films of two of the most-studied perovskites, La0.7Ca0.3MnO₃ (LCMO) and YBa₂Cu₃O₇ (YBCO), at the L2,3 edges of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length {λ}, and ratio λ/d have been independently determined between 50 and 300 K. The extracted sampling depth dLCMO≈3 nm for LCMO films at high temperatures in the polaronic insulator state (150--300 K) is near the values reported for some transition metals (dX=1.7--2.5 nm, where X = Fe, Co, Ni) at room temperature, and it is much smaller than dYBCO≈5 nm measured for YBCO films and the value previously reported for Fe₃O₄ (dFe3O4=4.5nm). The measured dLCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that the sampling depth in oxides is strongly material dependent and can be measurably influenced by electronic phase transitions deriving from strong correlations.