Atomic-level evolution of lattice variation and strontium redistribution in Sr-δ-doped La2CuO4
Superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration . In the case of La2CuO4 (LCO), high-Tc superconductivity is obtained either by substituting La3+ with Sr2+ or by inserting interstitial O2- . Recently, by using atomic layer-by-layer oxide molecular beam epitaxy, we have fabricated Sr-δ-doped LCO multilayered structures, in which some atomic layers of LaO have been fully substituted by SrO layers. By varying the spacing between the LCO and SrO layers high-Tc superconductivity (~ 40 K) has been obtained.
The local variation of in-plane and out-of plane atomic lattice parameters and the strontium redistribution in multilayers grown on a LaSrAlO4 (LSAO) substrate are investigated in this study. Cross-sectional HAADF images of the full multilayer, from the substrate to the vacuum, show no structural defects (see figure). The Sr redistribution at the interface was investigated using atomic resolution HAADF imaging in combination with EDX and EELS. An averaged image intensity profile along the growth direction shows that in the Sr-d-doped region the image intensity shows a relatively sharp intensity drop followed by a slowly increasing intensity, i.e.an asymmetric distribution of Sr along the growth direction. This asymmetric Sr distribution is confirmed by EDX and EELS line-scan profiles.
 P. A. Lee, N. Nagaosa, and X. G. Wen, Rev. Mod. Phys. 78 (2006) 17
 B. O. Wells, Y. S. Lee, M. A. Kastner, R. J. Christianson, R. J. Birgeneau, K. Yamada, Y. Endoh, and G. Shirane, Science 277 (1997) 1067
This research has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement no. 312483 (ESTEEM2). Researchers from Dept. Maier collaborate in this project.