Type of presentation: Oral

MS-12-O-2007 Atomic level evolution of lattice variation and strontium redistribution in Sr-δ-doped La2CuO4

Wang Y.1, Sigle W.1, Baiutti F.2, Gregori G.2, Logvenov G.2, Maier J.2, van Aken P. A.1
1Max Planck Institute for Intelligent Systems, Stuttgart, Germany, 2Max Planck Institute for Solid State Research, Stuttgart, Germany
yi.wang@is.mpg.de

Superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration [1]. In the case of La2CuO4 (LCO), high-Tc superconductivity is obtained either by substituting La3+ with Sr2+ or by inserting interstitial O2- [2]. 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, and by varying the spacing between the LCO and SrO layers high-Tc superconductivity (~ 40 K) has been obtained. In this contribution, the local variation of in-plane and out-of plane atomic lattice parameters and strontium redistribution in the Sr-δ-doped LCO multilayers on LaSrAlO4 (LSAO) substrate was investigated using a JEOL ARM 200CF scanning transmission electron microscope (STEM) equipped with a cold field-emission electron source, a probe corrector, a large-solid-angle SDD-type EDX detector, and a Gatan GIF Quantum ERS spectrometer. The microscope was operated at 200 kV, a semi-convergence angle of 30 mrad, and 90 - 370 mrad and 11-23 mrad collection angles were used to obtain high angle annular dark-field (HAADF) and annular bright-field (ABF) images.

Figure 1 shows the crystal structure model of LCO (a) and LSAO (b) and their epitaxial orientation relationship, where the atomic positions are assigned in the HAADF (c) and ABF (d) images. Figure 2a represents a typical cross-sectional HAADF image from the substrate to the vacuum, where no structure defects are observed. Atomically resolved HAADF and ABF images, which were simultaneously acquired of the Sr-δ-doped region, are presented in Figure 2b and 2c. The local lattice and oxygen octahedral distortion were evaluated by the image analysis.

A detailed study on the Sr redistribution at the interface was performed by atomic resolution HAADF imaging in combination with EDX and EELS, as shown in Figure 3. Due to the difference in atomic number (ZSr=38, ZLa=57), the atomic columns dominated either by La or Sr give rise to different contrast in the HAADF image. In the Sr-δ-doped region the atomic column intensity is significantly lower than in pure LCO. An averaged image intensity profile along the growth direction shows that in the Sr-δ-doped region the image intensity has a relatively sharp intensity drop followed by a slowly increasing intensity pointing to an asymmetric distribution of Sr along the growth direction. This asymmetric Sr distribution is confirmed by Sr-L EDX and Sr-L2,3 EELS (insert in Fig.3) line-scan profiles.

[1] P.A.Lee, N.Nagaosa, and X.G.Wen, Rev.Mod.Phys. 78 (2006) 17.
[2] B.O.Wells, et al., Science 277 (1997) 1067.

The research leading to these results has received funding from the European Union Seventh Framework Programme [FP/2007-2013] under grant agreement no312483 (ESTEEM2). U. Salzberger is particularly acknowledged for TEM specimen preparation.

Fig. 1: The projected crystal structure of (a) LCO and (b) LSAO. Atomic-resolution HAADF (c) and ABF (d) images showing the orientation relationship between LCO and LSAO.

Fig. 2: (a) HAADF image of Sr-δ-doped LCO on LSAO. Enlargement of simultaneously acquired (b) HAADF and (c) ABF images of the Sr-δ-doped area (Schematically highlighted by the yellow box).

Fig. 3: HAADF image of Sr-δ-doped LCO. The inset shows the integrated Sr-L EDX and Sr-L2,3 EELS line profiles.