Structural and Chemical Investigations of Superconducting Lanthanum Cuprate Bilayer Interfaces
Authors: Suyolcu Y. Eren , Wang Yi , Sigle Wilfried , Cristiani Georg , Logvenov Gennady , van Aken Peter A. 
Published: December 2016

Highly adaptable crystal structures of complex oxide materials enable changes in composition and provide the opportunity of fabricating them in different forms e.g., thin films and/or heterostructures. Complex oxide heterostructures show noteworthy electronic properties which are absent in bulk forms, and the phenomena occurring at their interfaces are intriguing. The interest attributed to these structures and interfaces is also related to the wide variety of the functionalities such as ferroelectricity, magnetism, superconductivity etc. [1]. One of the most exciting interface effects is high temperature interfacial superconductivity discovered in heterostructures consisting of non‐superconducting layers of insulating La
2
CuO
4
(LCO) and metallic La
1.55
Sr
0.45
CuO
4
(LSCO). Moreover, cuprate bilayers are also attractive in which the critical temperature can be enhanced remarkably in comparison to single phase systems [2].

Atomic‐layer‐by‐layer oxide molecular‐beam epitaxy (ALL‐oxide MBE) with its unique capabilities allowed us to synthesize bilayers consisting of 3 unit cells of metallic La
1.6
M
0.4
CuO
4
layer and 3 unit cells of the undoped insulating (I) LCO layer neither of which is superconducting by its own, where M represents the dopant, namely Sr
2+
, Ca
2+
, and Ba
2+
which have the same ionic charge but different ionic radii. The bilayers are superconducting with a critical temperature of ~17.0 K, ~37.0 K and ~38.5 K for Ca, Sr and Ba dopants, respectively.

For STEM investigations, a JEOL‐ARM 200F STEM equipped with a cold field‐emission electron source, a probe
C
s
corrector (DCOR, CEOS GmbH), a 100 mm
2
JEOL Centurio SDD‐type EDX detector, and a Gatan GIF Quantum ERS spectrometer was used. High‐angle annular dark‐field (HAADF) imaging and electron energy‐loss spectroscopy (EELS) were performed at probe semi‐convergence angles of 20 mrad and 28 mrad, respectively. Collection angles for HAADF and annular bright‐field (ABF) images were 75‐310 mrad and 11‐24 mrad, respectively.

Atomically resolved ABF and HAADF images were simultaneously acquired. HAADF images unveil not only the perfect interfaces between substrate and the bilayers but also the high structural quality (Fig 1a‐b). Atomic resolution EELS and energy‐dispersive x‐ray spectroscopy (EDXS) analyses were conducted to examine elemental distributions across the interfaces. EDXS analyses and EEL spectrum images (SI) reveal different cation redistribution behaviors and lengths depending on the cation size mismatch with respect to the La ionic radius. In Fig.1c the distribution of La and Sr across the interfaces of a Sr‐doped bilayer sample is presented, which reveals relatively homogeneous distribution of Sr and also the redistribution of Sr near the interface.

Through ABF imaging, oxygen positions were determined, interatomic distortions (cation – cation and CuO
6
octahedra) were measured and correlated with the dopant distributions obtained via chemical analyses. In Fig. 2a two of the quantitatively studied images (i.e. HAADF and ABF), are presented as an overlay and the arrows indicate the nominal interfaces. Figure 2b shows the measured interatomic distances which are indicated with arrows. Results for the different dopants will be presented and correlations with high‐temperature interfacial superconductivity will be discussed [3].