Generic Buffer Layers for Fe-based Superconductors:
Epitaxial FeTe_1-xSe_x Thin Films

K. Iida¹, J. Hänisch¹, E. Reich¹, F. Kurth¹, L. Schultz¹, B. Holzapfel¹, M. Schulze¹,
S. Aswartham², S. Haindl², S. Wurmehl², B. Büchner²,
S. Takeda³, S. Ueda³, M. Naito³, C. Tarantini⁴, J. Jaroszynski⁴

¹Institute for Metallic Materials, IFW Dresden
²Institute for Solid State Research, IFW Dresden, 01069 Germany
³Tokyo University of Agriculture &Technology, Tokyo 184-8588, Japan
⁴ National High Magnetic Field Laboratory, FL 32310, USA

Abstract - Iron chalcogenide superconductors are very sensitive to strain. Hence the superconducting transition temperature, T_c, of FeTe_1-xSe_x films can be tuned by strain and even higher T_c values can be realized in films than in bulk material. The biaxial strain is usually induced by a lattice mismatch between film and substrate. However, the correlation between T_c and lattice mismatch for FeTe_1-xSe_x is controversial. A fundamental problem is the formation of an interfacial layer between FeTe_1-xSe_x and oxide substrates, which compromises the epitaxial growth. Therefore, we proposed the implementation of a Fe buffer layer to grow epitaxial FeTe_1-xSe_x thin films. Similar to the Ba-122/Fe bilayers, 1) epitaxial
FeTe_1-xSe_x films with sharp out-of-plane and in-plane texture have been realized on Fe-buffered single crystalline MgO substrates. 2) The clean interface between Fe and FeTe_1-xSe_x has been also confirmed by transmission electron microscopy. These results indicate that Fe can work as generic buffer layer for epitaxial growth of Fe-based superconductors. The FeTe_1-xSe_x/Fe bilayer with a high T_c of 19 K showed strong intrinsic pinning from correlated ab-planes, since the Se(Te)-Se(Te) interlayer distance is almost identical to the out-of-plane coherence length at low temperatures. This work is partially supported by DFG. Project, No. BE 1749/13 and HA 5934/3-1. We also acknowledge the EU (IRON-SEA and SUPERIRON) under project No. FP7-283141 and FP7-283204. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy.

Keywords – Fe-based superconductors, FeTe_1-xSe_x, thin film, epitaxy, buffer layer, iron buffer.

[1] T. Thersleff et al., Appl. Phys. Lett. 97, 022506 (2010).
[2] K. Iida et al., Appl. Phys. Lett. 99, 202503 (2011).

IEEE/CSC & ESAS European Superconductivity News Forum (ESNF) No. 22 October/November 2012.  ESNF Reference No.STP300 Categories 2, 5.
This is an invited ASC 2012 presentation 4LF-01 not submitted to IEEE Trans. Appl. Supercond. (2013) for possible publication.