Influence of strain on superconducting properties in cuprates and pnictides

The influence of strain on electronic properties of functional materials like superconductors or ferroelectrics has drawn much attention in the scientific community during the last years. The major approaches to study this influence are the application of static pressure experiments or the use of epitaxial strain in thin films having a defined lattice mismatch with the underlying substrate.

The application of piezoelectric substrates is a promising alternative to these approaches for the investigation of strain-dependent properties of functional materials. The applied strain can be changed continuously and reversibly by using an electric field on such substrates due to the inverse piezoelectric effect (Fig. 1). Therefore, superconducting YBa₂Cu₃O_7-x (YBCO), La_1.85Sr_0.15CuO₄ (LSCO) and BaFe_1.8Co_0.2As₂ (Ba122) layers were grown on buffered single crystalline (001) oriented Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃ (PMN-PT) substrates using pulsed laser deposition. X-ray measurements revealed an undisturbed epitaxial growth of these materials on the piezocrystals and verified the strain transfer from the substrate to the superconductor under an applied electric field. The applicable strain values at low temperatures were determined by a Pt strain gauge deposited on the buffered PMN-PT substrates to enable a correlation between the change of the superconducting properties and the applied electrical field. A reversible shift of the superconducting transition temperature T_c of 0.4 K for LSCO and 0.2 K for Ba122 was observed on applying biaxial strains of 0.022% and 0.017%, respectively (Fig. 2). The change of the transition temperature is almost one order of magnitude smaller for optimally doped YBCO (Fig. 3). The observed T_c shift in LSCO is in good agreement to literature data, which confirms the suitability of the piezocrystalline approach for the study of strain-dependent properties in superconductors.

Research Funding: DFG - Deutsche Forschungsgemeinschaft (German Research Foundation)

Figure1: Sketch of the film architecture	Figure 2: Shift of the superconducting transition temperature for LSCO and Ba-122 in dependence on the applied electrical field at the piezocrystal	Figure 3: Dependence of the superconducting transition temperature on the in-plane strain for a YBCO layer deposited on a PMN-PT piezocrystal

recent publications:

S. Trommler, R. Hühne, K. Iida, S. Haindl, P. Pahlke, L. Schultz, B. Holzapfel
Reversible shift in the superconducting transition for La_1.85Sr_0.15CuO₄ and BaFe_1.8Co_0.2As₂ using piezoelectric substrates
New J. Phys. 12 (2010) 103030

S. Trommler, P. Pahlke, R. Hühne, L. Schultz, B. Holzapfel
Preparation of epitaxial La_2‑xSr_xCuO₄ thin films for dynamic investigations of epitaxial strain
J. Phys.: Conf. Ser. 234 (2010) 012045

Hühne, R., Okai, D., Dörr, K., Trommler, S., Herklotz, A., Holzapfel, B., Schultz, L.
Dynamic investigations on the influence of epitaxial strain on the superconducting transition in YBa₂Cu₃O_7-x
Supercond. Sci. Technol. 21 (2008) 075020