Department of Applied Physics

The biaxially textured growth of superconducting Co-doped BaFe2As2 (Ba-122) thin films has been realized on ion beam assisted deposition (IBAD) MgO coated conductor templates by employing an iron buffer architecture. The iron pnictide coated conductor showed a superconducting transition temperature of 21.5 K, which is slightly lower than that of Co-doped Ba-122 films on single crystalline MgO substrates. A self-field critical current density of over 10 5 A·cm −2 has already been achieved even at 8 K. The current experiment highlights the potential of possible coated conductor applications of the iron-based superconductors.

Since the discovery of Fe-based superconductors, great effort has been made to determine their fundamental properties such as width of energy gaps and pairing symmetry. Experimental results obtained by different groups and methods on various sample forms are however inconsistent. For instance, shapes of the differential conductance spectra obtained by point contact spectroscopy (PCS) and width of energy gaps extracted from them for Co-doped BaFe 2 As 2 (Ba-122) are varying from one measurement to another, and the results are interpreted within single gap or multi-gap models. 1-5 The pairing symmetry in Fe-based superconductors in general is also still under dispute. 6 A possible fundamental reason for such discrepancy is that the surface of Co-doped Ba-122 is highly sensitive against ambient air. Point contacts as well as other types of junctions on Fe-based superconductors are often prepared and measured ex-situ, which leads to undefined interface of such junctions and varying experimental results. It should be noted that recently suggested phase-sensitive experiments designed to test a pairing symmetry in Fe-based superconductors 7 are also based on point contacts and tunnel junctions, which are both highly surface-sensitive. Undefined surfaces and interfaces are also a major drawback for fabrication of various cryo-electronic devices such as Josephson junctions and SQUIDs, although short coherence length remains the main problem in this material class.

A low and almost temperature independent resistance in the normal state and an anomalous peak effect within the normal-superconducting transition have been observed in BaFe 1.8 Co 0.2 As 2 /Fe bilayers, prepared on IBAD-MgO/Y 2 O 3 buffered technical substrates. A resistor network array sufficiently reproduces this effect, assuming an increase of the electrical conductance between tape and film with decreasing buffer layer thickness. Based on this model we evaluated the influence of this effect on the critical current density and successfully reconstructed the superconducting transition of the bilayer.

We demonstrate the growth of epitaxial Fe/Ba(Fe 1−x Co x ) 2 As 2 (Fe/Ba-122) bilayers on MgO(001) and LSAT(001) single crystal substrates using Pulsed Laser Deposition (PLD). By exploiting the metallic nature of the FeAs tetrahedron in the Ba-122 crystal structure, we achieve a coherent interfacial bond between bcc iron and Co-doped Ba-122. T c values for both bilayers were close to that of the PLD target. Direct observation of interfacial bonding between Fe and the Ba-122 FeAs sublattice by atomic resolution transmission electron microscopy implies that this bilayer architecture may work for other iron pnictide systems and pave the way for the fabrication of superconducting/ferromagnetic heterostructures.

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe 1.8 Co 0.2 As 2 thin film electrode, an Au barrier and a PbIn counter electrode. The junctions show resistively shunted junction-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2 K. The temperature dependence of the critical current shows nearly linear behavior near T C . Well-pronounced Shapiro steps are observed at microwave frequencies of 10−18 GHz. Assuming an excess current of 200 µA at 4.2 K the effective I C R N product calculates to 7.9 µV.

Angle-resolved transport measurements revealed that planar defects dominate flux pinning in the investigated Co-doped BaFe 2 As 2 thin film. For any given field and temperature, the critical current depends only on the angle between the crystallographic c-axis and the applied magnetic field but not on the angle between the current and the field. The critical current is therefore limited only by the in-plane component of the Lorentz force but independent of the out-of-plane component, which is entirely balanced by the pinning force exerted by the planar defects. This one-dimensional pinning behavior shows similarities and differences to intrinsic pinning in layered superconductors.

Surface properties of Co-doped BaFe 2 As 2 (Ba-122) thin films prepared by pulsed laser deposition on MgO with Fe buffer layer and CaF 2 substrates were inspected by atomic force microscopy, scanning spreading resistance microscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy, auger electron spectroscopy/microscopy and point contact spectroscopy (PCS). Selected PCS spectra were fitted by extended 1D BTK model. The measurements were done on as-received as well as ion beam etched surfaces. Our results show that the substrate is considerably influencing the surface properties of the films, particularly the topography and surface conductivity distribution, what can affect results obtained by surface-sensitive techniques like PCS.

The use of piezoelectric substrates enables a dynamic observation of strain dependent properties of functional materials. Based on studies with La 1.85 Sr 0.15 CuO 4 we extended this approach to the iron arsenic superconductors represented by BaFe 2−x Co x As 2 to investigate strain driven changes in detail. We demonstrate that epitaxial thin films can be prepared on (001) Pb(Mg 1/3 Nb 2/3 ) 0.72 Ti 0.28 O 3 substrates using pulsed laser deposition. The structural as well as the electric properties of the grown films were characterized in detail. A reversible shift of the superconducting transition of 0.44 K for La 1.85 Sr 0.15 CuO 4 and 0.2 K for BaFe 1.8 Co 0.2 As 2 was observed applying a biaxial strain of 0.022% and 0.017% respectively.

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin film junctions with BaFe 1.8 Co 0.2 As 2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. Temperature dependent measurements of the electrical conductivity were strongly influenced by the properties of the electrodes. We developed a junction structure that allows us to characterize the electrodes, too, including the behavior of their normal state resistance in order to correct their influences on the conduction spectra. The corrected conductivity of the junction was described within an extended BTK-model and shows a behavior dominated by Andreev reflexion.