No abstract

The discovery of superconductivity (SC) with a transition temperature, T c , up to 65 K in single-layer FeSe (bulk T c = 8 K) films grown on SrTiO 3 substrates has attracted special attention to Fe-based thin films. The high T c is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of SC in the parent compound BaFe 2 As 2 (no bulk T c ) just by tensile lattice strain without charge doping. We investigate the interplay between strain and SC in epitaxial BaFe 2 As 2 thin films on Fe-buffered MgAl 2 O 4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit which leads to a suppression of the antiferromagnetic spin density wave and induces SC with bulk-T c ≈ 10 K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors.

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by , where e is the charge of the carrier, and n is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe1−xCox)2As2 with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe2As2 system.

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La 0.18 Sr 0.82 )(Al 0.59 Ta 0.41 )O 3 (LSAT), CaF 2 -buffered LSAT and bare CaF 2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe 0.7 Te 0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe 0.7 Te 0.3films varies significantly. We found that the superconducting transition temperature (T c ) of FeSe 0.7 Te 0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest T c of over 19 K was observed for the film on bare CaF 2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe 0.7 Te 0.3 film and the substrate.