Preparation And Superconducting Properties Of Fe(Se,Te)Thin Films

After the discovery of superconductivity at 26 K in LaFeAs(O,F),extensive research on iron-based superconductors has been carried out.As a member of the iron-based high temperature superconductors,iron chalcogenide Fe(Se,Te)attracted much attention due to its simple structure,which is favorable for probing the superconducting mechanism.Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors.Compared to single crystals,thin films are suitable for investigating transport properties and superconducting electronics applications thanks to their geometry.In this dissertation,we investigated the influence of substrate type on transport properties of superconducting Fe(Se,Te)thin films to find the suitable substrate.Furthermore,we found that CaF2,as the buffer layer,improved the structural and superconducting properties of Fe(Se,Te)thin films.Moreover,the influence of excess Fe and the mechanism of the improvement of superconducting properties were performed.FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2,LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties.The type of substrate affects the crystalline quality of the films and,therefore,the superconducting properties.The crystalline quality of the film on MgO is inferior to other films.Defects parallel to the c axis are found in the film on MgO.The lowest Tc and Jc are measured for the film on MgO with extended defects and the poorest degree of texture.The film on CaF2 exhibits the highest Tc and Jc.Epitaxial FeSe0.7Te0.3 thin films were prepared by pulsed laser deposition on(La0.18Sr0.82)(Al0.59Ta0.41)O3(LSAT),CaF2-buffered LSAT and bare CaF2 substrates,which exhibit an almost identical in-plane lattice parameter.The composition of all thin films were determined to be FeSe0.7Te0.3 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 FeSe0.7Te0.3 films varies significantly.We found that the superconducting transition temperature of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter.The highest Tc of over 19 K was observed for the film on bare CaF2 substrate,which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.Revealing the universal behaviors of iron-based superconductors(FBS)is important to elucidate the microscopic theory of superconductivity.In this work,we investigate the effect of in-plane strain on the slope of the upper critical field Hc2 at the superconducting transition temperature Tc(i.e.-dHc2/dT)for FeSe0.7Te0.3 thin films.The in-plane strain tunes Tc in a broad range,while the composition and disorder are almost unchanged.We show that-dHc2/dT scales linearly with Tc,indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS.The observed behavior is consistent with a sign change s± superconductivity in this material.The superconductivity was improved in FeSe0.5Te0.5 thin films by oxygen incorporation via exposing to the air.The transition temperature Tc increased from 14.4 K to 19.3 K with increasing exposure time.Furthermore,no shift of(00l)peaks was observed between the as-grown sample and the exposed sample,in spite of the dramatic improvement of superconducting property.These results suggested that the incorporated oxygen partially occupied the interstitial site and improved superconductivity.