Enhanced Superconducting Properties of Iron Chalcogenide Thin Films

Among the newly discovered iron-based superconductor, FeSe with the simplest structure and a transition temperature (T_c) around 8 K arouses much research interest. Although its Tc is much lower than that of the cuprates, iron chalcogenide has low anisotropy, slow decrease of the critical current density (J_c) with increasing magnetic field and high upper critical field H_c2 as well as easy composition control, which makes it a promising candidate to substitute NbSn/NbTi for high field applications. Compared with its bulk counterpart, iron-based superconductor thin film has a great potential in developing the ordered quasi-2D structure and is suitable for coating technology which has already been applied in YBa_2Cu_3O_7-x coated conductors.

In this thesis, we first optimized pure FeSe thin films by different growth conditions using pulsed laser deposition (PLD) and post-annealing procedures. The microstructure properties of the films including the epitaxial quality, interface structure and secondary phase have been studied and correlated with the superconducting properties.

Second, we reported our initial attempt on introducing the flux pinning centers into FeSe_0.5Te_0.5 thin films either under a controlled oxygen atmosphere or with a thin CeO_2 interlayer. The microstructure of the FeSe_0.5Te_0.5 films including the epitaxial quality, the interface structure and the secondary phase have been studied and correlated with the in-field performance of the superconducting thin films to explore the pinning properties of these nanoscale defects.

Very recently, ion beam assisted deposition (IBAD) substrates have been used to grow high quality FeSe_0.5Te_0.5 tape with excellent in-field performance. The film on IBAD substrate involves multiple steps of seed layer and buffer layer deposition to establish the epitaxial growth template. Therefore a simplified and cost effective iron-based coated conductor is more desirable. Towards the practical application, we demonstrated the growth of superconducting FeSe_0.5Te_0.5 film on amorphous glass substrates for the first time. The film is highly textured with excellent superconducting properties, e.g., T_c of 10 K and J_c under self-field as high as 1.2?10^4 A/cm^2 at 4 K. Further optimization of the film growth with various nanoscale interlayers has been carried out.

In addition the Te rich iron chalcogenide thin film with composition close to the composition with antiferromagnetic (AFM) transition has been demonstrated. Compared to the FeSe_0.5Te_0.5 which claimed to be the optimum composition from the literature report, the FeSe_0.1Te_0.9 is even more promising for the high field application with its coexistence of super high upper critical field and high critical current density.