Tuning superconductivity in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ thin films by tetrahedral geometry design

Significant progress has been made in fabricating high-quality epitaxial thin films of iron-based superconductors. Strain engineering offers the possibility of tailoring the structural distortions at the atomic scale and enhancing superconducting properties. Here, we report that tetrahedral geometry driven by thin film strain leads to a significant enhancement of the superconducting transition temperature (T$_{\mathrm{c}})$ of optimal Co-doped epitaxial BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ thin films above the value of the bulk single crystals. We have found that the As-Fe-As bond angles were strongly modified by both epitaxial and thermal strains caused by the temperature-dependent lattice mismatch between BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ thin films and the substrates. Synchrotron x-ray diffraction and resonant scattering demonstrate that the As-Fe-As bond angle and T$_{\mathrm{c}}$ are systematically tuned by in-plane strain and reach maximum T$_{\mathrm{c}}$ at the optimum bond angle of 109.5$^{\mathrm{o}}$. Strain engineering can provide a path toward tailoring superconducting properties and understanding superconductivity in other Fe-based superconducting thin films such as monolayer FeSe.