Quasi-two-dimensional superconductivity in FeSe$_{\mathrm{0.3}}$Te$_{\mathrm{0.7}}$ thin films and electric-field modulation of superconducting transition

Heterostructures composed of superconductors and ferroelectrics are important for studying the coupling between superconductivity and ferroelectricity, especially the modulation of superconductivity by ferroelectricity. We study the structural and superconducting properties of FeSe$_{\mathrm{0.3}}$Te$_{\mathrm{0.7}}$ (FST) thin films with different thicknesses grown on ferroelectric Pb(Mg$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})_{\mathrm{0.7}}$Ti$_{\mathrm{0.3}}$O$_{\mathrm{3}}$ substrates. The FST films undergo biaxial tensile strains which are fully relaxed for films above 200 nm, and the ultrathin and thicker films exhibit an insulating behavior and superconductivity, respectively. The current-voltage curves around the superconducting transition follow the Berezinskii-Kosterlitz-Thouless (BKT) transition behavior, revealing quasi-two-dimensional phase fluctuation. Upon applying electric field to the heterostructure, Tc of FST thin film increases due to the reduction of the tensile strain in FST. This work sheds light on the superconductivity, strain effect as well as electric-field modulation of superconductivity in FST films.