Superconductivity of Iron Selenide Thin Films

Near stoichiometry FeSe films were successfully grown on MgO, SrTiO$_{3}$, and LaAlO$_{3}$ single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction analysis showed that the FeSe films have a tetragonal structure on SrTiO$_{3}$ and LaAlO$_{3}$ substrates. A mixture of tetragonal and hexagonal structures was observed on MgO substrates due to the larger lattice constant misfit. The superconductivity of films exhibited a strong dependence on epitaxial strain and thickness. Thicker films ($\sim $ 100 nm and $\sim $ 200 nm) are fully relaxed and have a clear superconducting transition near that of the bulk FeSe. Thinner films ($\sim $ 50 nm) are strained. Films on nearly lattice-matched LaAlO$_{3}$ are superconducting, while films under tension on SrTiO$_{3}$ or MgO are metallic but not superconducting down to 5K. The onset temperature for superconductivity have a near linear magnetic field dependence with dH/dT = - 2.8 T/K for fields up to 9T.