The Meissner and Mesoscopic Superconducting States in 1-4 Unit-Cell FeSe-Films up to 80 K

Recent reports of interface-induced superconductivity in the unit-cell films of FeSe on SrTiO3 with a Tc up to ~ 65+-5 K, highest among the Fe-based superconductors and second only to that of the cuprates, have generated great excitement. Here we show results of the first magnetic and resistive investigation on the 1-4 unit-cell FeSe-films on SrTiO3. The samples display the Meissner state below ~ 20 K with a penetration field as low as 0.1 Oe due to large edge effect at 2 K, a mesoscopic superconducting state comprising patches of dimension < 1 micrometer up to 45~55 K and an unusual relaxation of the diamagnetic signal up to 80 K, suggesting possible superconductivity at this high temperature. The observations demonstrate the heterogeneous nature of the superconducting state in these films and offer new insights into the role of interfaces in high temperature superconductivity proposed and future superconducting electronics fabricated.