The Meissner and Mesoscopic Superconducting States in the Ultrathin FeSe-Films

We carried out a detailed investigation on the superconductivity in eight 1-4 unit-cell FeSe-films on SrTiO3(STO) substrates by measuring their magnetization and resistivity in a field between 5E2 and 7E4 Oe over the last one and half years as a function of temperature and frequency, from 2 to 300 K and 0 to 1.5 kHz, respectively. Systematic aging effect for these samples was also well studied. The results show that samples display a complex superconducting structure, i.e. a Meissner state but populated with weak-links below 20 K, and an unusual superconducting mesostructure up to 45 K. A model is proposed to account for such a superconducting mesoscopic structure, similar to the Andreev reflection between the normal and superconducting carriers. Above 45 K, collective glass-like excitations are evident although their nature is yet to be determined.