Meissner and mesoscopic superconducting states in 1-4 unit-cell FeSe films

We report a detailed investigation on the superconductivity in eight 1-4 unit-cell (UC) FeSe films on SrTiO3 substrates by measuring their magnetization and sheet resistance in a field between 5 x 10(-2) and 7 x 10(4) Oe over the last one and a half years as a function of temperature and frequency, from 2 to 300 K and from 0 to 1.5 kHz, respectively. The results show that samples of 1-4-UC FeSe films all 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, strongly suggesting that superconductivity may be of an interfacial nature. A model is proposed to account for the magnetizations in such a superconducting mesoscopic structure, similar to the Andreev reflection between the normal and superconducting carriers. Above 45 K, collective glasslike excitations are evident, although their nature is yet to be determined. The complex superconducting structure observed is consistent with the challenges in synthesizing ultrathin FeSe films with a superconducting temperature much higher than that of a bulk FeSe.