Increasing transition temperature (Tc) and understanding paring mechanism may be the most important two scientific goals in the study of high-Tc superconductivity. In experiments, carrier doping is an effective approach to the realization of the former goal, while measurements of electronic structure and superconducting properties including zero resistance and Meissner effect are very helpful for the later. To the best of my knowledge, however, there is no experimental tool can do all the measurements while tuning carrier density in one instruments. Recently, we developed a multi-functional STM that has four electrodes set in its piezo tube, so that the STM tip can be in situ replaced by a four-point probe or a two-coil head, which enables measurements of electrical transport and diamagnetic properties of a sample surface, respectively, in addition to the usual STM and STS functionality [1,2]. Combined with a molecule beam epitaxy chamber, the multi-functional STM was applied to the study of superconductivity in ultrathin FeSe films grown on SrTiO3(100) substrate. Intrigued by the discovery of high-Tc superconductivity in single layer of FeSe film on SrTiO3 [3,4], researchers recently found large superconducting-like energy gaps in K-adsorbed multilayer FeSe films by ARPES and STM [5-6]. However, the existence and nature of the superconductivity inferred by the spectroscopic studies has not been investigated by measurements of zero resistance or Meissner effect due to the fragility of K atoms in air. With the newly developed multi-functional STM, we succeeded in observing the diamagnetic response of the K-adsorbed multilayer FeSe films, and thus find a dome-shaped relation between Tc and K coverage. Intriguingly, Tc exhibits an approximately linear dependence on the superfluid density in both underdoped and overdoped regions. Moreover, the temperature dependence of upper critical field implies a two-band nature, and the quadratic low-temperature variation in the London penetration depth indicate an s order parameter. These results provide compelling information towards further understanding of high-temperature superconductivity in FeSe-derived superconductors.

Reference
1. J. F. Ge, et al., Review of Scientific Instruments 86, 053903 (2015).
2. M. C. Duan, et al., Review of Scientific Instruments 88, 073902 (2017).
3. W. H. Zhang et al., Chinese Phys Lett 31, 017401 (2014).
4. J. Ge et al., Nature Materials 14, 285-289 (2015).
5. Y. Miyata, et al., Nature Materials 14, 775 (2015).
6. C. L. Song et al., Physical Review Letters 116, 157001 (2016).