Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of CaFe2As2 Films on SRTIO3 Substrate and Investigation on the Reported Superconductivity in Intercalated Black Phosphorus

CaFe2As2 (Ca122) has attracted lots of interest recently due to the discovery of the interface-enhanced superconductivity in this system. Although molecular beam epitaxy (MBE) growth of epitaxial thin film of Ca122 was reported, no detailed study of the growth mode and surface reconstructions were conducted in-situ during the growth. Therefore, we decided to use our newly purchased MBE – LT-STM combined system to explore this new material system. We have successfully obtained epitaxial CaFe2As2 films grown on SrTiO3 substrate by choosing a Ca : Fe flux ratio of 1 : 1. No impurity peaks were detected from X-ray measurements. The growth mode of Ca122 films was observed to be island like instead of a layer-by-layer growth mode. In addition, a √2 × √2 surface reconstruction was observed. Interestingly, by drastically decreasing the Ca : Fe flux ratio, we discovered that the resulting films were epitaxially grown Fe dominated films covered by thin layers of surface reconstructed Ca122 whose quality was significant better than pure Fe grown on SrTiO3 substrate. The growth mode of the Fe dominated films was quasi-2D film like, whereas the pure Fe on SrTiO3 had 3D isolated islands growth mode. The surface reconstructed Ca122 was essential in improving the Fe film quality. A model was proposed to explain the discovery. On a separate work, we decided to conduct a systematic investigation of the reported superconductivity in intercalated black phosphorus (BP) which we questioned. We have reproduced the superconducting transition at 3.8 K in Li- and Na-intercalated BP crystals that consist of minute amounts of free Sn (reduced from the residual non-superconducting Sn compounds by Li or Na) when prepared by the vapor transport technique but not in BP crystals that are free of Sn when prepared by the high-pressure technique. The superconducting transition takes place at the same temperature as pure Sn and the field effect on the transition is similar to that on Sn. The magnetic anisotropy of the superconducting state in the Li- and Na-intercalated BP is rather small. We have therefore concluded that the superconducting transition reported by Zhang et al. is associated with the Sn but not intrinsic to the intercalated BP.