Exploring Electronic Structures And Superconductivity In Superconducting Interfaces

Interface superconductivity has drawn intense attention from point of view of both fundamental research and applications since its discovery.Exotic and emergent phe-nomena at various hetero-interfaces have revealed them a cornucopia of new physics,some of which have been employed to develop novel devices.Here we report successful growth of several heterostructures by molecular beam epitaxy（MBE）,and characteri-zation of their electronic structure and superconductivity by in-situ scanning tunneling microscopy/spectroscopy（STM/STS）.We have carried out a systematic study of the 1 UC-FeSe/STO system under post-growth low-temperature annealing in ultra high vacuum.A superconducting-insulating-metallic-superconducting transition and new surface reconstructions have been observed.With different annealing conditions,it is found that 2 UC-FeSe,which is usually invisible to STM and speculated to desorb,has diffused to low-temperature region in the high-temperature annealing process.Upon proper low temperature annealing,it can be observed steadily by STM due to the existence of concentration gradient.We also found that excess Se,rather than Fe vacancies,is the key factor for the surface reconstruction in FeSe.The study has opened a new window for understanding the growth kinetics and superconductivity mechanism of 1 UC-FeSe/STO system.We have successfully grown monolayer SnSe₂on epitaxial bilayer graphene（BLG）/Si C substrate.Superconductivity with T_c=4.84 K and coherence lengthξ=17.2nm have been determined by STM/STS.By engineering the heterostructure,we have found the superconductivity originates from the SnSe₂/BLG interface.Two-dimensional electron gas formation due to band bending and charge transfer at the hetero-interface plays a key role in the superconductivity observed herein.It is the first report on su-perconductivity in semiconductor SnSe₂based van der Waals heterostrcture.Our results shed light on designing new superconducting heterostructures and formation mechanism of interface superconductivity.To explore possible interface superconductivity,we have also grown monolayer SnSe₂thin films on Si（111）and STO（001）substrates.Typical semiconducting character,rather than superconductivity,has been observed in the two heterostructures.Interestingly,both systems exhibit charge density wave（CDW）with 2×2 periodicity has been found,although CDW is not present in bulk SnSe₂.The formation of CDW cannot be attributed to conventional Fermi surface nesting scenario because of nearly zero density of states at E_F.Enhanced electron-phonon interaction at the interface as well as the charge transfer are found to play dominating role in the formation of CDW herein.By delicate control of the substrate and thus interface,we are able to engineer the physical properties of monolayer SnSe₂-based heterostructures.Both interface induced superconductivity and CDW have been observed.The study illustrates the relationships between CDW and superconductivity,and new physics at artificial heterostructures.