Preparation Of Heterostructures Of Topological Insulators,Superconductors And Oxides And Investigation Of Their Exotic Behaviors

Searching for new exotic quantum phenomena is one of the fundamental goals and driving forces of condensed matter physics. Recent advances in the atomic-scale controlled synthesis techniques enable us to focus on designing and fabricating heterostructures for generating new quantum states. In this thesis, by using molecular beam epitaxy?MBE? technique, we have prepared topological insulator /high-temperature?high-Tc? superconductor, and iron-based superconductor/oxide heterostructures, and investigated their exotic behaviors by using low-temperaure scanning tunneling microscopy?STM? with high magnetic field, as well as angle-resolved photoemission spectroscopy?ARPES?.Interface between a topological insulator and an s-wave superconductor can host Majorana femions, which obey robust non-Abelian statistics, therefore have potential application as building blocks for topological quantum computers. Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization. However, identifying Majorana fermions experimentally has been challenging mainly because of the small pairing gap induced by typical conventional superconductors. Here, by growing high-quality topological insulator Bi₂Se₃ films on a d-wave high-Tc superconductor Bi₂Sr₂CaCu₂O_8+? using MBE, we have successfully induced high-Tc superconductivity on the surface states of Bi₂Se₃ films with a large pairing gap up to 15 me V. Interestingly, distinct from the d-wave pairing of Bi₂Sr₂CaCu₂O_8+?, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by ARPES. Our work could provide a critical step towards the realization and application of the long sought Majorana fermions.The recently discovered single-unit-cell?1UC? Fe Se/SrTiO₃ interface high-Tc superconductor has provided an ideal platform for understanding the pairing mechanism of high-Tc superconductivity, for its simple crystal and electronic structures. To elucidate the roles of different interfacial effects and find new interface high-Tc superconductors, we have grown 1UC-Fe Se/anatase-TiO₂?001? heterostructures on SrTiO₃?001? surface by using MBE. Scanning tunneling spectroscopy?STS? has revealed a superconducing gap with a typical size of 17 me V and a maximum size of 21 me V, implying it is a new high-Tc superconductor. By comparing it with 1UC-FeSe/SrTiO₃, we have found the most plausible scenario for the origin of high-Tc superconductivity is charge transfer effect plus interfacial electron-phonon interaction, rather than tensile strain effect or screening effect of ferroelectric phonons. Moreover, oxygen vacancies at the interface seem unlikely the main source of charge transfer effect. Our work could provide critical insights for understanding the mechanism of high-Tc superconductivity and fabricating new high-Tc superconductors.The superconducting properties of 1UC-Fe Se/oxide heterostructures are also quite puzzling: the STS study shows a “double-gap” spectrum, while there are only two nearly degenerated electron pockets around M point in the Brillouin zone without hybridization, which are hardly opening two distinct superconducting gaps. By using low-temperature STM with high magnetic field to investigate the vortex structures of MBE-grown 1UC-Fe Se/TiO₂?001? heterostructures, we have observed a nearly square vortex lattice, and two distinct Andreev bound states inside vortex cores at 0.6 me V and ±1.3 me V, respectively. These findings highly suggest that the superconducting gap in this system is strongly anisotropic with four-fold symmetry. Together with the “double-gap” structure in STS, we have proposed a superconducting gap function which could nicely account for all the phenomena above. Our work could provide important information for understanding the superconducting properties and pairing mechanism in such systems.