Investigation Of The Interfaces Between Topological Insulators And Superconductors

Majorana fermions(MFs) are particles which are identical to their own antiparticles, and they obey non-Abelian statistics. MFs in solid state systems can be used to develop fault-tolerant topological computations, and this issue has attracted a number of researches to take part in the family in searching for MFs. The theoretical physicists predict that MFs bound states can host in topological superconductors, which can be realized by introducing superconductivity(Cooper-pairs) of conventional superconductors into the surface states of topological insulators. In this thesis, we study the superconducting proximity effect of topological insulators films and the properties of Bi films grown on conventional superconductor substrate NbSe2, using molecular beam epitaxy(MBE), scanning tunneling spectroscopy(STM) and angular resolved photoemission electron spectroscopy(ARPES). The main results are as follows:(1) By means of MBE, we grow topological insulator Bi2Se3 thin films on an s-wave superconductor substrate NbSe2 and build the single crystal topological insulator/superconductor heterostructure successfully. Using STM, we study the initial growth stage of the Bi2Se3 epitaxial films in detail and make the crystal interface structure clear. We find a BiSe interfacial layer forms spontaneously under the first Bi2Se3 quintuple layer(QL). Combined with first-principles calculation and our experimental results, the interfacial structure is determined which is in NaCl-type lattices. The existence of the BiSe interfacial layer can reduce the lattice mismatch dramatically between 1QL Bi2Se3 and NbSe2substrate. Using STS, we find the proximity-induced superconducting gaps decrease dramatically with increasing of Bi2Se3 layer thickness.The superconducting gaps decrease and their coherence peaks broadened with increasing the applied magnetic fields. Compare the STS data obtained at 0.4K and 4.2K,we find the coherence peaks at 0.4K are stronger than that at 4.2K, therefore the gaps are more obvious at 0.4K. All the experimental data mentioned above prove the superconductivity in Bi2Se3 films. Furthermore, we prove the topological surface states in Bi2Se3 thin films. Therefore, we observe the coexistence of superconductivity and topological surface states in Bi2Se3 films on NbSe2 substrate, this observation lays the groundwork for experimentally realizing MF in condensed matter physics.(2) We study and optimize the MBE growth process of Sb2Te3 thin films on NbSe2 substrate, and obtain high quality Sb2Te3 films. We determine the critical thickness for dirac point(DP) is 4QL, through the zeroth energy peak of laudau levels. We further study the relationship between the energy position of DP and the layer thickness of the films. After that, we tune the fermi-level gradually into the bulk band gaps near DP by Bi doping and research the influence of fermi level to the proximity induced gaps and its corresponding vortex core states. We find that the position of the fermi level referred to its bulk band has strong influence to superconducting proximity effect, even it is stronger than that caused by changing layer thickness. This reveals that the bulk states have main contribution and play an important role to the proximity effect. However,the contribution of topological bands is the most critical part in forming topological superconductors and MFs.(3) At last, we study the growth of Bi films on NbSe2 substrate. We find that the1 st bi-layer(BL) of Bi films are in(110) orientation. We obtain its atomic resolution images and moiré patterns. When continue depositing Bi atoms, the orientation transit from(110) to(111). Using STM, we study the electronic properties during transition process in detail. We carry out ARPES experiments to measure the band dispersion of the Bi films during the transition process and find that for Bi(110) film, it is hard to get any information, while for Bi(111) film, its band are very clear. We thus obtain the band structure and information of fermi surface of Bi(111) films. The 1BL of Bi films were theoretically predicted to be a 2D topological insulators, therefore, our study is helpful for investigations about the electronic and topological properties of Bi films.