Studies On Iron-based Superconductor Fe(Te,Se) And Its Related Topological Superconducting Platforms

In recent years,the unconventional high-temperature superconductivity and the topological superconductivity are two frontier topics in condensed matter physics.As the second family of unconventional high-temperature superconductors after cuprates,iron-based superconductors have attracted widespread attention after discovered.Due to the failed application of BCS theory based on the electron-phonon coupling to iron-based superconductors,s^±pairing was proposed based on antiferromagnetic fluctua-tion.In the s^±pairing picture,the superconducting gaps have opposite signs on hole and electron pockets.As a member of iron-based superconductors,Fe(Te,Se)is an ideal platform to study pairing symmetry owing to almost the same size of hole and electron pockets.And it is also a suitable superconducting substrate for fabricating topological insulator/superconductor heterostructures because of a relatively high crit-ical temperature.The topological insulator/superconductor heterostructure is one of the ways to realize topological superconductors,in which one can find Majorana fermions which has potential application in topological quantum computation.In addition,the vortex and vortex bound states in superconductors can provide the information about the mechanism of superconductivity.Majorana zero modes may exist in the vortex cores in topological superconductors.In the thesis,I will focus on the studies based on iron-based superconductor Fe(Te,Se),including the pairing symmetry,the vortex and vortex bound states,and the exploration of topological superconducting heterostructures base on Fe(Te,Se).The related results are summarized as follows:1.Direct visualization of sign-reversal s^±superconducting gaps in Fe(Te,Se).By measuring a series of quasiparticle interference and analyzing the data with phase-reference quasiparticle interference method,we successfully visualize the sign rever-sal of superconducting gaps between hole and electron pockets.Further theoretical researches show that the phase-reference quasiparticle interference method is also ap-plicable for data measured for multiple nonmagnetic impurities without extra phase correction.2.Observation of discrete energy levels of vortex bound states.By using scanning tunneling microscope,we investigate vortex bound states in iron-based superconductor Fe(Te,Se).Discrete energy levels of vortex bound states are clearly visualized in the vortex center.The peak energy ratio is about 1:3:5,and these peaks refer to the vortex bound states(E_?=±??²/E_F)with?equals to 1/2,3/2 and 5/2.The further analysis indicates that the Fermi energy in Fe(Te,Se)is very small.Our results show the first observation of discrete energy levels of Caroli-de Gennes-Matricon vortex bound states proposed in 1964.3.Discovery of superconductivity with twofold symmetry in Bi₂Te₃/Fe Te_0.55Se_0.45heterostructures.By using molecular beam epitaxy,we fabricated Bi₂Te₃/Fe Te_0.55Se_0.45heterostructures with very high quality.The observation of the superconducting gap on Bi₂Te₃thin films indicates that the suerpconductivity has been successfully induced by proximity effect on top of Fe Te_0.55Se_0.45.Quasiparticle interference results mea-sured on 2 quintuple layer(QL)films show the evidence of superconducting gap with twofold symmetry,and the gap minimum is along one of the main crystalline axes,which follows the?_4ynotation.The twofold symmetry of the superconducting gap is also supported by elongated vortices measured on 2 QL films and the elongation is observed on 3QL films but is absent on 1QL films.Our results provide an easily accessible platform based on iron-based superconductors for investigating topological superconductivity.