Molecular Beam Epitaxy Growth And Angle Resolved Photoemission Spectroscopy Studies Of Single-Crystal FeSe Thin Films

Since the discovery of high temperature superconductivity in ion-based superconductors in 2008, searching for superconducting materials with a higher transition temperature (Tc) and the thorough understanding of the superconducting mechanism have always been a dream for the scientists. Iron selenium is structurally the simplest example of an iron-based high-temperature superconductor, yet it is perhaps the least studied one duo to the lack of high quality single crystals. Recently, in single unit cell(UC) FeSe thin films grown on SrTiO3 (STO) substrates by molecular beam epitaxy (MBE), both scanning tunneling spectroscopy(Q.K.Xue’s group in Tsinghua university) and angle-resolved photoemission spectroscopy (ARPES)(X.J.Zhou’s group in IOP) experiments have observed the largest superconducting gap in Fe-HTS, which probably closes at a new record of 65 K. A comprehensive study of the electronic structure of FeSe will be crucial important for the better understanding of the high temperature superconductivity and further increasing the Tc. In this dissertation, I will introduce the two complicated oxide MBE systems and a special aimed MBE system for the growth of FeSe thin films designed by myself. Using this FeSe-MBE system, we have successfully grown high quality FeSe thin films, and their electronic structures, superconductivity and magnetic property have been studied. The electronic structures of hexagonal FeSe and Na2Ti2Sb2O single crystal have also been studied. The corresponding results are as follow:1. Interface induced superconductivity and the magnetic property of FeSe/STO thin films.The single layer FeSe thin films were fabricated by MBE and then transferred to ARPES chamber in-situ for the electronic structure measurement. We have confirmed the 60 K superconducting transition temperature with reliable in-situ data, and the origin of the electrons for the heavily electron doped single layer FeSe has been identified. We substantiate the presence of spin density waves (SDWs) in all FeSe films thicker than 2 UC, and we find that this weakens with increased thickness or reduced tensile strain. In the thick films, we give the first experimental bulk electronic structure of FeSe and the phase diagram as a function of lattice constant, which possesses all the same generic features as those of the iron pnictides. The general trend from the phase diagram suggests that higher Tc could be achieved in heavily electron-doped FeSe compounds that have a larger lattice constant. With the simplest structure, cleanest composition and single tuning parameter, FeSe is an ideal system for testing theories of Fe-HTS.2. Doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopantWe have grown FeSe thin films on STO substrates using MBE, and studied the doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopant. For single layer FeSe, The electron pockets at M point are enlarged when doped with cobalt, and the Tc is increased form 60 K to 70 K. When the 2.5 UC,3.25 UC and 4 UC FeSe sample are electron doped by cobalt, the SDW is suppressed and superconductivity can be detected. For the multi-layer FeSe thin films, the electronic structure turns out to be alike of single layer FeSe when they are effectively Co doped. But it is not superconducting at the largest doping level, which may indicate that the FeSe/STO interface effect plays an important role.3. The electronic structure and magnetism of hexagonal FeSe thin films.We have grown hexagonal FeSe thin films on STO substrates and studied the temperature and thickness dependence of their electronic structures. The single layer hexagonal FeSe is heavily electron doped, and the Fermi surface consists of six elliptic electron pockets. With increased temperature, part of the bands shift downward to the high binding energy while some bands shift upwards to Ep. The bands shift gradually between 30 K and 300 K, and then keep unchanged over 300 K. The magnetic susceptibility shows an anomalous peak at about 290 K, indicating a magnetic transition in hexagonal FeSe thin films. We found that the electronic structure of hexagonal FeSe thin film keeps unchanged when it is gown and annealed at low temperature. While grown and annealed at high temperature, the electronic structure shows dramatic change which may be caused by the formation of Se vacancies.4. The electronic structure of Na2Ti2Sb2O single crystal.The electronic structure of Na2Ti2Sb2O single crystal is studied by photon energy and polarization dependent ARPES. The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicating that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is week. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES results suggest that the kz of Na2Ti2Sb2O is week。...