Molecular Beam Epitaxy Growth And Scanning Tunneling Microscopy Study On Ferroelectric And Fe-based Superconducting Thin Films

Ultra-thin films often exhibitnovelelectronic, optical and transport properties that are different from their bulk counterparts.Controllable growthand appropriate characterization of those filmswith available technqiues are essential to understanding of the novel properties and exploring their applications. In this dissertation, by using low-temperature scanning tunneling microscope/spectroscopy(STM/STS), we have investigated ferroelectric Pb1-xSnxTe(001) and superconducting Li Fe As(001) ultra-thin films prepared by molecular beam epitaxy(MBE). The main results of this dissertation are as follows:(1) We have grown stress-free ultra-thin filmsof SnTe on the graphitized 6H-SiC(0001) substrate by MBE. It is found that the SnTe films maintain the in-plane spontaneous polarization(SP) even when the thickness is only 1 unit cell(UC). The ferroelectric transition temperature TC of 1 UC film is 270 K, nearly two times higher than the bulk material. The 2~4 UC films exhibit robust ferroelectricity even at room temperature. The variable-temperature STM experiment shows that thephase transition near TCin 1 UC thick SnTe film is second-order. We propose a ferroelectric memory deviceprototype based on the 3UC thick films and find that its ON/OFF ratio can reach 3000 at 4.7 K.(2) The ferroelectric Pb1-xSnxTe film is a non-centrosymmetric two-dimensional(2D) system with strong spin-orbit coupling(SOC). Based on its crystal symmetry, we propose that the valleys perpendicular to the SP will have opposite spin splitting, while those parallel to the SP will be spin degenerate. We have observed the valley-selective scattering induced by the valley-dependent spin splitting in the quasiparticle interference(QPI) patterns on the ferroelectric Pb0.5Sn0.5Te film surface. The energy splitting magnitudeis over 220 meV. Our study implies that ultra-thin ferroelectric films could be a new class of materials for valleytronics.(3) The combination ofcentrosymmetry breaking and strong SOC can induce valley-dependent spin splitting in one-dimensional(1D) systems. This is indeed observed: ultra-thin SnTe films exhibit band-bending induced edge states, and corresponding 1D QPI patternssuggest thatscattering is band-selective and can be explained bya valley-dependent spin-splitting band model.(4) We have systematically studied the molecular beam epitaxial growth of high quality single crystal Li FeAs(001) films on the Nb-doped SrTiO3(100) substrate. It is found that when the film thickness reaches 4 QL, superconducting gap emerges at 4.7 K. The films thicker than 13 QL show similar superconducting properties as bulk materials with a superconducting gap of about 7 meV at 4.7K.Ex situ transport measurement shows that for 100 QL film the superconducting transition temperature Tc= 16 K and upper critical field Hc2 = 13.0 T.