Study On The Superconductivity Of Two-dimensional Heteroepitaxial Pb Thin Films And FeSe Thin Films

The study of two-dimensional electronic properties requires the preparation ofsingle-crystal thin films with atomically-flat surface over a macroscopic scale. In thisthesis, we realize the heteroepitaxial growth of large-scale single-crystal Pb and FeSethin film on Si substrate and SrTiO₃substrate using molecular beam epitaxy （MBE）,respectively. The thickness-dependent superconductivity of Pb and FeSe thin films isstudied by transport combined with scanning tunneling microscopy （STM）. The thesismainly consists of the following two parts:（1） Using Pb striped incommensurate phase （SIC） decorating the interface betweenPb films and Si（111） substrates, we have achieved the layer-by-layer growth of large-scale single-crystal Pb thin films. By studying the thickness-dependent superconduc-tivity, We firstly demonstrate that the superconducting transition is two-dimensionalBerezinskii-Kousterlitz-Thouless （BKT） phase transition for Pb films with thickness of4-9atomic layers. By increasing the thickness of single-crystal Pb thin films layer bylayer, the crossover from BKT transition to mean-field transition is firstly demonstrat-ed. Thus, we have accomplished the thickness-dependent superconductivity diagram forsingle-crystal Pb thin films.（2） By realizing the preparation of large-scale single-crystal FeSe thin films onTiO₂-terminated SrTiO₃（100） substrates, we have demonstrated the high temperaturesuperconductivity in1unit-cell （UC） thick FeSe films. The superconducting gap showsa strong dependence on the treatment of SrTiO₃（100） substrate. For1UC FeSe filmgrown on Se-etched SrTiO₃（100） substrate, the superconducting gap is20.1meV, al-most one order of magnitude larger than the free-standing FeSe films （the supercon-ducting gap2.2meV with corresponding critical temperature9.4K）. If the same su-perconducting mechanism is held as in free-standing FeSe films, Tc would exceed77K-the liquid nitrogen boiling point. For1UC FeSe film on insulating SrTiO₃（100） sub-strate without Se-etching treatment, the superconducting gap is10meV and the highestTc is50K confirmed by the transport measurement. With further increasing the filmthickness, we find that the second UC and thicker films do not superconduct at all, andthe observed superconductivity behavior is limited to the very first unit cell of the film above the interface.