Growth, Electronic Structure And Superconductivity Of Transition Metal Chalcogenide Thin Films

Transition metal chalcogenides with layered structures exhibit very rich physics,such as superconductivity,topologically nontrivial state,new type of quasiparticles,band engineering and spintronics,and can be employed to explore many emerging frontiers in modern condensed matter physics.Molecular beam epitaxy（MBE）is well known as an ideal technique for preparation of two-dimensional materials and heterosturctures with precise control of composition and atomic structure.In this work,we use MBE to prepare thin films of four kinds of transition metal chalcogenides,and investigate their atomic structure,band structure,superconductivity and other properties using low temperature scanning tunneling microscopy（STM）,transport measurement,angle-resolved photoemission spectroscopy（ARPES）in combination with theoretical calculations.The main results are summarized as follows:（1）We use STM to systematically investigate the impurities and defects in monolayer FeSe films grown on SrTiO₃（001）substrate.The impurities and defects include intrinsic defects and those induced by chemical doping,surface adsorption and substrate surface modification,which generate various effects on the electronic states and superconducting gaps.The observed impurity bound states present constraints on theoretical scenarios.This study enhances the comprehension of the superconducting pairing symmetry of monolayer FeSe films.（2）Combining STM and transport measurement,we investigate the electronic properties of PdTe₂ thin films on SrTiO₃（001）substrate.A narrow-gap semiconductor to metal transition occurs when the film thickness increases from single layer to multiple layers.All the multilayer films exhibit superconductivity transition.The transition temperatures for films in ultrathin limit are obviously higher than expected and can be further enhanced through Mg intercalation.First-principles calculations reveal thickness dependence of lattice parameters and band structures caused by interlayer orbital coupling,which is consistent with the experimental observation.The calculations predict the existence of topologically nontrivial states as well.Our work implies that PdTe₂ thin film is a promising candidate for exploring two-dimensional topological superconductivity.（3）We conduct STM and ARPES study on 1–5 layer anti-PbO-type CoSe thin films grown on SrTiO₃（001）substrate.In contrast to the bulk counterpart,CoSe films don’t exhibit signs of ferromagnetism.Compared with monolayer FeSe films on SrTiO₃,CoSe films show similar band structures.However,its Fermi level is higher,Fermi surfaces more complex,and bandwidth larger.The results suggest that superconductivity might be induced in the CoSe films through hole doping.（4）We conduct STM study on pyrite-type CuS₂（111）thin films grown on both SrTiO₃（001）and Bi₂Sr₂CaCu₂O₈₊δsubstrates.For ultrathin films on both substrates,we observe an energy gap at the Fermi level,which degrades with increasing temperature or film thickness.It might be attributed to the Coulomb gap induced by electron-electron interaction.The electronic states are not much affected by the substrates.In contrast,the sulfur content plays a key role in the growth and the electronic properties.