Molecular Beam Epitaxial Growth And Scanning Tunneling Microscopic Investigation Of Conventional Superconductors And ?-? Compounds

As the dimension of material reduces,electronic structures located at surface or interface usually play a critical role in novel physical properties.The interaction at interface between low dimensional material and substrate will have great impact on the property,suggesting an effective pathway to modulate the low dimensional materials by heterostructure-interface engineering.In this thesis,several heterostructures are prepared by growing conventional superconductors and IV-VI compounds on SrTiO₃?STO?substrate using molecular beam epitaxy?MBE?technology and investigated by scanning tunneling microscope/scanning tunneling spectroscopy?STM/STS?.The main results are summarized below:?1?Well-crystallized weak coupling?tin?and strong coupling?lead?superconductor islands have been grown STO substrate by MBE.By means of low temperature scanning tunneling spectroscopy,enlarged superconducting gaps have been observed in both superconductor islands,which can be attributed to interfacial charge transfer and enhanced electron-phonon coupling from STO substrate.The investigation of temperature dependence of tunneling spectra shows that enlarged superconducting gaps in Sn islands?2.05 meV?exhibit enhanced superconducting transition temperature?T_c?of 8.2 K,obviously higher than the bulk value?3.72 K?.However,those enlarged superconducting gaps?7.1 meV,4.6 meV?in Pb islands do not induce a prominent T_c enhancement?9.8K,5.4 K?due to the existence of the competition between superconductivity and electron correlation.Our experiment demonstrates that the interface effects of STO substrate can effectively improve the T_c of weak coupling superconductor.?2?Few-layers PbSe have been epitaxially grown on STO substrate.From the investigation of different thickness films,we discover that interfacial strain can induce structure buckling,resulting in the formation of?2×?2 reconstruction.As the thickness increases,compressive strain releases and eventually Dirac-like edge states at the edge of the 7^th-layer PbSe are observed by STS measurement.Collaborated by the first-principle calculations,we found that compressive-strain-induced structure buckling will modulate the size of the inverted gap and leads to topological phase transition from nontrivial two-dimensional topological crystalline insulator to trivial insulator.Therefore,our simulation calculations and experimental observations demonstrate the existence of a potential 2D TCI phase of epitaxial PbSe and strain can effectively modulate the topological property.?3?We have successfully synthesized continuous layer-controllable SnSe₂ films on SrTiO₃ substrate using MBE.The investigation of STM and Raman measurements shows that strong compressive strain can be intrinsically built in few-layers films by interface strain and lead to the formation of 2×2 charge order.As the compressive strain releases,the region with the coexistence of 1×1 hexagonal lattice and 2×2 charge order appears on bilayer.The spatially resolved STS measurements demonstrate that enhanced interface superconductivity is only observed in 1×1 region.In conjunction with experimental observation and first-principle calculation,enhanced interface superconductivity should be a consequence of the combined effect of interface charge transfer and strain effect.