Scanning Tunneling Microscopy Studies Of Unconventional High-Temperature Superconductivity

Understanding the exotic properties and pairing mechanism of unconventional superconductors are among the most wanted fundamental issues in condensed matter physics and material science.In this thesis,by using molecular beam epitaxy?MBE?,argon-ion bombardment and annealing?IBA?and low temperature scanning tunneling microscopy/spectroscopy?STM/STS?,we have prepared and studied the topological superconductor?-Bi₂Pd,Bi-based cuprate superconductors and a variety of copper oxide films on the atomic scale.Topological superconductor is a novel quantum state with gapless boundaries,which host the long-sought Majorana fermions.By using MBE technique,we have successfully grown?-Bi₂Pd films with controlled impurities concentration and lateral size on SrTiO₃?001?substrate.In situ STS measurements reveal two superconducting gaps with different energy scales,which we demonstrate originate from the bulk bands and topological surface states,respectively.The observation implies that two-dimensional topological superconductivity occurs on the surface of?-Bi₂Pd films.By applying magnetic field perpendicular to the sample surface,an obvious zero-bias conductance peak is invariably observed in the vicinity of magnetic vortices,which shows no splitting even at regions far from the vortex centers and anticipated for a Majorana zero mode.Moreover,the surface superconducting gap outside the vortex core cannot regain its value at the zero magnetic field,characteristic of the topological superconductivity under magnetic field.Furthermore,we find that the topological superconductivity and Majorana zero modes are immune to intrinsic nonmagnetic impurities,which are in line with the time-reversal-invariant property for a topological superconductor.High-temperature cuprate superconductors have attracted much attention in condensed matter physics since they were discovered in 1986.Due to the apparent complexity of their multilayered crystal structure,the pairing mechanism remains mysterious so far.In this thesis,by using top-down strategy?namely IBA technique?,we have prepared each ingredient oxide layer of Bi-based cuprates?Bi₂Sr₂CaCu₂O8+?and Bi₂Sr₂CuO6+?samples?for the first time,which allow us to investigate systematically their properties by STM/STS.We find that the pseudogap is in intrinsic property of doped metal oxides?the charge reservoirs?,and is not directly connected to the occurrence of superconductivity in cuprates.We further find that the energy gap associated with the superconductivity in cuprates exists exclusively on the CuO₂ layers.By a comparison study of the two materials,we have clarified the physical mechanism why Bi₂Sr₂CaCu₂O8+?exhibits a higher transition temperature than Bi₂Sr₂CuO6+?.In addition,by using ozone-assisted MBE technique?bottom-up strategy?,we have grown several copper oxides films,such as CuO,CaCu_xO_y,CaCu_xO_y and LaCu_xO_y,on SrTiO₃?001?substrate.We have used in situ STM/STS to study their topographies and electronic properties.Although no superconducting films have been obtained,the results pave an important material foundation for searching for superconductivity in oxides thin films.