Proximity Effect and Tunneling Spectroscopy of High-temperature Superconductor-semiconductor Hybrid Structures

This thesis presents Andreev scattering studies of a variety of high-temperature superconductor Bi2Sr2CaCu2O8+x-normal material (SN) junctions. The SN devices were fabricated with our newly-developed technique, called the mechanical-bonding method. High-temperature superconductivity was observed in topological insulators Bi2Se3 and Bi2Te3 (as well as graphite) by proximity to the high-temperature superconductor Bi2Sr2CaCu2O8+x. This proximity effect was further confirmed by the application of a magnetic field and observation of a shifting in the energy of the Andreev-reflected quasiparticles as a result of diamagnetic screening supercurrents. However, the ability to probe the topological surface states in Bi2Se3 and Bi2Te3 was limited, due to the Fermi energy being deep in the bulk conduction band. Thus we turned to Bi2Te2Se, which is quite insulating in the bulk. The differential conductance measurements on Bi2Sr2CaCu2O8+x/Bi2Te2Se devices demonstrated a zero-bias conductance peak, suggesting evidences of a new excitation emerging at the interface between the two materials. Furthermore, the mechanical bonding method was employed as a technique to probe the density of states of Bi2Sr2CaCu2O8+x at various doping levels, observing the superconducting gap and the pseudogap of Bi2Sr2CaCu2O8+x at various temperatures.Ph.D