Studies of Superconductors Using a Low-Temperature Scanning Tunneling Microscope

A low-temperature scanning tunneling microscope (STM) was designed and constructed in this thesis project. It is capable of achieving atomic resolution in the topographic mode, and a resolution of 1.5 meV in the spectroscopic mode. It can operate in temperatures ranging from room temperature down to 4.2 K, as well as operate in magnetic fields as high as 9 T. The displacement/voltage (A/V) response of its piezoelectric scanning element was calibrated as a function of temperature and voltage. This STM was dedicated to the study of high temperature superconductors (HTS). The first of two seperate studies consisted of measuring the temperature dependence of the energy gap of bulk polycrystalline Ba_{ rm 1-x}K_{rm x}BiO_3 (BKBO). Shortly after exposing a clean sample surface, BKBO, as well as the other HTS, has a tendency to form a topical insulating layer. As a result, the measurements were obtained with the STM in the point contact mode. At 4.2 K, the tunneling conductance shows nearly ideal BCS behavior, however at higher temperatures, the lifetime broadening term was required to fit the data. Gamma(T) displays a surprising T^3 temperature dependence and the magnitude is significantly larger than the calculated theoretical recombination lifetimes. Comparisons with other conductance data obtained from grain boundary and sandwich-type BKBO junctions showed similar results. The second study consisted of developing a method to consistently obtain topographic and spectroscopic data of HTS while in the vacuum tunneling mode. A normal metal overlayer was applied to the freshly cleaved surface of a HTS. Via the proximity effect, the superconducting information could then be transmitted to the normal metal overlayer. Proximity effect energy gaps were observed consistently in Nb/Au, Bi_2Sr_2CaCu _2O_{rm 8+x} (BSSCO)/Au, and HgBa_2 CuO_{rm 4+x}/Ag bilayers. The conductance curves were able to be fit by the well-established Arnold proximity effect model. Cleavage steps of varying sizes were observed on the BSSCO surface and energy gaps were measured at various positions on these steps. The observation of energy gaps in the HgBa _2CuO_{rm 4+x} /Ag sample confirmed the effectiveness of this overlayer method.