Josephson scanning tunneling microscope: A local probe of the superconducting pair amplitude.

摘要

The scanning tunneling microscope (STM) has proved to be an important tool in the study of superconductors, offering a local probe of the superconducting density of states. STM experiments have so far employed normal-metal tips, thus probing only the quasiparticle spectrum-excitations on the superconducting state. In unconventional superconductors, the lack of a theory that would help correlate between quasiparticle spectra and superconductivity calls for a direct measurement of the local pair amplitude. This can be achieved by using superconducting STM tips to form a scannable Josephson junction—the Josephson STM.; This thesis describes the implementation of such a device, from the fabrication of reproducible superconducting Pb/Ag tips to the study of the Josephson effect in STM based, ultra-small vacuum junctions. It was found that the pair current in these junctions appear as a peak in the current-voltage characteristics near zero bias, for junction resistances smaller than 100 kΩ. The shape of the current-voltage characteristics, the magnitude of the supercurrent peak as a function of the junction resistance, and the microwave response of the junctions, are in excellent agreement with the classical phase diffusion model. A procedure that was developed for the measurement of the Josephson product, I_cR_N, from the tunneling characteristics in superconducting STM junctions, will be discussed. Results will be presented for tunneling into Pb films, and NbSe₂ and Bi₂Sr ₂CaCu₂O₈ crystals.