Electron tunneling: Inelastic electron tunneling spectroscopy and low-temperature scanning tunneling microscopy.

摘要

This thesis describes experiments encompassing both Inelastic Electron Tunneling Spectroscopy (IETS) and Scanning Tunneling Microscopy (STM). IETS is used to study the tunneling characteristics of undoped Mg-MgO-metal tunnel junctions. The development of a low temperature STM, and it's use to study high T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors is discussed.; The IETS experiments indicate the tunneling characteristics can be improved by increasing the uniformity of the barrier. Uniform oxidation was achieved by oxidizing in a wet environment, or by pretreating the Mg with hydrogen. The pretreated junctions exhibited a 124 meV peak in the IET spectra indicating chemisorbed hydrogen at the magnesium surface. Hydrogen acted as a nucleation site for oxidation, promoting uniform growth. Using different cover electrodes it was found that metals with smaller ionic radii penetrate into the oxide at pinholes and defects. The effect was reduced in hydrogen junctions indicating a smaller degree of imperfections. Different cover electrodes shifted the IETS hydroxyl peak to lower energies due to an image dipole effect{dollar}sp1.{dollar} The shift was larger with Mg negatively biassed.; Design and operation of a low temperature microscope is presented. Measurements of NbSe{dollar}sb2{dollar} at 4.2 K indicate that the microscope is capable of atomic resolution, and spectroscopy with millivolt resolution. The STM was used to measure a number of {dollar}rm YBasb2Cusb3Osb{lcub}6.5+x{rcub}(YBC),{dollar} and BiSrCaCu{dollar}sb2{lcub}rm Osb{lcub}x{rcub}{rcub}{dollar} samples at 4.2 K. A non-conducting layer on most of the samples meant the tip had to be driven into the surface to tunnel, preventing scanning. The short coherence lengths produced large spatial variations in the tunneling characteristics. Many curves exhibited charging effects. Curves exhibiting a Coulomb Blockade or the Coulomb staircase{dollar}sp2{dollar} were often observed. Many curves were dominated by tunneling between grains rather than tunneling between sample and tip. Ambiguity regarding the exact tunneling mechanism makes unambiguous estimate of the energy gap impossible. A 56 meV gap was observed in the YBC ceramics indicating a low temperature phase. The non-conducting layer was not observed on a number of YBC films. The surface exhibited a thin conducting shell, and measurements indicated that superconductivity could persist out to the surface. Energy gap values as high as 20 meV were observed. ftn{dollar}sp1{dollar}J. R. Kirtley and P. K. Hansma, Phys. Rev. B 13, 2910 (1976). {dollar}sp2{dollar}K. A. Mullen, E. Ben-Jacob, R. C. Jaklevic and Z. Schuss, Phys. Rev. B. 37, 98 (1988).