[en] The status of our project on single-electron tunneling is at this point excellent. As outlined in our original proposal, a key goal in the development of this project was the demonstration and exploration of the microwave properties of single-electron systems. As discussed here, such work has to date been carried out

[en] Finely layered Niobium/Germanium composites with unique physical and superconducting properties are discussed. These materials are studied as model systems for quasi-two-dimensional superconductors and have provided quantitative verification of dimensional crossover behavior in complete agreement with the Josephson-coupling theory. The composites consist of alternating sputter-deposited thin films of Nb and Ge. Individual layer thickness ranges from 5 A to 100 A; typical samples contain 50 Nb/Ge layer pairs of fixed thickness ratio. We have investigated the structural and transport properties of the composites as a function of layer thickness. Likewise the critical temperature and normal-state properties, including the heat capacity and density of states (inferred from critical field measurements), have been studied. Measurements have been made of the parallel upper critical fields below T_c and the fluctuation conductivity above T_c of samples with selected Ge thicknesses. These results have revealed a systematic variation in the dimensional nature of these systems both with Ge layer thickness and, in the quasi-2D systems, temperature. Systematic behavior in anomalous positive curvature of the perpendicular upper critical fields has been examined and discussed in the context of a possible manifestation of 2D vortex-lattice melting

[en] The status of our project on single-electron tunneling is, again, excellent. As outlined in our original proposal, a key goal for this project has been the development of a scanning tunneling instrument for the purpose of imaging individual particles and tunneling into these particles at high magnetic fields. Further progress is discussed in this report

[en] The status of our project on single-electron tunneling is, again, excellent. As outlined in our original proposal, a key goal in the development of this project was the demonstration and exploration of the microwave properties of single-electron system. As discussed in this paper such work has to data been carried out. Also as discussed in our previous progress report, the next step in the experimental evolution of the project will be to use lithographically-defined small dots as capacitors as outlined in our proposal. At this point we have made such microdotsdots as will be discussed. We have also continued our work with metal droplets to form single-electron tunnel systems

[en] We discuss results on tunneling in barriers consisting of both pure fullerene films and layered composites of fullerenes and dielectric materials. This work focuses on C₆₀ films, which ranged from 50 to 600 Angstrom in thickness and were layered with both Al₂O₃ and Ge films 10 to 40 Angstrom in thickness. These studies reveal that for the deposition conditions used here, incomplete C₆₀ coverage occurred for film thicknesses less than ∼400 Angstrom. For composites of C₆₀ with Al₂O₃ or Ge, we observed isolated clusters of C₆₀ molecules and Coulomb blockade behavior consistent with the size scale of the clusters. Interesting dynamical effects were also observed in conductance characteristics that were both dramatic and in some cases entirely reproducible. copyright 1998 The American Physical Society

[en] We summarize a continuing investigation into using ion implantation to alter the transition temperature of superconducting thin films. The primary motivation for the work presented here was to study the feasibility of using magnetic ion doping to replace the bi-layer T_c control process currently used for certain cryogenic detector applications at National Institute for Standards and Technology. The results from work with various ion species implanted into aluminum, molybdenum, titanium and tungsten host films are presented

[en] The authors have investigated the critical current, I /SUB c/ R, and oxide barrier shape in Nb/Ta/Ox/C.E. tunnel junctions. Here, layers of Ta in the thickness range O< D /SUB Ta/ <1000A were deposited, in situ, on 2000A thick Nb underlayers. Junctions were completed with Pb, PbBi, and Ag counter-electrodes. They find that as D /SUB Ta/ is increased, there is a more rapid decrease of I /SUB c/ R compared with the effective energy gap, in accord with an extended version of the Gallagher theory including strong-coupling and electron-scattering effects. In addition, we have investigated the average barrier height, /phi/, and width, s, of the oxide barriers which form on the Ta overlayers. It is observed empirically that /phi/ about 6/(s-s /SUB o/ ) where s /SUB o/ about 10A and /phi/ is measured in eV. This relationship is also found to hold for barrier formation on a wide variety of pure and composite metallic systems. These results are discussed in conjunction with the Fromhold-Mott-Cabrera theory for self-limiting oxide growth on metal surfaces

[en] We have observed a tunneling structure for systems of the type Ag--barrier--metal particles--barrier--Ag, which shows gap structure and multiple-conductance peaks as a function of voltage which closely resembles the point-contact tunneling characteristics reported for the high-T/sub c/ superconductors. These data suggest that the appearance of multiple conductance peaks is not due to an intrinsic multiplicity of gaps in the high-T/sub c/ superconductors but to charging effects related to the granularity of the material

[en] We have made layered superconductors composed of alternating thin films of sputter-deposited Nb and Ge. Individual layer thicknesses range from 5 to 100 A, each sample containing from 50 to 80 Nb/Ge-layer pairs with a fixed thickness ratio, D_NbD_Ge. As revealed by x-ray analysis, the samples have a distinct superlattice structure consisting of amorphous Ge layers and, except for those with D_Nb<30 A, (110) textured polycrystalline Nb layers. Transport studies reveal that each Nb layer itself consists of a clean central portion with damaged regions at the Nb/Ge interfaces. A proximity-effect analysis of the samples indicates that the presence of this damaged Nb interfacial layer largely governs the observed transition temperatures of these multilayers. Measurement of the upper critical fields and fluctuation conductivity have confirmed both the dimensional and the temperature-dependent dimensional crossover effects expected in Josephson-coupled quasi-two-dimensional superconductors. The observed degree of control over interlayer coupling possible with these multilayers suggests they are a model system for the study of quasi-two-dimensional superconductivity

[en] A Kaufman ion-beam source has been used to sputter deposit high quality superconducting Nb (T_c = 9.1 K) and Ta (T_c = 4.3 K) films. Superconducting and electrical properties were studied as a function of deposition conditions and an optimum set of conditions was found. Nb films always had the bulk bcc crystal structure. The Nb film deposition rates ranged from 1 to 5 A/s with a Xe or Ar sputtering gas pressure of approx.1 to 4 x 10-4 Torr and a beam current density of 5--20 mA/cm₂. Values of T_c above 8.3 K for Nb films were obtained with the use of Xe rather than Ar gas. In the case of Ta films, the deposition of a thin (> or =3 A) Nb underlayer was required for the nucleation and growth of Ta in the bulk bcc crystal structure. Ta films deposited without a Nb underlayer always had high resistivity, approx.150 μΩ cm, and a tetragonal ß-Ta crystal structure. The Nb and Ta targets and Si substrates all remained below 70 ₀C during deposition and the films were easily patterned by standard photoresist liftoff