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Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (${\mathrm{C}}_{60}$) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of ${\mathrm{C}}_{60}$ reduces the critical thickness at which Al becomes conducting from \ensuremath{\sim}35 to \ensuremath{\sim}20 \AA{}. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of ${\mathrm{C}}_{60}$. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active ${\mathit{A}}_{\mathit{g}}$(2) pentagonal-pinch mode, imply that up to six electrons per ${\mathrm{C}}_{60}$ are transferred from the Al to the ${\mathrm{C}}_{60}$ layer. This demonstration of charge transfer across planar metal-${\mathrm{C}}_{60}$ interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.

Transport in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ was studied by modulating the resistance and superconductive kinetic inductance via capacitively charging (001) surfaces. The mobility varies inversely with temperature, the effective mass is 5(\ifmmode\pm\else\textpm\fi{}1)${\mathit{m}}_{\mathit{e}}$ at low tmeperature, and hole carrier densities are temperature independent and nearly the same in normal and superconducting states. Transition temperatures at ideal surfaces are equal to the bulk and show notably weak modulation with carrier density.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

Local cation order approaching ideal bulk termination was found for cleaned (001) surfaces of YBa2Cu3O7 crystals by ion‐channeling surface‐peak analysis. Surfaces etched in dilute CH3OH:Br2 and ultrasonically agitated in CH3OH to dislodge the Br2 appear terminated preferentially by the Cu‐O ‘‘double‐plane’’ sequence, containing less than 1/2‐cation monolayer of surface disorder. Planar polishing with ion beams incident at a glancing angle on a rotating crystal leaves residual disorder as low as ∼8 A, damage which is readily removed by a light chemical etch.

The authors give a revised effective mass of the cooper pairs and its temperature dependence for the YBCO high{minus}Tc superconductors. (AIP)

Glancing angle ion beam milling has been used to successively thin c‐axis oriented Ba2YCu3O7 films fabricated using an in situ pulsed laser deposition process. The systematics of the thinning process are characterized by scanning electron microscopy, ion channeling by Rutherford back‐scattering spectrometry, and measurements of the resistance transitions. Comparisons are made with the results of previously published work in which ex situ annealed films, fabricated in a three‐source evaporator and having a significantly different surface morphology, were subjected to a similar thinning treatment.

Milling with Xenon ions directed at grazing angles onto the surfaces of c-axis oriented YBa2Cu3O7 films is shown to give smooth films which superconduct at thicknesses approaching a lattice constant. The observed linear dependence of the room temperature sheet conductance with milling time allows a determination of the effective electrical thickness after each milling stage. The resistance transitions of all films having a superconducting transition show a metallic behavior with a positive temperature coefficient of resistance. Degradation of superconducting properties becomes significant for films less than 100A thick although this degradation is appreciably less for films which initially (before milling) have sharp resistance transitions and a strong c-axis orientation. Reproducible and controlled fabrication of ultrathin films using this milling technique shows promise as an important step in the post-deposition processing of high- Tc thin films for electronic device applications.

The prospect of incorporating Josephson tunnel junctions into future device technologies requires that fabrication procedures be developed which assure high junction yield, uniform quality, and reproducibility. Critical‐current distributions which are extracted from the I‐V characteristics of series‐connected lead and lead‐alloy junction arrays provide a quantitative measure of the yield, as revealed by a direct count of the 2Δ increments, and the quality or uniformity, as revealed by the width of the distribution. Examples of the effects of oxidation parameters, junction area, electrode thickness, magnetic fields, and thermal cycling on the critical‐current distributions demonstrate the ease in which significant statistical information on simultaneously prepared junctions is obtained. As a further illustration of the use of this diagnostic method, we present results on flux‐trapping experiments together with a preliminary survey of the effectiveness of a simple technique in which lead alloys are evaporat...

Trilayer structures, comprising a thin-film In/InO x superconductor separated from an Al gate electrode by an overgrown dielectric, have been studied to ascertain the feasibility of electric-field control of superconductivity for device applications, Modulation of the areal charge density of 50-A thick In/InO x films has been found to cause more than a 350Ω/ box$^b$ change in the sheet resistance near the midpoint of the resistive transition in one film and the creation of ~10Ω/ box$^b$ of resistance from the superconducting state of a second film. We report on efforts to increase this modulation by decreasing the electron density of unperturbed films, improving the charge storage capabilities of the thin-film gate dielectrics, and improving the carrier mobility which has been found to be sensitive to interface preparation. Device implications, based on these results, are also discussed.

Discrete conductance increments in the current-voltage characteristics of superconductor-insulator-superconductor tunnel junctions (of area ⪕25 μ m 2 ) associated with single flux quanta (“vortices”) trapped in the thin film electrodes have been observed. These tunneling results were obtained for both Sn and Pb and provide, for the first time, a direct measure of the density of quasiparticle states corresponding to a single vortex. Simultaneous measurements of the Josephson critical current suppresion indicate that the vortex cores are on the average not aligned across the barrier on the scale of the coherence lenght. The measured vortex density of states is in disagreement with predictions of the normal cores model and previously published data.

Sputtering of a magnesium target with a beam of argon ions in the presence of a partial pressure of reactive oxygen gas has been found to yield smooth MgOx thin‐film dielectrics with low electrical loss, good mechanical stability, and excellent reproducibility. The films consist of polycrystalline MgO with a grain size ≲50 A and an OH‐containing component. The thickness dependence of the areal capacitance is discussed in the context of a two‐layer model in which this oxygen‐rich phase (x≂1.4) overlays a thin (≲40 A) stoichiometric phase (x≂1.0). Observation of superconducting tunneling characteristics in trilayer Au‐MgOx‐Pb structures confirms pinhole‐free coverage which may have possible application as artificial tunnel barriers.

An electrodynamic balance has been constructed in which the perturbative forces from applied magnetic fields are used to probe the superconducting properties of single levitated charged grains of high Tc superconductors with diameters ≲10 μm. The low‐temperature magnetization of selected Ba2YCu3O7 grains has been observed to exhibit a flux exclusion near 80% of the theoretical maximum. Hysteresis, flux creep, and the lower critical field are identified.

Successive ion beam milling at grazing angles of a 2400‐A‐thick, c‐axis‐oriented YBa2Cu3O7 film is shown to give smooth films which superconduct at thicknesses on the order of tens of angstroms. The thinning and polishing process is characterized at successive milling stages using resistance transitions, x‐ray analysis, scanning electron microscopy, and Rutherford backscattering and channeling analysis of composition and surface quality. As thinning proceeds, scanning electron microscopy and diffraction features associated with a‐axis texture are removed and crystalline quality, as measured by x rays and channeling, markedly improves.

A dual‐gun reactive ion beam sputtering technique has been used to reproducibly fabricate Al2O3 dielectrics with low electrical loss for controlled thickness ranging from approximately 10 to 360 A. The linear dependence of the reciprocal capacitance on dielectric thickness of Al‐Al2O3‐Al trilayer structures incorporating this dielectric reveals a significant contribution from an interfacial capacitance in series with the geometric capacitance. Room‐temperature measurements of both the dc resistance and the frequency‐dependent complex impedance demonstrate that, with respect to bulk, there is an enhanced frequency‐dependent dielectric loss associated with this interfacial capacitance.

We describe a method for measuring the position of a Josephson junction from outside the cryostat. An inhomogeneous ac magnetic field from two opposed coils modulates the critical current of the junction unless it is in the central region where the field is small. We use the technique to measure the critical current of individual junctions in a series‐connected array, using electrical contacts across the entire string.