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A magnetic field is used to tune through a new superconducting-insulating transition of amorphouscomposite indium oxide films at various stages of disorder. The results are in accord with scaling theory which identifies a universal sheet resistance separating a superconducting phase of localized vortices and Bose-condensed electron pairs from an insulating phase of Bose-condensed vortices and localized electron pairs. A unity dynamical exponent is confirmed and scaling behavior of the resistance over a wide range of temperatures and magnetic fields is found. PACS numbers: 74.65.+n, 74.70.Mq, 74.75.+t Superconductivity in two dimensions (2D) provides a unique arena in which a wide variety of novel and fundamental physical phenomena occur. There is now, for example, almost unanimous agreement that the seminal ideas of Berezinskii 1 and Kosterlitz and Thouless 2 are applicable both to the melting of the Abrikosov vortex lattice and to the zero-magnetic-field vortex-antivortex phase transition. In addition, the systematic introduction and control of disorder provides an opportunity to explore the validity of predictions 3,4 and the suggestions of experiment 5 that a superconducting-insulating transition should have universal properties, including a universal metallic sheet resistance near A/4e 2=ss 6450 ft/D at the transition.In experimental systems, the presence of disorder can introduce complications related to specific materials properties such as microstructure, contamination, chemical composition, and defect concentration. If the microscopic disorder is sufficiently homogeneous in the sense that the length scale characterizing the uniformity of disorder is longer than the length scale used by the theory to model the superconducting behavior, then there is an opportunity for a realistic comparison of experimental behavior and theoretical prediction. In this Letter, we present such a comparison using scaling theory of the magnetic-field-tuned superconductor to insulating transition, discussed in a companion paper, 6 to model the temperature-and field-dependent behavior of the resistance of amorphous-composite indium oxide (aInO*) films fabricated at different stages of disorder.In this theory, disorder is measured by a variable A which is assumed to have continuous behavior through the transition. The superconducting coherence length £, introduced in the theory, diverges as | A c -A | ~~w when disorder approaches criticality, A-• A c . The exponent v, not measured in our experiments, is predicted to have a lower bound of unity in 2D. The identification of the Kosterlitz-Thouless transition temperature T c with a characteristic frequency (energy) which scales as £~z together with the expectation that the critical field B c will vanish as £ ~2 implies the relation B c~~T } lz , where z is the dynamical exponent. Our experimental determination of T c and B c for films with varying amounts of disorder confirms the predicted value z = l. Furthermore, we verify the scaling dependence of the sheet resistance R on both...

N-channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on-off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.

We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segregated secondary phases, and strongly suggests that a near-room-temperature dilute magnetic semiconducting oxide has been realized. Based on these results, ZnO doped with Mn and Sn may prove promising as a ferromagnetic semiconductor for spintronics.

Meissner-eA'ect and microwave-absorption measurements on bulk samples show that Rb"C60is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the Kdoped material. Only Ba06K04Bi03 and the cuprate superconductors have higher transition temperatures.