A. F. Hebard

Meissner-effect and microwave-absorption measurements on bulk samples show that ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$ is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ and the cuprate superconductors have higher transition temperatures.

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.

A magnetic field is used to tune through a new superconducting-insulating transition of amorphous-composite 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.

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.