Synthesis, superconductivity and ESR characterization of Rb 3C 60 crystals

Abstract

A one-step doping process of C 60 crystals with three equivalents of rubidium to produce Rb 3C 60 crystals was developed. The T c's for all Rb 3C 60 crystalline samples ( ∼ 30.5 K ) were higher than those of the powder samples (28 to 29.6 K), but consistent with the reported four-probe resistivity result for a doped C 60 crystal (transition midpoint = 30.2 K). High superconducting shielding fractions (between 60 and 90%) and sharp magnetic transition widths (Δ T 10–90 between ∼ 3 K and 0.7 K) were observed for the samples doped with Rb at temperatures of 300 to 450°C. The doping process, as well as the composition, of these samples were studied with the use of an ESR spectrometer. Two major components, RbC 60 and Rb 3C 60, were quantitatively characterized. The superconducting shielding fractions of these samples were found to be linearly correlated with the Rb 3C 60 content obtained from ESR measurements. Higher-temperature doping (> 400°C) favored the formation of Rb 3C 60 at the expense of severe C 60 crystal fragmentation. The optimal doping condition to produce Rb 3C 60 crystals was found to be around 300°C.