Effect of Rb and Cs doping on superconducting properties ofMgB2 thin films

Abstract

A Rutherford backscattering spectrometry (RBS) study has found thatconcentrations up to 7 at.% of Rb and Cs can be introduced to a depth of∼700 Å inMgB2 thin films by annealing in quartz ampoules containing the elemental alkali metals at<350 °C. No significant change in transition temperature(Tc) (determined resistively) was observed, in contrast to an earlier report of very highTcs (>50 K) from susceptibilitymeasurements on MgB2 powders. The lack of a significant change inTc and intra-granular carrier scattering suggests that Rb and Cs diffuse into the film, but do not enterthe grains. Instead, the observed changes in the electrical properties, including a significant drop inJc and anincrease in Δρ(ρ300−ρ40), arise from a decrease in inter-granular connectivity due to segregation of the heavyalkaline metals and other impurities (i.e. C and O) introduced into the grain boundaryregions during the anneals.