  Our Rutherford backscattering spectrometry (RBS) study has found that
concentrations up to 7 atomic percent of Rb and Cs can be introduced to a depth
of ~700 A in MgB2 thin films by annealing in quartz ampoules containing
elemental alkali metals at <350 degree centigrade. No significant change in
transition temperature (Tc) was observed, in contrast to an earlier report of
very high Tc (>50 K) for similar experiments on MgB2 powders. The lack of a
significant change in Tc and intra-granular carrier scattering suggests that Rb
and Cs diffuse into the film, but do not enter the grains. Instead, the
observed changes in the electrical properties, including a significant drop in
Jc and an increase in delta rho (rho300-rho40), arise from a decrease in
inter-granular connectivity due to segregation of the heavy alkaline metals and
other impurities (i.e. C and O) introduced into the grain boundary regions
during the anneals.
