Effect of Rb and Cs-doping on superconducting properties of MgB2 thin films

TL;DR

This study investigates how Rb and Cs doping affects MgB2 thin films, revealing that doping alters electrical properties mainly through grain boundary modifications without significantly changing the superconducting transition temperature.

Contribution

It provides detailed analysis of alkali metal doping effects on MgB2 thin films, highlighting the role of grain boundary segregation in electrical property changes.

Findings

Rb and Cs can be incorporated up to 7 at.% without changing Tc.

Doping decreases inter-granular connectivity, reducing Jc.

Electrical property changes are due to grain boundary segregation.

Abstract

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.