Pair-breaking critical current density of magnesium diboride

We report the investigation of the pair-breaking current density j(d) in magnesium diboride. At low current densities j, the transition temperature T-c shifts in the classic DeltaT(c)(j)/T-c(0)proportional to-[j/j(d)(0)](2/3) manner, with a projected j(d)(0)approximate to2x10(7) A/cm(2). Current-voltage curves at fixed temperatures yield a similar value for j(d)(0), with an overall j(d)(T) dependence consistent with Ginzburg-Landau theory. To our knowledge this is the first complete investigation of j(d)(T) down to Tapproximate to0 in any superconductor.