The critical current density and upper critical field of MgB2 wire. (a) Field dependence of the critical current density for the malic acid-doped sample at various temperatures. (b) Temperature dependence of the upper critical field extracted from the percolation model analysis. In (a), solid symbols are experimental data from the standard four-probe method using the electric field criterion of 1 μV/cm. Lines are calculated by the percolation model. In (b), lines are calculated by the two-band dirty-limit theory.