  We evaluated the doping effects of carbohydrate (malic acid, C4H6O5), from
0wt% to 30wt% of total MgB2, on the phase, lattice parameters, critical
temperature, resistivity, and upper critical field of MgB2 superconductor. The
lattice parameters calculated show a large decrease in the a-axis for MgB2 +
C4H6O5 samples, but no change in the c-axis. This is an indication of the
carbon substitution into boron coming from C4H6O5, resulting in enhancement of
resistivity, critical current density, and upper critical field. Specifically,
the critical current density value of 25000 Acm2 at 5 K and 8 T for the MgB2 +
30wt% C4H6O5 sample is higher than that of the un-doped MgB2 by a factor of 21.
In addition, resistivity value for all the MgB2 + C4H6O5 samples ranged from 80
to 90 microohm centimeter at 40 K, which is higher than for un-doped MgB2. The
increased resistivity indicates increased impurity scattering due to carbon,
resulting in enhanced upper critical field.
