  A comprehensive study of the effects of carbohydrate doping on the
superconductivity of MgB2 has been conducted. In accordance with the dual
reaction model, more carbon substitution is achieved at lower sintering
temperature. As the sintering temperature is lowered, lattice disorder is
increased. Disorder is an important factor determining the transition
temperature for the samples studied in this work, as evidenced from the
correlations among the lattice strain, the resistivity and the transition
temperature. It is further shown that the increased critical current density in
the high field region can be understood by a recently-proposed percolation
model [Phys. Rev. Lett. 90 (2003) 247002]. For the critical current density
analysis, the upper critical field is estimated from a correlation that has
been reported in a recent review article [Supercond. Sci. Technol. 20 (2007)
R47], where a sharp increase in the upper critical field by doping is mainly
due to an increase in lattice disorder or impurity scattering. On the other
hand, it is shown that the observed reduction in self-field critical current
density is related to the reduction in the pinning force density by
carbohydrate doping.
