  Extremely large positive magnetoresistance (XMR) was found in a nonmagnetic
semimetal InBi. Using several single crystals with different residual
resistivity ratios (RRRs), we revealed that the XMR strongly depended on the
RRR (sample quality). Assuming that there were no changes in effective mass m*
and carrier concentrations in these single crystals, this dependence was
explained by a semiclassical two-carrier model. First-principle calculations
including the spin-orbit interactions (SOI) unveiled that InBi had a
compensated carrier balance and SOI-induced "hidden" three-dimensional (3D)
Dirac bands at the M and R points. Because the small m* and the large carrier
mobilities will be realized, these hidden 3D Dirac bands should play an
important role for the XMR in InBi. We suggest that this feature can be
employed as a novel strategy for the creation of XMR semimetals.
