  The electronic and magnetic properties of spin-1/2 pyrochlores R2V2O7 were
investigated on the basis of density-functional calculations. Contrary to the
common belief, the spin-1/2 V4+ ions are found to have a substantial easy-axis
single-ion anisotropy. The |D/J| ratio deduced from the magnon quantum Hall
effect of Lu2V2O7, where J is the nearest-neighbor spin exchange and D is the
Dzyaloshinskii-Moriya parameter, is much greater than the value estimated from
our calculations (i.e., 0.32 vs. 0.05). We show that this discrepancy is due to
the neglect of the single-ion anisotropy of the V4+ ions, and the negative
magnetoresistance observed for R2V2O7 arises from a new mechanism.
