  Inspired from an experimentally examined organic conductor, a novel mechanism
for negative magnetoresistance is proposed for repulsively interacting
electrons on a lattice whose band dispersion contains a flat portion (a flat
bottom below a dispersive part here). When the Fermi level lies in the flat
part, the electron correlation should cause ferromagnetic spin fluctuations to
develop with an enhanced susceptibility. A relatively small magnetic field will
then shift the majority-spin Fermi level to the dispersive part, resulting in a
negative magnetoresistance. We have actually confirmed the idea by calculating
the conductivity in magnetic fields, with the fluctuation exchange
approximation, for the repulsive Hubbard model on a square lattice having a
large second nearest-neighbor hopping.
