  An AlAs two-dimensional electron system patterned with an anti-dot lattice
exhibits a giant piezoresistance (GPR) effect, with a sign opposite to the
piezoresistance observed in the unpatterned region. We trace the origin of this
anomalous GPR to the non-uniform strain in the anti-dot lattice and the
exclusion of electrons occupying the two conduction band valleys from different
regions of the sample. This is analogous to the well-known giant
magnetoresistance (GMR) effect, with valley playing the role of spin and strain
the role of magnetic field.
