  We report here an experimental and theoretical study on the magnetoresistance
properties of heavily phosphorous doped germanium on the metallic side of the
metal-nonmetal transition. An anomalous regime, formed by negative values of
the magnetoresistance, was observed by performing low-temperature measurements
and explained within the generalized Drude model, due to the many-body effects.
It reveals a key mechanism behind the magnetoresistance properties at low
temperatures and, therefore, constitutes a path to its manipulation in such
materials of great interest in fundamental physics and technological
applications
