  We present a model for the positive extreme magnetoresistance (XMR), recently
observed in a plethora of metallic systems, such as PtSn$_4$, PtBi$_2$,
PdCoO$_2$, WTe$_2$, NbSb$_2$, NbP, TaSb$_2$, LaSb, LaBi, ZrSiS and MoTe$_2$.
The model is an extension of our earlier work on positive giant
magnetoresistance, and uses an elaborate diagrammatic formulation. XMR is a
bulk effect (not a surface effect), due to the dramatic sensitivity of the
conductivity to the finite magnetic field $H$. This is possible at low
temperatures, in the presence of finite disorder elastic spin scattering, and
for a special value, predicted from the theory, of the material-dependent
effective Coulomb repulsion. Good agreement with experiments is obtained.
According to our model XMR is higher in cleaner samples, and anisotropic with
regards to the direction of $H$. We discuss in particular compounds containing
the elements Pt, Sc, and Rh.
