  We describe a generic mechanism by which a system of Dirac fermions in
thermal equilibrium acquires electric charge in an external magnetic field. To
this end the fermions should have an additional quantum number, isospin or
color, and should be subject to a second magnetic field, which distinguishes
the isospin/color, as well as to a corresponding isospin chemical potential.
The role of the latter can be also played by a nontrivial holonomy (Polyakov
loop) along the Euclidean time direction. The charge is accumulated since the
degeneracies of occupied lowest Landau levels for particles of positive isospin
and anti-particles of negative isospin are different. We discuss two physical
systems, where this phenomenon can be realized. One is monolayer graphene,
where the isospin is associated with two valleys in the Brillouin zone and the
strain-induced pseudo-magnetic field acts differently on charge carriers in
different valleys. Another is hot QCD, for which the relevant non-Abelian field
configurations with both nonzero chromo-magnetic field and a nontrivial
Polyakov loop can be realized as calorons - topological solutions of Yang-Mills
equations at finite temperature. The induced electric charge on the caloron
field configuration is studied numerically. We argue that due to the
fluctuations of holonomy external magnetic field should tend to suppress charge
fluctuations in the quark-gluon plasma and estimate the importance of this
effect for off-central heavy-ion collisions.
