  We study the Fermi level structure of 2+1-dimensional strongly interacting
electron systems in external magnetic field using the AdS/CFT correspondence.
The gravity dual of a finite density fermion system is a Dirac field in the
background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit
the magnetic system can be reduced to the non-magnetic one, with
Landau-quantized momenta and rescaled thermodynamical variables. We find that
at strong enough magnetic fields, the Fermi surface vanishes and the
quasiparticle is lost either through a crossover to conformal regime or through
a phase transition to an unstable Fermi surface. In the latter case, the
vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi
liquid regime with unstable quasiparticles and a change in transport properties
of the system. We associate it with a metal-"strange metal" phase transition.
Next we compute the DC Hall and longitudinal conductivities using the
gravity-dressed fermion propagators. For dual fermions with a large charge,
many different Fermi surfaces contribute and the Hall conductivity is quantized
as expected for integer Quantum Hall Effect (QHE). At strong magnetic fields,
as additional Fermi surfaces open up, new plateaus typical for the fractional
QHE appear. The somewhat irregular pattern in the length of fractional QHE
plateaus resemble the outcomes of experiments on thin graphite in a strong
magnetic field. Finally, motivated by the absence of the sign problem in
holography, we suggest a lattice approach to the AdS calculations of finite
density systems.
