  We investigate disorder effects on Landau levels in Dirac electron systems
with the use of a non-Hermitian quasiparticle Hamiltonian formalism. This
formalism reveals that spin-dependent scattering rates induce the spectrum
collapse of Landau levels, i.e., the disappearance of the energy gaps between
n-th and -n-th levels under a finite external magnetic field. The spectrum
collapse occurs in both weak and strong magnetic field regimes, thus showing a
reentrant behavior. Particularly, in the strong magnetic field regime, in
contrast to naive expectation, the increase of a magnetic field stabilizes the
spectrum collapse of Dirac Landau levels. Furthermore, it is revealed that the
spectrum collapse is associated with the emergence of a vortex texture with a
topological winding number of a complex energy spectrum of the non-Hermitian
system.
