  We investigate disordered graphene with strong long-range impurities.
Contrary to the common belief that delocalization should persist in such a
system against any disorder, as the system is ex-pected to be equivalent to a
disordered two-dimensional Dirac Fermionic system, we find that states near the
Dirac points are localized for sufficiently strong disorder and the transition
between the localized and delocalized states is of Kosterlitz-Thouless type.
Our results show that the transition originates from bounding and unbounding of
local current vortices.
