Transitions between the quantum Hall state and the Anderson insulator are s
tudied in a two-dimensional tight-binding model with a uniform magnetic fie
ld and a random potential. Focus is put on a weak magnetic-held regime. It
is closely related to the continuum model. By applying topological argument
s and the Thouless-number study, the Nay how the Hall conductance changes i
s elucidated and light is shed on the trajectory of the delocalized states.
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