  The reason why the half-integer quantum Hall effect (QHE) is suppressed in
graphene grown by chemical vapor deposition (CVD) is unclear. We propose that
it might be connected to extended defects in the material and present results
for the quantum Hall effect in graphene with [0001] tilt grain boundaries
connecting opposite sides of Hall bar devices. Such grain boundaries contain
5-7 ring complexes that host defect states that hybridize to form bands with
varying degree of metallicity depending on grain boundary defect density. In a
magnetic field, edge states on opposite sides of the Hall bar can be connected
by the defect states along the grain boundary. This destroys Hall resistance
quantization and leads to non-zero longitudinal resistance. Anderson disorder
can partly recover quantization, where current instead flows along returning
paths along the grain boundary depending on defect density in the grain
boundary and on disorder strength. Since grain sizes in graphene made by
chemical vapor deposition are usually small, this may help explain why the
quantum Hall effect is usually poorly developed in devices made of this
material.
