  We report fabrication of graphene devices in a Corbino geometry consisting of
concentric circular electrodes with no physical edge connecting the inner and
outer electrodes. High device mobility is realized using boron nitride
encapsulation together with a dual-graphite gate structure. Bulk conductance
measurement in the quantum Hall effect (QHE) regime outperforms previously
reported Hall bar measurements, with improved resolution observed for both the
integer and fractional QHE states. We identify apparent phase transitions in
the fractional sequence in both the lowest and first excited Landau levels
(LLs) and observed features consistent with electron solid phases in higher
LLs.
