Observation of even denominator fractional quantum Hall states in monolayer graphene

We report magneto-capacitance measurements of high quality monolayer graphene encapsulated in boron-nitride (hBN) and few-layer graphite. In addition to the usual odd-denominator hierarchy states at partial fillings ν=p/(2p±1), we observe even-denominator ν=±½ and ±¼ states, which appear only for a narrow range of magnetic fields. Similar behavior is observed in 3 different samples, but at dramatically different B: in two of the samples, the even-denominator states appear at B≈28 T, while in the third sample they appear at B≈5.5 T. We trace the difference to the measured zero-magnetic field sublattice splitting, which arises due to the interaction with the hBN subtrate. The ν=±½,¼ FQH states are accompanied by a series of phase transitions and crossovers in the neighboring odd-denominator states, indicative of an underlying isospin phase transition. Mean field analysis suggests this transition is between a low-field valley ordered Kekule phase and a high field canted antiferromagnetic phase. The observed behavior cannot be explained within the composite fermion picture, suggesting a possible intermediate phase between the Kekule distorted phase and the antiferromagnet.