Trigonal warping and even denominator fractional quantum Hall effect in trilayer graphene

We report magneto-capacitance measurements of high quality trilayer graphene devices fabricated by encapsulating the trilayer in both boron nitride and few-layer-thick graphite. The resulting devices are exceptionally low-disorder. We resolve Landau levels at magnetic fields well below 1 T and odd-denominator fractional quantum Hall (FQH) states at B = 6 T. At high displacement fields and low magnetic fields, we observe the emergence of three-fold quasi-degenerate Landau levels, signifying the emergence of new massless Dirac points. At high magnetic fields, we observe numerous fractional quantum Hall states between filling factors ν = -6 and +6. In addition to the expected hierarchy of odd-denominator states, we also observe even denominator states at ν =9/2 (at B = 30 T) and 9/2 and 11/2 (at B = 40 T).