The experimental observation of quantum Hall effect of l=3 chiral quasiparticles in trilayer graphene

The linear dispersion of the low-energy electronic structure of monolayer graphene supports chiral quasiparticles that obey the relativistic Dirac equation and have a Berry phase of pi (refs 1,2). In bilayer graphene(3), the shape of the energy bands is quadratic, and its quasiparticles have a chiral degree, l = 2, and a Berry phase of 2 pi. These characteristics are usually determined from quantum Hall effect (QHE) measurements in which the Berry phase causes shifts in Shubnikov-de Haas (SdH) resistance oscillations. The QHE in graphene also exhibits an unconventional sequence of plateaux of Hall conductivity, sigma(xy), with quantized steps of 4e(2)/h, except for the first plateau, where it is governed by the Berry phase. Here, we report magnetotransport measurements in ABC-stacked trilayer graphene, and their variation with carrier density, magnetic field and temperature. Our results provide the first evidence of the presence of l = 3 chiral quasiparticles with cubic dispersion, predicted to occur in ABC-stacked trilayer graphene(4-12). The SdH oscillations we observe suggest Landau levels with four-fold degeneracy, a Berry phase of 3 pi, and the marked increase of cyclotron mass near charge neutrality. We also observe the predicted unconventional sequence of QHE plateaux, sigma(xy) = +/- 6e(2)/h, +/- 10e(2)/h, and so on.