▎ 摘　　要

The physics of Dirac fermions in condensed-matter systems has received extraordinary attention following the discoveries of two new types of quantum Hall effect in single-layer and bilayer graphene(1-3). The electronic structure of trilayer graphene (TLG) has been predicted to consist of both massless single-layer-graphene-like and massive bilayer-graphene-like Dirac subbands(4-7), which should result in new types of mesoscopic and quantum Hall phenomena. However, the low mobility exhibited by TLG devices on conventional substrates has led to few experimental studies(8,9). Here we investigate electronic transport in high-mobility (>100,000 cm(2) V(-1) s(-1)) TLG devices on hexagonal boron nitride, which enables the observation of Shubnikov-de Haas oscillations and an unconventional quantum Hall effect. The massless and massive characters of the TLG subbands lead to a set of Landau-level crossings, whose magnetic-field and filling-factor coordinates enable the determination of the Slonczewski-Weiss-McClure (SWMcC) parameters(10) used to describe the peculiar electronic structure of TLG. Moreover, at high magnetic fields, the degenerate crossing points split into manifolds, indicating the existence of broken-symmetry quantum Hall states.