  Dirac fermions in condensed matter physics hold great promise for novel
fundamental physics, quantum devices and data storage applications. IV-VI
semiconductors, in the inverted regime, have been recently shown to exhibit
massless topological surface Dirac fermions protected by crystalline symmetry,
as well as massive bulk Dirac fermions. Under a strong magnetic field (B), both
surface and bulk states are quantized into Landau levels that disperse as
B^1/2, and are thus difficult to distinguish. In this work, magneto-optical
absorption is used to probe the Landau levels of high mobility Bi-doped
Pb0.54Sn0.46Te topological crystalline insulator (111)-oriented films. The high
mobility achieved in these thin film structures allows us to probe and
distinguish the Landau levels of both surface and bulk Dirac fermions and
extract valuable quantitative information about their physical properties. This
work paves the way for future magnetooptical and electronic transport
experiments aimed at manipulating the band topology of such materials.
