  When a Dirac fermion system acquires an energy-gap, it is said to have either
trivial (positive energy-gap) or non-trivial (negative energy-gap) topology,
depending on the parity ordering of its conduction and valence bands. The
non-trivial regime is identified by the presence of topological surface or
edge-state dispersing in the energy gap of the bulk and is attributed a
non-zero topological index. In this work, we show that such topological indices
can be determined experimentally via an accurate measurement of the effective
velocity of bulk massive Dirac fermions. We demonstrate this analytically
starting from the Bernevig-Hughes-Zhang Hamiltonian (BHZ) to show how the
topological index depends on this velocity. We then experimentally extract the
topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical
Landau level spectroscopy. This approach is argued to be universal to all
material classes that can be described by a BHZ-like model and that host a
topological phase transition.
