  In the recently discovered topological crystalline insulators (TCIs),
topology and crystal symmetry intertwine to create surface states with a unique
set of characteristics. Among the theoretical predictions for TCIs is the
possibility of imparting mass to the massless Dirac fermions by breaking
crystal symmetry, as well as a Lifshitz transition with a change of Fermi
surface topology. Here we report high resolution scanning tunneling microscopy
studies of a TCI, Pb1-xSnxSe. We demonstrate the formation of zero mass Dirac
fermions protected by crystal symmetry and the mechanism of mass generation via
symmetry breaking, which constitute the defining characteristics of TCIs. In
addition, we show two distinct regimes of fermiology separated by a Van-Hove
singularity at the Lifshitz transition point. Our work paves the way for
engineering the Dirac band gap and realizing interaction-driven topological
quantum phenomena in TCIs.
