  It has recently been shown that the electronic states in bulk gapless HgCdTe
offer another realization of pseudo-relativistic three-dimensional particles in
a condensed matter system. These single valley relativistic states, referred to
as massless Kane fermions, cannot be described by any other well-known
relativistic massless particles. Furthermore, the HgCdTe band structure can be
continuously tailored by modifying either the cadmium content or temperature.
At the critical concentration or temperature, the bandgap, Eg, collapses as the
system undergoes a semimetal-to-semiconductor topological phase transition
between the inverted and normal alignments. Here, using far-infrared
magneto-spectroscopy we explore the continuous evolution of band structure of
bulk HgCdTe as temperature is tuned across the topological phase transition. We
demonstrate that the rest-mass of the Dirac-like Kane fermions, m changes sign
at the critical temperature, while their velocity, c remains constant. The
relation Eg = 2mc2 with the universal value of c = (1.07 +- 0.05)10x6 m/s
remains valid in a broad range of temperatures and Cd concentrations,
indicating a striking universality of the pseudo-relativistic description of
the Dirac-like Kane fermions in HgCdTe.
