  We consider the low-temperature thermal transport properties of the 2D
proximity-induced superconducting state formed at the interface between a 3D
strong topological insulator (TI) and a d-wave superconductor (dSC). This
system is a playground for studying massless Dirac fermions, as they enter both
as quasiparticles of the dSC and as surface states of the TI. For TI surface
states with a single Dirac point, the four nodes in the interface-state
quasiparticle excitation spectrum coalesce into a single node as the chemical
potential, $\mu$, is tuned from above the impurity scattering rate ($|\mu| \gg
\Gamma_0$) to below ($|\mu| \ll \Gamma_0$). We calculate, via Kubo formula, the
universal limit ($T \rightarrow 0$) thermal conductivity, $\kappa_0$, as a
function of $\mu$, as it is tuned through this transition. In the large and
small $|\mu|$ limits, we obtain disorder-independent, closed-form expressions
for $\kappa_0/T$. The large-$|\mu|$ expression is exactly half the value
expected for a d-wave superconductor, a demonstration of the sense in which the
TI surface topological metal is half of an ordinary 2D electron gas. Our
numerical results for intermediate $|\mu|$ illustrate the nature of the
transition between these limits, which is shown to depend on disorder in a
well-defined manner.
