  A key challenge in condensed matter research is the optimization of
topological insulator (TI) compounds for the study and future application of
their unique surface states. Truly insulating bulk states would allow the
exploitation of predicted surface state properties, such as protection from
backscattering, dissipationless spin-polarized currents, and the emergence of
novel particles. Towards this end, major progress was recently made with the
introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state
conductance and quantum oscillations are observed at low temperatures.
Nevertheless, an unresolved and pivotal question remains: while room
temperature ARPES studies reveal clear evidence of TI surface states, their
observation in transport experiments is limited to low temperatures. A better
understanding of this surface state suppression at elevated temperatures is of
fundamental interest, and crucial for pushing the boundary of device
applications towards room-temperature operation. In this work, we
simultaneously measure TI bulk and surface states via temperature dependent
optical spectroscopy, in conjunction with transport and ARPES measurements. We
find evidence of coherent surface state transport at low temperatures, and
propose that phonon mediated coupling between bulk and surface states
suppresses surface conductance as temperature rises.
