Since the advent of topological insulators hosting symmetry-protected Diracsurface states, efforts have been made to gap these states in a controllableway. A new route to accomplish this was opened up by the discovery oftopological crystalline insulators (TCIs) where the topological states areprotected by real space crystal symmetries and thus prone to gap formation bystructural changes of the lattice. Here, we show for the first time atemperature-driven gap opening in Dirac surface states within the TCI phase in(Pb,Sn)Se. By using angle-resolved photoelectron spectroscopy, the gapformation and mass acquisition is studied as a function of composition andtemperature. The resulting observations lead to the addition of a temperature-and composition-dependent boundary between massless and massive Dirac states inthe topological phase diagram for (Pb,Sn)Se (001). Overall, our resultsexperimentally establish the possibility to tune between a massless and massivetopological state on the surface of a topological system.
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