We present an infrared spectroscopy study of ${\mathrm{ZrTe}}_5$, which confirms a recent theoretical proposal that this material exhibits a temperature-driven topological quantum phase transition from a weak to a strong topological insulating state with an intermediate Dirac semimetal state around $T_p\simeq 138\mathrm{K}$. Our study details the temperature evolution of the energy gap in the bulk electronic structure. We found that the energy gap closes around $T_p$, where the optical response exhibits characteristic signatures of a Dirac semimetal state, i.e., a linear frequency-dependent optical conductivity extrapolating to the origin (after subtracting a weak Drude response). This finding allows us to reconcile previous diverging reports about the topological nature of ${\mathrm{ZrTe}}_5$ in terms of a variation of $T_p$ that depends on the crystal growth condition.

• Received 25 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.187401