Optically driven ultrafast lattice contraction in a topological crystalline insulator
The ability to dynamically control topological order remains a central challenge in the exploration of quantum materials. Here, we investigate the lattice and electronic response of the topological crystalline insulator (TCI) Pb$_{1-x}$Sn$_x$Se to ultrafast optical excitation using time- and angle-resolved photoemission spectroscopy (tr-ARPES) in combination with X-ray diffraction (XRD) and first-principles calculations. Static XRD measurements establish the lattice constant as the key control parameter for the topological transition, with a critical value $a_\mathrm{c}=6.073$ Åseparating the normal-insulator and topological phases. Despite generating electronic temperatures far above the topological-to-normal transition temperature $T_\mathrm{c}$, optical excitation in the TCI phase does not induce a phase transition. Instead, it drives an ultrafast lattice contraction of $Δa \approx -0.45$ pm within less than 1 ps that counteracts thermal expansion and prevents the system from reaching $a_\mathrm{c}$. The optical excitation therefore pushes the system deeper into the topological phase, providing a direct explanation for the absence of an optically driven transition. Total-energy calculations reveal that this behavior originates from the band inversion in the topological regime, which reverses the character of the photoinduced charge transfer and transiently strengthens atomic bonding. These findings identify band inversion as a mechanism that fundamentally alters the nonequilibrium lattice response, with general implications for ultrafast optical control of topological materials.