Origin of light-induced metastability in ${\mathrm{ZrTe}}_5$

We study the nonequilibrium electronic structure of a model Dirac semimetal ${\mathrm{ZrTe}}_5$ by using time-and-angle resolved photoemission spectroscopy and density functional theory–based electron and phonon calculations. By measuring the electronic dispersion near the $\Gamma$ point at time delays up to 10 picoseconds, we discovered that the band spectral weight does not recover during the measured temporal window, revealing the existence of a light-induced metastable state in the electronic structure of this material. Our calculations find that the photoexcited $A_{1g}$ phonon mode leads to a band renormalization that both supports our experimental observations at the zone center and predicts changes to the band structure outside of our experimental window, ultimately showing the evolution from a direct to an indirect gap semimetal; such band renormalization dramatically reduces the electron-hole recombination rate giving rise to the metastability in this system.