  A grand challenge underlies the entire field of topology-enabled quantum
logic and information science: how to establish topological control principles
driven by quantum coherence and understand the time-dependence of such periodic
driving? Here we demonstrate a THz pulse-induced phase transition in Dirac
materials that is periodically driven by vibrational coherence due to
excitation of the lowest Raman-active mode. Above a critical field threshold,
there emerges a long-lived metastable phase with unique Raman coherent
phonon-assisted switching dynamics, absent for optical pumping. The switching
also manifest itself by non-thermal spectral shape, relaxation slowing down
near the Lifshitz transition where the critical Dirac point (DP) occurs, and
diminishing signals at the same temperature that the Berry curvature induced
Anomalous Hall Effect varnishes. These results, together with first-principles
modeling, identify a mode-selective Raman coupling that drives the system from
strong to weak topological insulators, STI to WTI, with a Dirac semimetal phase
established at a critical atomic displacement controlled by the phonon pumping.
Harnessing of vibrational coherence can be extended to steer symmetry-breaking
transitions, i.e., Dirac to Weyl ones, with implications on THz topological
quantum gate and error correction applications.
