Exploring photoinduced transitions and lattice dynamics in strongly correlated systems

Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices for information technology. Here, we use MeV ultrafast electron diffraction (UED) to show a transient 3D Dirac semimetal state can be induced by a femtosecond laser pulse in a topological insulator ZrTe5. Combining accurate measurements of photoinduced lattice distortions via structural refinement and DFT calculations we reveal in the photoinduced transient state how the gap is closed in the presence of strong spin-orbit coupling and the origins of the two timescales of the relaxation dynamics. Other results on electron-phonon interplay and dynamics in Bi₂Sr₂CaCu₂O₈ and FeSe superconductors will also be discussed. Recent development and applications of the GHz pulser device implemented into a 200keV commercial transmission electron microscope at BNL for ultrafast stroboscopic imaging will be reported.