"By photo-exciting electrons in a material causing subsequent electron-electron and electron-phonon interactions, an energy landscape is created that is very different from that in thermal equilibrium. This distinction sets the baseline that photo-induced phase transitions (PIPT) could go through very different pathways compared with the thermally-induced phase transitions in equilibrium. This opens up a new dimension for complex materials research in non-equilibrium with ultrafast tools. In recent years, with the discovery of photo-induced superconductivity (SC) and other hidden quantum states, the study of these metastable and hidden phases in quantum materials has drawn intense interest in the ultrafast community.With femtosecond electron diffraction, we have observed intriguing phenomena in a few charge-density wave (CDW) materials (CeTe3, ErTe3 and 1T-TaS2) and iron chalcogenide systems (FeTe, FeSe0.44Te0.56 and FeSe). We explore the metastable hidden phases and observe universal dynamics in these materials far from equilibrium.CeTe3 exhibits uni-directional stripe CDW order in thermal equilibrium. Bi-directional CDW is thermodynamically forbidden. After femtosecond laser pulse excitation, the system is driven to a bi-directional order as it crosses a nonthermal fixed point. The new state is formed through associated symmetry changes that cause softening/hardening of CDW-related phonons. The CDW wavevector change proves that Fermi surface nesting (FSN)-enhanced electron-phonon coupling plays a central role in driving the CDW instabilities. Based on these results, we propose a nonthermal phase transition pathway in the non-equilibrium phase diagram.The work in ErTe3 is one step further based on the CeTe3 results. ErTe3 is on the opposite side of the rare-earth tritelluride (RETe3) series to host two orthogonal CDW orders at low temperature. Together with CeTe3 data, the ultrafast results at various temperatures in ErTe3 indicate that the system becomes more symmetric after laser excitation. Given the robustness of the data, the conclusion here may be extended to similar systems as well.The generic features of CDW dynamics in 1T-TaS2 are very similar to those in the quench dynamics of isolated quantum systems (e.g. cold atoms). After laser excitation, the system goes to the prethermalization plateau region before the thermalization stage. We find that both regimes follow universal scalings due to the existence of two nonthermal fixed points. Microscopically, we propose a chiral-symmetry-breaking mechanism that mediates the phase transformation. With a 2500 nm excitation laser, we emphasize the photo-doping, instead of the photo-thermal effect, in driving the phase transition. Due to the lack of thermal energy, the phase transition induced by the 2500 nm laser is more first order-like with faster switching speeds than 800 nm excitation. This high-speed switching with little thermal energy deposition holds promise for better future optoelectronic applications.In FeTe, we directly observed the ultrafast structural transition by ultrafast electron diffraction (UED) for the first time. In the studies of optimally-doped compound FeSe0.44Te0.56, we observed the large-amplitude acoustic phonon excitations at right above the transition temperature Tc. While experiments with better spatial-temporal resolutions are needed, the acoustic phonon observed here might be important for SC in iron chalcogenide. In FeSe, we observed the laser-enhanced local stress that is known to be coupled to the nematic phase and superconductivity. Future UED experiments targeted for understanding the local stress would be very helpful."--Pages ii-iii.
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