Femtosecond imaging of nonequilibrium phase transitions: Symmetry breaking and prethermalization in interacting CDW systems

In correlated many-body systems, the fundamental processes of spontaneous symmetry breaking (SSB) to form long-range order may be intercepted by competing interactions between particles. In such cases, the transition becomes sharp over the control parameters, and surprising synchronicity develops to manifest macroscopic switching between two stable long-range orders. This departure from SSB behavior has been the hallmark of macroscopic switching of quantum materials, but never before have the microscopic processes been captured with atomic detail. Here we demonstrate a surprising post-laser-quench self-organization of an interacting CDW system exhibiting critical slowdown near thermal and interaction driven critical points. Unlike in SSB system, such photoinduced criticalities primarily emerged in the time domain and manifested in the prethermalization and dynamical phase transition. Using femtosecond (fs) imaging, we capture these highly tunable, nonequilibrium processes and the emerging unconventional symmetry-broken intermediate state, opening up an entirely new dimension in studying and controlling phase transitions far from equilibrium.