Abstract
Quantum material systems upon applying ultrashort laser pulses provide a rich platform to access excited material phases and their transformations that are not entirely like their equilibrium counterparts. The addressability and potential controls of metastable or long-trapped out-of-equilibrium phases have motivated interests both for the purposes of understanding the nonequilibrium physics and advancing the quantum technologies. Thus far, the dynamical spectroscopic probes eminently focus on microscopic electronic and phonon responses. For characterizing the long-range dynamics, such as order parameter fields and fluctuation effects, the ultrafast scattering probes offer direct sensitivity. Bridging the connections between the microscopic dynamics and macroscopic responses is central toward establishing the nonequilibrium physics behind the light-induced phases. Here, we present a path toward such understanding by cross-examining the structure factors associated with different dynamical states obtained from ultrafast electrons scattering, imaging, and modeling. We give the basic theoretical framework on describing the non-equilibrium scattering problems and briefly describe how such framework relates to the out-of-equilibrium phenomena. We give effective models outlining the emergences of nonthermal critical points, hidden phases, and non-equilibrium relaxational responses from vacuum-suspended rare?earth tritellurides, tantalum disulfides thin films, and vanadium dioxide nanocrystalline materials upon light excitations.
Highlights
The interests for exploring light-induced new functional phases or properties of matters are motivated by practical endeavors, dubbed as materials on demand [1,2], as a new direction of material research
We point to the fact that with the above-the-gap excitation, the pump does not couple to the order parameter directly; rather it heats up the carriers first and that suppresses the charge-density wave (CDW) spectroscopic gap on a shorter timescale than the long-wave response associated with the lattice order parameter
We discussed a set of results that provide important insights into ultrafast nonthermal control of quantum materials, in particular, the photoinduced phase transitions to thermodynamically inaccessible states by ultrashort optical excitation in RTe3, TaS2 and VO2
Summary
The interests for exploring light-induced new functional phases or properties of matters are motivated by practical endeavors, dubbed as materials on demand [1,2], as a new direction of material research. We will attempt to establish a unified framework to treat ultrafast scattering from the nonequilibrium states of quantum materials In this case, the system we refer to is the broken-symmetry order expressed in the lattice field with distinct order parameter that can be measured by the scattering approach. We ask how a quantum material containing long-range broken-symmetry states may effectively switch under an ultrafast “quench” [13, 43] enforced by laser pulse in routes distinctively different from a thermal state [44, 45] In this central aspect, excited quantum material transformation is akin to the femtochemistry problem [46] where the ultrafast electronic excitation sets the new bonding landscape before the heavier molecular nuclear dynamics can follow. The second is to explore the ideas of using the light-excited quantum material as a platform to study the nonequilibrium physics
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