Quantum materials in the time domain

The equilibrium ground state of a quantum material is only one point in a much broader landscape of possible electronic phases and interactions. By extending angle-resolved photoemission spectroscopy (ARPES) and resonant x-ray scattering (RXS) into the time domain, it becomes possible to follow how quantum materials respond to ultrafast perturbations, to disentangle coupled degrees of freedom through their characteristic timescales, and, in some cases, to manipulate electronic structure and collective order in ways that have no equilibrium analogue. This provides a powerful route to uncovering microscopic interactions and to exploring how light can be used not only to probe, but also to control quantum matter.

Our group has pursued this program across a range of systems and techniques. Using laser-based time-resolved ARPES, we established a new way to determine mode-projected electron-phonon coupling directly in the time domain in graphite, and went on to reveal optical manipulation of Rashba-split electronic states in the topological insulator Bi2​Se3​, as well as electronic-interaction-driven dynamics in Ta2​NiSe5​. In cuprates, ARPES showed that superconductivity can collapse through ultrafast loss of phase coherence, and that the pseudogap in electron-doped compounds can vanish through quenching of the spin-correlation length. Complementing these photoemission studies, FEL-based time-resolved RXS demonstrated that light can enhance charge-density-wave coherence in a high-temperature superconductor and, more recently, that charge-density-wave correlations in La-based cuprates strongly enhance a channel of electron-phonon coupling that can be dynamically suppressed in less than 100 fs by manipulating the electronic system alone. Together, these studies illustrate our broader effort to use ultrafast probes not only to observe nonequilibrium quantum matter, but to uncover and control the interactions that define it.