Host: Cui-Zu Chang
Interactions of fundamental degrees of freedom in solids lead to a plethora of collective quantum phenomena, such as superconductivity, density waves, and topological states of matter. Equilibrium spectroscopies including angle-resolved photoemission, scanning tunneling spectroscopy, and scattering experiments have yielded enormous information on the interactions between charge, orbital, spin and lattice, yet these interactions are often difficult to be disentangled. In this talk, I will use exemplary studies on cuprate superconductors and FeSe thin films to elucidate how time-resolved photoemission spectroscopy provides new insight on cooperative interactions. Using femtosecond laser pulses, we launch coherent lattice vibrations and observe oscillations of the electronic band structure. This experiment enables a direct quantification of electron-phonon interactions without resorting to theoretical calculations. In the second part, I will demonstrate our latest results using in situ photoemission and in situ transport to understand high temperature superconductivity in monolayer iron selenide. Our experiments suggest that while spectroscopic signatures of superconductivity are found above 60 K, the Berezinski-Kosterlitz-Thouless transition dictates the zero-resistance transition near 40 K.