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 the first part of my 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 and multilayer iron selenide. Our experiments suggest that high temperature superconductivity is confined to the interfacial layer, and survives even when additional FeSe layers are deposited. The implications on the nature of superconductivity in monolayer FeSe will be discussed.

Bio: Shuolong Yang obtained his B.S. in Physics and Mathematics, with a minor in Electrical Engineering at Stanford University. He continued to pursue his Ph.D. in Applied Physics at Stanford. He began his postdoctoral study at Cornell University as a Kavli postdoctoral fellow starting in 2016, working at the intersection between condensed matter physics and materials science. The main theme of his research has been using molecular beam epitaxy to design functional materials, and employing angle-resolved photoemission spectroscopy to characterize the electronic properties of these materials. Moreover, he combined femtosecond laser techniques with photoemission spectroscopy, and was able to directly excite and probe the molecular vibrations of complex materials in real time. This advancement allowed him to quantify the electron-lattice coupling in iron selenide superconductors, copper-oxide superconductors, and topological insulators. His research will potentially lead to the discovery of novel functional materials for electronic and energy applications, and enable the characterization of these materials at nonequilibrium.