ABSTRACT: In this talk, I will introduce two state-of-the-art ultrafast techniques — time-resolved electron diffraction and extreme-ultraviolet (XUV) transient absorption spectroscopy — and discuss how they shed new light on a fifty-year-old quasi-2D material, 1T-TiSe2, which is alleged to exhibit exciton condensation below a rather high transition temperature of Tc ≈ 200 K. From ultrafast electron diffraction, I will demonstrate that exciton correlations maintain the out-of-plane phase coherence in a ground state that features a commensurate charge density wave. Using XUV transient absorption, I will show evidence for short-range excitonic fluctuations above Tc and an enhanced excitonic susceptibility near Tc, which are expected in a quasi-2D system but have not been experimentally observed to date.

BIO: Alfred started as a faculty at Stanford in September 2024, and his group is interested in studying light-induced non-equilibrium phenomena in quantum materials, exploring the intersection between condensed matter physics and ultrafast science. Prior to Stanford, Alfred was a Miller Research Fellow at UC Berkeley. He received his PhD in Physics from MIT, and BS in Physics and MS in Computer Science from Stanford.