: Escaping the Phase Diagram Hidden States of Quantum Matter

About this event

Speaker: Anshul Kogar, UCLA

When we think of phases of matter, we usually imagine an equilibrium phase diagram, where distinct phases emerge as temperature, pressure, or other external parameters are varied. But such diagrams also limit our imagination: what would water look like if we could cool it all the way to zero Kelvin while somehow preventing it from freezing?In this talk, I will show how ultrafast pulses of light allow us to circumvent the constraints of equilibrium phase diagrams by driving phase transitions on femtosecond timescales. This approach enables access to transient and hidden states of matter that cannot exist under equilibrium conditions.A familiar example of a phase transition is the transition from a solid to a liquid. In the 1990s, it was predicted that an analogous transition could occur within the electronic degrees of freedom of certain materials: a charge density wave, an electronically ordered “solid,” could melt into a liquid state while the atomic lattice remains intact. Although charge density waves have been studied for decades, clear evidence for their liquid counterpart remained elusive due to the constraints of the equilibrium phase diagram.By using ultrafast laser excitation to drive the system out of equilibrium, we demonstrate that liquid charge density waves exist in nature. Finally, I will discuss how this work points toward a broader class of hidden states of matter that may be waiting within phase diagrams, accessible only through nonequilibrium control.