Researchers from UCLA have used ultrafast electron diffraction to reveal a hidden electronic phase that cannot be reached by ordinary heating. The study focuses on 1T-TaS₂, a layered material in which electrons naturally organize into a periodic charge pattern known as a charge density wave. Theory has long suggested that such ordered electronic patterns could “melt” into a liquid-like state, but in this material a competing structural transformation occurs near 600 K and normally prevents that regime from being observed. By using femtosecond laser pulses, the experiment bypassed this limitation: the material was driven into a transient, superheated state while compressed electron pulses captured how the electronic order evolved in real time.
The diffraction measurements show that, after laser excitation, the charge order first disappears and then partially reforms through a sequence of intermediate states controlled by defects. At lower starting temperature, the diffraction pattern indicates a partially melted state: the regular spacing of the charge pattern is lost, but a preferred angular alignment remains. At higher temperature, the diffraction peaks spread into a nearly uniform ring, showing that both positional and angular order have disappeared. This is the experimental signature of a liquid charge density wave. More broadly, the work demonstrates how ultrafast electron diffraction can uncover short-lived phases of matter that are hidden from conventional equilibrium experiments.