Topological phase transition to a hidden charge density wave liquid

Charge density waves (CDWs), electronic crystals that form within a host
solid, have long been speculated to melt into a spatially textured electronic liquid.
Though they have not been previously detected, liquid CDWs may nonetheless be
fundamental to the phase diagrams of many correlated electron systems, including
high temperature superconductors and quantum Hall states. In 1𝑻-TaS2, a leading
candidate material, access to the liquid state is obstructed by a structural phase
transition. Using femtosecond light pulses, we bypass this transition to uncover
hidden CDW correlations via ultrafast electron diffraction. Upon photoexcitation,
the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state.
At higher temperatures, photoexcitation completely destroys translational and
orientational order producing a diffuse ring at the CDW wavevector—a hallmark
of a liquid CDW. Our work provides compelling evidence for a spatially modulated
electronic liquid and establishes a pathway for accessing hidden quantum phases.