Light-induced liquid charge-density-wave state in 1T-TaS2

Nearly all materials will undergo a phase transition from an isotropic fluid to a crystalline solid at sufficiently low temperatures, characterized by a spontaneous loss of long-range translational and rotational order. The solid-to-liquid phase transition is especially interesting in two dimensions due to the enhanced effect of thermal fluctuations and topological defects. Here, using ultrafast electron diffraction, we study the melting and reformation of a quasi-two-dimensional incommensurate charge-density-wave (CDW) in 1T-TaS2. We observe a complete loss of orientational order of the CDW, i.e. a liquid CDW state, during its subsequent reformation after photoexcitation. Surprisingly, the orientational isotropy of the CDW superlattice is manifested despite the presence of the six-fold symmetry of the underlying parent lattice. These results provide insight into the recombination dynamics of dislocation and disclination-type topological defects induced by a light pulse in low-dimensional solids.