Ultrafast Dynamics of Charge Density Waves in 4H(b)-TaSe2 Probed by Femtosecond Electron Diffraction

The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H(b)-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation, the phase transition is achieved, showing a second-order character, in contrast to the first-order behavior in thermal equilibrium.