Ultrafast dynamics of charge density waves in 4Hb-TaSe2 probed by   femtosecond electron diffraction
  The dynamics of the photoinduced commensurate to incommensurate charge density wave (CDW) phase transition in 4Hb-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime the CDW reforms on a 150 ps timescale, 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. 
N. Erasmus, M. Eichberger, K. Haupt, I. Boshoff, G. Kassier, R.   Birmurske, H. Berger, J. Demsar, and H. Schwoerer
cond-mat.str-el cond-mat.mtrl-sci