Dynamics of charge density wave order in the quasi one dimensional conductor (TaSe4)2I probed by femtosecond optical spectroscopy

Abstract

Carrier and collective mode dynamics in the quasi one-dimensional charge density wave (CDW) system (TaSe4)2I have been investigated by means of time-resolved optical pump-probe spectroscopy. In the low excitation, linear, regime we focus on the temperature dependence of the collective amplitude modes, originating from linear coupling of the electronic modulation to phonons at q CDW. Numerous amplitude modes are observed, ranging from 100 GHz to several THz. The modes’ softening near T c is rather weak, which could be related to strong decoupling of electronic and lattice subsystems. Alternatively, the data could be reconciled also in case the CDW phase transition is of the first-order type where a nearly constant order parameter below T c would prevent softening. In the high excitation regime we investigated the energetics of the photoinduced CDW-normal phase transition. Similarly to the elaborately investigated one-dimensional CDW system K0.3MoO3 we observe two characteristic energy scales, related to melting the electronic modulation alone (100 meV per unit cell) and to the overall (electronic modulation and the periodic lattice distortion) collapse of the CDW (> 400 meV per unit cell). While the latter coincides with the thermal energy needed to heat the sample from 5 K above T c the former is consistent with the mean field estimate for the electronic condensation energy, suggesting that the weak coupling description of the CDW in (TaSe4)2I is appropriate.