Eur. Phys. J. Special Topics 222, 1005-1016 (2013)
https://doi.org/10.1140/epjst/e2013-01902-4

Regular Article

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

¹ Dept. of Physics and Zukunftskolleg, Univ. of Konstanz, 78457, Germany
² Complex Matter Department, Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia
³ Institute of Physics, Hr-10000 Zagreb, Croatia
⁴ Physics Department, EPFL, 1015 Lausanne, Switzerland
⁵ Ilmenau University of Technology, Institute of Physics, 98693 Ilmenau, Germany

Received: 18 March 2013
Revised: 12 May 2013
Published online: 15 July 2013

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 K_0.3MoO₃ 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.