Femtosecond nonequilibrium dynamics in quasi-1D CDW systems K 0:3 MoO 3 and RB 0:3 MoO 3 A. Tomeljak a , B. Kavcic a , H. Scha ¨fer b , V.V. Kabanov a , D. Mihailovic a , D. Staresinic c , K. Biljakovic c , J. Demsar a,b,Ã a Department of Complex Matter, Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia b Department of Physics and Center for Applied Optics, University of Konstanz, D-78457 Konstanz, Germany c Institute of Physics, 10000 Zagreb, Croatia article info PACS: 71.45.Lr 72.15.Nj 78.47.+p Keywords: Charge density waves Femtosecond real-time dynamics Collective modes abstract Here we report on detailed studies of the photoexcited carrier and collective mode dynamics in prototype quasi-1D charge density wave (CDW) systems K 0:3 MoO 3 and Rb 0:3 MoO 3 focusing on their low temperature and low excitation range. We show that the order parameter relaxation dynamics are inconsistent with the Rothwarf–Taylor model describing the relaxation of the coupled quasi-particle and high frequency phonon system in a system with a narrow gap in the density of states. Therefore we propose an alternative model that accounts for the photoexcited carrier relaxation phenomena commonly observed in CDW systems. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Femtosecond time-resolved (TR) optical spectroscopy is an emerging new tool for studying dynamics and associated low energy electronic structure in correlated electron systems. In this technique the sample under investigation is photoexcited by a femtosecond optical pulse and the resulting relaxation dynamics is measured by following the dynamics of changes in the dielectric function as a function of the time delay after perturbation. The first TR measurements of the photoexcited relaxation dynamics on the charge density wave (CDW) compounds were performed on the prototype quasi-1D CDW semiconductor blue bronze K 0:3 MoO 3 [1]. These experiments showed dramatic changes in the dynamics upon cooling through the CDW transition tempera- ture T 3D c ¼ 183 K. Four distinct components have been observed in a photoinduced reflectivity transient. The first was an oscillatory component attributed to the coherently excited amplitude mode of the CDW. In addition, there were also two distinct fast decay components which were well fitted by a double exponential decay with decay times 0.5 and 10ps, respectively. The fast decaying component displayed critical behavior near T 3D c tying it to the dynamics of the CDW order parameter. Its temperature depen- dence was very similar to the dynamics in cuprate super- conductors, and was therefore attributed to the single particle recombination dynamics, which is in the phonon bottleneck scenario governed by the decay of phonons with energy higher than 2D, D being the magnitude of the single particle gap in the DOS. The slowly decaying component was tentatively attributed to the overdamped phase mode of the CDW since the damping constant matched the width of the phase mode line. Finally, the long lived component was observed with decay time much longer than 10 ns, which was attributed to photogenerated in-gap localized states. Since the first report [1] several experimental papers have been published on ultrafast dynamics in a variety of CDW systems and the experimental results and the underlying model was widely accepted [2–6]. On the other hand, in the last couple of years major progress has been made in theoretical under- standing of the relaxation phenomena in systems with a narrow gap in the DOS. In particular, it was shown that the Rothwarf– Taylor (RT) captures the main underlying physics of the relaxa- tion processes in superconductors [2,7] as well as in heavy electron systems [8]. Since analytical solutions of the RT model enable comparison of the model with experiment and enable one to determine several important microscopic parameters, e.g. the electron–phonon coupling constant; we have performed detailed temperature and excitation intensity dependent mea- surements to determine whether the assignment of various relaxation processes made in Ref. [1] holds or an alternative model is needed. Moreover, studies of the relaxation dynamics under the applied external electric field have been made to check the assignment of the 10 ps component with the overdamped phase mode. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.049 Ã Corresponding author at: Department of Physics and Center for Applied Optics, University of Konstanz, D-78457 Konstanz, Germany. E-mail address: jure.demsar@uni-konstanz.de (J. Demsar). Physica B 404 (2009) 548–551