Structural Dynamics in the Charge Density Wave Compound 1T-TaS2

This Diploma thesis is centered around the study of the structural dynamics in charge density wave (CDW) compounds. Owing to their quasi low dimensionality, CDWs present an ideal model system to investigate the delicate interplay between various degrees of freedom like spins, electrons, lattice, etc., common to macroscopic quantum phenomena such as high-temperature-superconductivity and colossal magnetoresistance. In this respect, femtosecond (fs) time resolved techniques are ideal tools to trigger ultrafast processes in these materials and to subsequently keep track of various relaxation pathways and interaction strengths of different subsystems [Ave01, Oga05, Kus08]. Particularly 1T -TaS2 hosts a wealth of correlated phenomena ranging from Mott-insulating behavior [Tos76, Faz79] and superconductivity under pressure [Sip08, Liu09] to the formation of charge density waves with different degrees of commensurability [Wil75, Spi97]. Here, photoinduced transient changes in reflectivity and transmission of 1T -TaS2 at different CDW phases are presented. The experimental observations include, inter alia, a coherently driven CDW amplitude mode and two distinct relaxation timescales on the order of τfast ∼ 200 fs and τslow ∼ 4ps, which are common to all CDW compounds [Dem99]. Moreover, the frequency shift of the amplitude mode and the change in relaxation timescales when overcoming the phase transition from the commensurate to the nearly commensurate CDW state are discussed. A prerequisite for the direct study of structural dynamics by means of fs electron diffraction is specimen availability with thicknesses on the order of tens of nanometers comprising lateral dimensions of a few hundred micrometers. The preparation of 30 nm × 100μm × 100μm dimensioned films of 1T -TaS2 is described, as well as their characterization by means of temperature dependent transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The generation of fs electron pulses can be achieved by a compact design of the electron gun [Siw04, Dwy05]. In this work, the development of a compact back illuminated 30 kV electron gun is outlined. Though a complete characterization is still under way, the main features of the design are presented. Finally, direct atomic level insights into the structural dynamics of 1T -TaS2 are provided by means of fs electron diffraction. The periodic lattice distortion of the CDW is found to collapse within τmelt ≈ 170 ± 40 fs which is faster than one half of the corresponding amplitude mode and thus indicative of an electronically-driven process. The energy transfer to optical phonons takes place within τe-ph ≈ 350 ± 50 fs. The recovery of the CDW proceeds on a timescale of τrec ∼ 4 ps, identical to τslow found in the all-optical pump probe experiment. Therefore, the order parameter relaxation in CDW compounds is attributed to the picosecond timescale. Applying pump fluences which are equivalent to the energy needed for heating 1T -TaS2 into the incommensurate CDW state, the phase transition is observed to take place on the sub picosecond timescale. This experiment demonstrates the complementary insights gained by fs electron diffraction in the study of complex phenomena like melting and recovery of the order parameter in CDW systems.