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TT: Fachverband Tiefe Temperaturen

TT 28: FS: Time-Resolved Spectroscopy in Correlated Electron Systems:
Experiment and Theory

TT 28.1: Topical Talk

Donnerstag, 25. März 2010, 09:30–10:00, H18

Angle- and time-resolved photoelectron spectroscopy of charge density wave materials — •Uwe Bovensiepen — University Duisburg-Essen, Faculty of Physics, Duisburg, Germany

In charge density wave (CDW) materials a band gap opens at sufficiently low temperatures because the system gains energy by forming a new periodic arrangement of the ion cores, which modulates the electron charge density. As shown femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) monitors the phonon mode that couples to electrons at the Fermi level, which is a direct example of electron-phonon coupling. To achieve this the material is pumped by an IR femtosecond laser pulse (hν=1.5 eV). The response to this excitation is probed by a delayed UV laser pulse (hν=6.0 eV). The latter pulse generates photoelectrons which are analyzed regarding parallel momentum and kinetic energy. The tritelluride TbTe₃ responds by electron-hole pair excitation and two distinct coherent phonon modes that result in periodic variations of the respective photoemission lines at the phonon frequency. One of them presents a pronounced momentum dependent dynamics and is responsible for closing (and opening) of the band gap. Hence it represents the amplitude mode of the CDW. In the dichalcogenide 1T -TaS₂ the amplitude mode is also excited, however, it does not couple strongly to the band gap. Accordingly, electronic correlations might contribute to the gap formation. This is confirmed by a considerably faster and shortlived closing of the gap.

Experiments have been performed at the Freie Universität Berlin.