Berlin 2015 – wissenschaftliches Programm
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HL: Fachverband Halbleiterphysik
HL 68: Low-dimensional systems: Molecular conductors (TT with CPP/HL/MA/O)
HL 68.1: Vortrag
Donnerstag, 19. März 2015, 09:30–09:45, H 3010
Nature of the empty electronic states of TCNQ and their thermal evolution due to the CDW instability of TTF-TCNQ revealed by NEXAFS — •Alisa Chernenkaya1, K. Medjanik1,2, P. Nagel3, M. Merz3, S. Schuppler3, E. Canadell4, J.-P. Pouget5, and G. Schönhense1 — 1JGU, Mainz, Germany — 2MAX-lab, Lund, Sweden — 3KIT, Karlsruhe, Germany — 4ICMAB, Bellaterra, Spain — 5Uni Paris-Sud, Orsay, France
The electronic structure of TTF-TCNQ was studied by near-edge x-ray absorption fine structure (NEXAFS) to detect a signature of the Peierls transition at 54 K [1]. All experimental unoccupied TCNQ orbitals predicted by first-principles calculations are clearly resolved, the σ*(π(ag, b3u)) orbital was observed for the first time [2]. The temperature dependence of NEXAFS peak intensities gives evidence of a subtle modification of the electronic structure when the charge density wave (CDW) fluctuations develop as the Peierls transition of the TCNQ stacks is approached from higher temperatures. These changes are explained on the basis of the charge transfer, the shape of the lower empty TCNQ molecular orbitals and the deformation of TCNQ during the pre-transitional CDW fluctuations. Finally the data suggest that the internal stack deformation consisting in a substantial out of plane displacement of the central ring with respect to the cyano-groups allows to gain C-C bonding energy which helps the stabilization of the Peierls transition on the TCNQ stack.
[1] J.P. Pouget, Z. Kristallogr. 219, 711, 2004.
[2] A. Chernenkaya et al., EPJB, accepted.