Dresden 2006 – scientific programme
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TT: Tiefe Temperaturen
TT 25: Correlated Electrons - Poster Session
TT 25.1: Poster
Wednesday, March 29, 2006, 14:30–18:30, P1
Charge-order metal-insulator transition in the two-dimensional α- (BEDT-TTF)2I3: a Raman scattering study — •M. Masino1, N. Drichko2, M. Dressel2, D. Schweitzer2, C. Ulrich3, and B. Keimer3 — 1Dip. Chimica GIAF, Universitá di Parma, Italy — 2Physikalisches Institut, Univetsität Stuttgart, Germany — 3MPI für Festkörperforschung, Stuttgart, Germany
A charge order metal-insulator transition is a widely studied phenomenon in low-dimensional solids. Two-dimensional organic conductor α-(BEDT-TTF)2I3 (TCO=135 K) is a model compound to investigate the change of the vibronic and electronic Raman response on this transition. We studied polarized Raman spectra of the single crystals as a function of temperature. We focus our attention on the low-frequency spectral region (10-500 cm−1) where the insulating single particle gap is expected in the electronic response. We found a strong dependence of the spectra on excitation-line and polarization: resonant behavior of the linear I3 anion is found with λ=514 nm excitation polarized parallel to I3 axis (i.e. approximately parallel to the a axis); while the Raman response from the BEDT-TTF conducting layer is seen with the λ=647 nm polarized in the perpendicular direction. This allows us to improve the assignment of the BEDT-TTF phonons. In the charge ordered phase the most strongly coupled molecular vibrations split due to the different charge distribution on the molecules. Moreover, the overall spectral background decreases, and a broad band at 400 cm−1 appears below Tc. The nature of these anomalous spectral features is discussed in terms of electronic scattering response, and vibronic Raman enhancement.