J. Phys. IV France
Volume 114, April 2004
Page(s) 183 - 189

J. Phys. IV France
114 (2004) 183
DOI: 10.1051/jp4:2004114042

Influence of electronic correlations and band-filling on the charge-ordering effects in two-dimensional organic conductors investigated by infrared spectroscopy

M. Dressel1, N. Drichko1, 2, J. Schlueter3, O. Bogdanova4, E. Zhilyaeva4, R. Lyubovskaya4, A. Greco5 and J. Merino6

1  1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany
2  Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia
3  Material Science Divisions, Argonne National Laboratory, Argonne, IL 60439-4831, USA
4  Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Russia
5  Facultad de Ciencias Exactas Ingeniería y Agrimensura e Instituto de Física Rosario, Rosario, Argentina
6  Max-Planck-Institut für Festkörperforschung, 70506 Stuttgart, Germany

A comparative study of the low-energy optical properties of the 1/4-filled two-dimensional organic conductors $\alpha
$ -(BEDT-TTF) 2MHg(SCN) 4 ( M = K, NH 4) and the 1/5-filled salt $\beta''$-(BEDO-TTF) 5[ CsHg(SCN) 4] 2 is presented. The superconducting $\alpha
$-(BEDT-TTF) 2NH 4Hg(SCN) 4 exhibits metallic-like optical properties down to 4 K. This is in contrast to the non-superconducting salt $\alpha
$-(BEDT-TTF) 2KHg(SCN) 4 where a strong feature appears at frequencies of about 200 cm -1 and temperatures below 200 K which indicates the opening of a pseudogap. In the anisotropic and poor metal $\beta''$-(BEDO-TTF) 5[ CsHg(SCN) 4] 2 we find a similar pseudogap in the optical spectra around 300 cm -1 up to room temperature and the presence of a strong Drude-like contribution. Based on exact diagonalisation calculations of the optical conductivity on an extended Hubbard model at quarter-filling we argue that the proximity of the $\alpha
$-phase BEDT-TTF salts to a charge-ordering transition is responsible for the observed pseudogap. Our proposed scenario suggests that the different ground states, including superconductivity, are a consequence of the fluctuations associated with short-range charge ordering which builds up close to the quantum phase transition. In a 1/5-filled system similar features like the short-range checkerboard charge fluctuations are expected, but the Drude contribution is always present because the charge ordered state is strongly modified by the presence of mobile empty sites. Key words. optical properties - charge ordering - electronic correlations - two-dimensional organic conductors.

© EDP Sciences 2004