J. Phys. IV France 114 (2004) 305
DOI: 10.1051/jp4:2004114066

A comparative mid-infrared study of superconductor BETS ₄Hg _2.84Br ₈ and metal BETS ₄Hg ₃Cl ₈

N. Drichko¹, B. Petrov¹, V.N. Semkin¹, R.M. Vlasova¹, O.A. Bogdanova², E.I. Zhilyaeva², R.N. Lyubovskaya², I. Olejniczak³, H. Kobayashi⁴ and A. Kobayashi<sup>5

1</sup>  Ioffe Physico-Technical Institute RAS, 194021 St. Petersburg, Russia
²  Institute of Problems of Chemical Physics, RAS, 142432 Chernogolovka, MD, Russia
³  Institute of Molecular Physics PAS, Poznan, Poland
⁴  Institute for Molecular Science, Okazaki, 444, Japan Spectrochemical Centre, Japan
⁵  University of Tokyo Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Abstract
Polarized reflectivity spectra of the 2 K organic superconductor BETS ₄Hg _2.84Br ₈ (T _c=2 K) and the metal down to 35 K BETS ₄Hg ₃Cl ₈ were measured in the 700-6000 cm ^-1 frequency range down to 10 K. Higher anysotropy of reflectivity in the conducting plane of these compounds compared to their BEDT-TTF-based analogues is explained by the differences in intermolecular overlap. No Drude-peak is observed in the optical conductivity spectra of these compounds in the measured frequency range down to 10 K. In BETS ₄Hg ₃Cl ₈ a simultaneous splitting of the electronic mid-infrared band and EMV-coupled (a _g) vibration at about 150 K might suggest that the charge-ordering takes place in this compound below 150 K. Key words. organic conductors, infrared spectroscopy.

© EDP Sciences 2004