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TT: Tiefe Temperaturen

TT 31: Correlated Electrons: Metal Insulator Transition - Part 2

TT 31.9: Talk

Thursday, March 30, 2006, 16:00–16:15, HSZ 301

Bandwidth-controlled Mott transition in κ-phase BEDT-TTF salts — •Martin Dressel¹, N. Drichko¹, M. Dumm¹, D. Faltermeier¹, C. Meziere², and P. Batail² — ¹1. Phys. Inst., Univ. Stuttgart, Germany — ²Lab. CIMI, CNRS-Univ. d’Angers, France

Infrared reflection measurements of the half-filled two-dimensional organic conductors κ-(BEDT-TTF)₂Cu[N(CN)₂]Br_xCl_1−x are performed as a function of temperature and Br-substitution (x=0%, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. The mid-infrared band centered around 3400 cm⁻¹ for E∥ c is assigned to localized charge transfer within the dimers which is barely visible along the a direction. As the charge becomes delocalized for increasing Br content and low temperatures, the band shifts to higher frequencies. In a similar way, intramolecular vibrations which are observed via electron-molecular vibration coupling change in frequency. As the temperature drops below 50 K an energy gap develop in the Cl-rich samples which increase for T→ 0. With increasing Br concentration spectral weight shifts into the gap region and eventually fills it up completely. As these samples (x=85% and 90%) become metallic at low temperatures, a Drude-like component develops due to the coherent quasiparticles. We perform a detailed analysis of the spectral weight transfer which gives us insight into the influence of electronic correlations on the dynamical properties. The observed behavior of the Drude spectral weight at the critical value of U/t (which is reached for x≈ 70%) agrees with the abrupt jump to a finite value predicted by theory. The series of alloys is a good model of a bandwidth-controlled Mott insulator.