# Dresden 2006 – wissenschaftliches Programm

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# TT: Tiefe Temperaturen

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

### TT 31.9: Vortrag

### Donnerstag, 30. März 2006, 16:00–16:15, HSZ 301

**Bandwidth-controlled Mott transition in ****κ****-phase BEDT-TTF salts** — •Martin Dressel^{1}, N. Drichko^{1}, M. Dumm^{1}, D. Faltermeier^{1}, C. Meziere^{2}, and P. Batail^{2} — ^{1}1. Phys. Inst., Univ. Stuttgart, Germany — ^{2}Lab. CIMI, CNRS-Univ. d’Angers, France

Infrared reflection measurements of the
half-filled two-dimensional organic conductors
κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br_{x}Cl_{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.