Bandwidth-controlled Mott transition inκ−(BEDT−TTF)2Cu[N(C<

Abstract

Infrared reflection measurements of the half-filled two-dimensional organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x were performed as a function of temperature (5 K < T < 300 K) and Br-substitution (x = 0%, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad mid-infrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)2 dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlations dependence. Calculations based on the cluster model support our interpretation.

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)Elsässer1, Wu2, Dressel3 et al. 2012Phys. Rev. BSelf Cite

“…The transition between the Hubbard bands peaks around 2100 cm −1 while the intra-dimer excitations center around 3000 cm −1 (more pronounced for E b); this is marginally lower in energy than for κ-Cl. 26 Calculations by density functional theory and tight binding methods yield an intra-dimer transfer integral t d ≈ 170 meV with a slight increase upon cooling, 21 in accord with our observations. The Hubbard band, on the other hand, shifts to lower energies as the temperature is reduced.…”

“…33 The Mott gap increases upon cooling and arrives at ∆ ρ (k B /hc) ≈ 500 cm −1 for T = 20 K and might become slightly larger in the T → 0 limit. Systematic investigations as a function of effective correlations U/t by applying chemical pressure 26,32,33 confirm this assignment.…”

“…2(a) demonstrates the insulating behavior of both compounds, but also important differences: cooling from room temperature κ-Cl crosses over from a semiconducting phase to a Mott insulator at T M ≈ 40 K and orders antiferromagnetically around T N = 25 K. 29,30 For κ-CN the resistivity continuously In Fig. 3 the frequency-dependent reflectivity and conductivity of κ-Cl is plotted for different temperatures and polarizations using data taken by Faltermeier et al 26 Since the optical properties of κ-(BEDT-TTF) 2 Cu[N-(CN) 2 ]Br x Cl 1−x (0 ≤ x < 1) have been presented and extensively analyzed during the last years, we refer to Refs. 26,32-34 for a detailed discussion.…”

“…Temperature dependent optical reflection experiments R(ν, T ) were performed in a wide frequency range using several Fourier transform infrared spectrometers (23−12 000 cm −1 ) with an infrared microscope attached to it; 25,26 in addition the high-frequency optical properties (up to 35 000 cm −1 ) were determined by spectroscopic ellipsometry at room temperature. For T < 100 K the optical transmission was measured in the THz range (18 − 46 cm −1 ) using a coherent source spectrometer.…”

“…They are best taken into account by the cluster model (see for example Rice 37 and Delhaes and Yartsev 38 ) that describes the optical properties of molecular clusters with arbitrary geometry and equilibrium charge density distribution. 26,39 In addition, vibrations of the anions as well as lattice vibrations are present. Due to strong electronic correlations, in these Mott insulators itinerant carriers are absent at low temperatures, as discussed in detail in Ref.…”