Dynamical properties of a strongly correlated model for quarter-filled layered organic molecular crystals

Abstract

The dynamical properties of an extended Hubbard model, which is relevant to quarter-filled layered organic molecular crystals, are analyzed. We have computed the dynamical charge correlation function, spectral density, and optical conductivity using Lanczos diagonalization and large-N techniques. As the ratio of the nearestneighbor Coulomb repulsion, V, to the hopping integral, t, increases there is a transition from a metallic phase to a charge-ordered phase. Dynamical properties close to the ordering transition are found to differ from the ones expected in a conventional metal. Large-N calculations display an enhancement of spectral weight at low frequencies as the system is driven closer to the charge-ordering transition in agreement with Lanczos calculations. As V is increased the charge correlation function displays a collective mode which, for wave vectors close to (,), increases in amplitude and softens as the charge-ordering transition is approached. We propose that inelastic x-ray scattering be used to detect this mode. Large-N calculations predict superconductivity with d xy symmetry close to the ordering transition. We find that this is consistent with Lanczos diagonalization calculations, on lattices of 20 sites, which find that the binding energy of two holes becomes negative close to the charge-ordering transition.

Signatures of Electron-Electron Correlations in the Optical Spectra of α-(BEDT-TTF)2 MHg(SCN)4 (M=NH4 and Rb)

“…Therefore, the observed shift of the spectral weight from the highfrequency features to the Drude peak on cooling (indicated by arrows in Fig. 1, 2) is consistent with predictions of the charge fluctuation scenario 7 . A discussion of the main ingredients of this theory is revised in the following section.…”

“…We attribute the observed electronic features to the spectral weight shifted from the Drude-peak to higher frequencies due to the influence of electronic correlations; a similar interpretation was proposed for the spectra of θ-phase compounds 7,12 , where a very intense feature and a pseudogap at 300 cm −1 was previously observed in the low temperature spectra of α-(BEDT-TTF) 2 KHg(SCN) 4 and interpreted as an indication of the fluctuating charge ordering 8 .…”

“…Slave-boson, CPA (Coherent Potential Approximations) and large-N calculations on the extended Hubbard model predict that sufficiently close to V c and within the metallic phase, increasing the temperature drives the system into the charge ordered phase. This reflects the unusual 'reentrant' character of the metal-charge ordered transition, which leads to an interesting temperature dependence of the electronic properties of the metal when being close to a checkerboard charge ordering transition at T=0 such as an increase in the electron effective mass with increasing temperature 7 . This behavior is different from the one found in a conventional metal in which increasing the temperature would tend to suppress the renormalization of the electrons due to their interaction with other electrons, phonons, etc, effectively decreasing their effective mass enhancement as T → T F , where T F is the Fermi temperature of the metal.…”

“…On the other hand, extensive theoretical studies were performed for the electronic correlations in 1/4-filled layered molecular metals close to the charge-order transition 5,6,7,8 . Here we present an optical study of 1/4 filled metals α-(BEDT-TTF) 2 M Hg(SCN) 4 (M =NH 4 ,Rb), aimed on obtaining the effective mass and scattering rate of charge carriers, in order to compare them with theory and to distinguish between the electronic correlations and other factors defining their behavior.…”

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

“…The charge fluctuation in a quarter-filled-band system was theoretically discussed in refs. [40,41] using the charge correlation function C(q,ω). If the system has CO instability, the charge correlation function has a strong peak at q 0 and ω 0~0 .…”