Anomalous temperature dependence of the single-particle spectrum in the organic conductor TTF-TCNQ

Abstract

The angle-resolved photoemission spectrum of the organic conductor TTF-TCNQ exhibits an unusual transfer of spectral weight over a wide energy range for temperatures 60K < T < 260K. In order to investigate the origin of this finding, here we report numerical results on the single-particle spectral weight A(k, ω) for the one-dimensional (1D) Hubbard model and, in addition, for the 1D extended Hubbard and the 1D Hubbard-Holstein models. Comparisons with the photoemission data suggest that the 1D Hubbard model is not sufficient for explaining the unusual T dependence, and the long-range part of the Coulomb repulsion also needs to be included.PACS numbers: 71.10. Fd, 79.60.Fr, 71.20.Rv, 72.15.Nj The low-dimensional interacting systems receive attention because of their unusual electronic properties [1]. In this respect, the high-resolution AngleResolved Photoemission Spectroscopy (ARPES) measurements on the quasi-one-dimensional organic conductor tetrathiofulvalene-tetracyanoquinodimethan (TTF-TCNQ) have provided evidence for non-Fermi liquid behavior in this compound [2,3,4,5]. In particular, the ARPES experiments have found that the single-particle spectral weight at the Fermi wavevector k F is transferred over an energy range of ≈ 1.3eV of the Fermi level in the TCNQ-derived band, as the temperature T decreases from 260K to 60K [3,4]. In a Fermi liquid the spectralweight transfer would have occurred within ∼ k B T of the Fermi level. Here, we investigate the origin of this unusual ARPES data and its meaning for the electronic structure of TTF-TCNQ by using the Dynamical Density Matrix Renormalization Group (DDMRG), Quantum Monte Carlo (QMC) and the exact diagonalization methods.There are various possibilities as to what might be the origin of the anomalous T dependence of the singleparticle spectral weight at the Fermi level, A(k F , ω), in TTF-TCNQ: (i) It has been suggested that the Tdependence of the ARPES data can be explained within the one-dimensional (1D) Hubbard model [3,4]. In this case, the anomalous T -dependence of A(k F , ω) over the conduction bandwidth has been attributed to the strongcorrelation effects. Indeed, by using the Bethe-Ansatz solution, the photoemission spectrum has been fitted excellently to the dispersion of the spinon and holon bands of the 1D Hubbard model with the parameters t = 0.4eV for the hopping matrix element and U = 2eV for the Coulomb repulsion. The recent observation of the 3k F structures in A(k, ω) by the ARPES [5] also supports this picture. (ii) An alternative point of view is that an extended Hubbard model with long-range Coulomb repulsion is necessary, particularly because the screening of the long-range Coulomb repulsion is expected to be weaker for the surface layer of TTF-TCNQ. (iii) Another possibility is that the electron-phonon interaction, in addition to the strong Coulomb repulsion, plays a role in producing the unusual T dependence. In this paper, our goal is to differentiate among these possibilities. For this purpose, we present DDMRG and finite-temper...

Structural vs electronic origin of renormalized band widths in TTF-TCNQ: An angular dependent NEXAFS study

“…This questions the assumption of a surface relaxation and hence an enhanced value of t to explain the experimental data. Results by exact diagonalization 6 for T = 0 found that the inclusion of the nearest-neighbor Coulomb repulsion V in the extended HM offers a possible loop hole since it effectively increases the band widths of the spinon and holon excitations to match the experimental dispersions while, at the same time, allowing for a smaller t and hence a smaller J ef f in better agreement with experiment. Indeed, Cano-Cortés et al 5 recently have reported on density-functional results which find that the long-ranged Coulomb repulsion even to the third nearest neighboring molecule within a stack is still about 20% of U.…”

“…3,4 On the other hand, further theoretical work has shown that the also observed transfer of spectral weight at k F over the entire conduction band width with increasing temperatures cannot be reconciled within this parameter set. 6,20 The effective magnetic exchange J ef f which defines a characteristic temperature for the onset of such an spectral-weight transfer would come out much too large within the 1D HM. This questions the assumption of a surface relaxation and hence an enhanced value of t to explain the experimental data.…”

“…An alternative proposition has recently been elaborated on, 5 namely, the necessity to incorporate the next-nearest-neighbor Coulomb repulsion V in a HM description in addition to the onsite Coulomb energy U. 6 This reduces the effective energy for two electrons to pass each other to U − V and hence enhances the effective hopping integral t ef f Ϸ tU / ͑U − V͒ without the need to assume a larger t. Indeed, it was shown by density-functional theory that V Ϸ U / 2 within a stack. 5 However, these results are based on the bulk crystal structure, and an experimental proof that no surface reconstruction is at work here is still lacking.…”

Spin, charge, and single-particle spectral functions of the one-dimensional quarter filled Holstein model

“…The temperature dependence in the Luther-Emery phase interestingly shows that above the temperature scale at which the single gap opens at the Fermi energy, features of the Luttinger liquid phase, namely a polaronic band crossing the Fermi energy and a gaped charge mode, are apparent. This observation should be set in the context of photoemission experiments carried out on TTF-TCNQ organics where measurements are carried out at a temperature scale above the Peierls transition and interpreted in terms of a Luttinger liquid model [1,3,4,5]. For the Holstein model given by Eq.…”

“…One can mention the quasi one-dimensional organics TTF-TCNQ where photoemission experiments are carried out down to 60 K just above the Peierls transition [1]. A detailed modeling of this experimental situation is bound to include both electronic correlations [2,3,4,5] as well as the phonon degrees of freedom [6]. In two dimensions the electron-phonon interactions leads to a delicate interplay of superconductivity and charge density waves depending on the partial nesting properties of the Fermi surface [7,8].…”