[Paper Review] Coulomb parameters and photoemission for the molecular metal TTF-TCNQ

Laura Cano-Cortés, Andreas Dolfen+5 more|arXiv (Cornell University)|Sep 18, 2006

Organic and Molecular Conductors Research22 citations

TL;DR

This study uses density-functional theory to compute realistic Coulomb parameters for the molecular metal TTF-TCNQ, revealing significant nearest-neighbor Coulomb repulsion (V ≈ 0.9–1.0 eV) and longer-range interactions along molecular stacks. Including V in an extended Hubbard model broadens the spectral function, resolving discrepancies in photoemission data without requiring enhanced hopping parameters, thus improving the description of spin-charge separation and electronic correlations.

ABSTRACT

We employ density-functional theory to calculate realistic parameters for an extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra- and intermolecular screening in the crystal, we find significant longer-range Coulomb interactions along the molecular stacks, as well as inter-stack coupling. We show that the long-range Coulomb term of the extended Hubbard model leads to a broadening of the spectral density, likely resolving the problems with the interpretation of photoemission experiments using a simple Hubbard model only.

Motivation & Objective

• To provide accurate, first-principles estimates of Coulomb parameters for TTF-TCNQ, especially beyond on-site U.
• To address the long-standing discrepancy between photoemission spectra and predictions from simple t-U models.
• To investigate the role of intermolecular and long-range Coulomb interactions (V, V', V'') in shaping the spectral function.
• To assess the impact of inter-stack coupling and screening on electronic correlations in this quasi-1D molecular metal.
• To clarify the origin of spectral broadening observed in ARPES experiments and its implications for spin-charge separation.

Proposed method

• Employed all-electron DFT with the PBE functional and Gaussian basis sets (NRLMOL) to compute HOMO and LUMO charge densities of TTF and TCNQ molecules.
• Calculated bare Coulomb integrals (U_bare, V_bare, etc.) via direct integration of electron densities over intermolecular distances.
• Accounted for intra-molecular screening by computing total energy differences for charged molecules and fitting to obtain U_0.
• Modeled inter-molecular screening using point polarizabilities and validated against constrained-DFT calculations on small clusters.
• Used the t-U-V Hamiltonian to simulate spectral functions via Lanczos diagonalization, with V treated as a perturbation.
• Applied first-order Rayleigh-Schrödinger perturbation theory to explain spectral broadening due to V.

Experimental results

Research questions

• RQ1 What are the realistic values of on-site (U) and nearest-neighbor (V) Coulomb interactions in TTF-TCNQ, including screening effects?
• RQ2 Why does the standard t-U model fail to reproduce the width of the spectral function in photoemission experiments?
• RQ3 How does the inclusion of long-range Coulomb interactions (V, V', V'') affect the spectral density and effective hopping in TTF-TCNQ?
• RQ4 To what extent do inter-stack Coulomb couplings influence electronic properties and correlations in this molecular metal?
• RQ5 Can the observed spectral broadening in ARPES be explained by V without increasing the hopping parameter t?

Key findings

• The on-site Coulomb interaction U is reduced to 1.7–2.0 eV after screening, consistent with prior estimates but now derived from first principles.
• The nearest-neighbor Coulomb repulsion V is substantial, with values of 0.9–1.0 eV for TCNQ and TTF chains, significantly larger than previously assumed.
• Including V in the t-U-V model leads to a broadening of the spectral function around the Fermi level, matching experimental ARPES data.
• The spectral broadening arises from a linear shift of spectral peaks with V, proportional to the local density of states, as predicted by first-order perturbation theory.
• The effective hopping t_eff is enhanced to approximately 0.38 eV due to the V-term, explaining why increasing t in the t-U model was previously needed to fit spectra.
• The system is near a ferroelectric instability, suggesting that electronic correlations could be drastically altered under hydrostatic pressure.