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

We have performed angle-dependent near-edge x-ray absorption fine structure measurements in the Auger electron yield mode on the correlated quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) in order to determine the orientation of the molecules in the topmost surface layer. We find that the tilt angles of the molecules with respect to the one-dimensional axis are essentially the same as in the bulk. Thus, we can rule out surface relaxation as the origin of the renormalized band widths which were inferred from the analysis of photoemission data within the one-dimensional Hubbard model. Thereby, recent theoretical results are corroborated which invoke long-range Coulomb repulsion as alternative explanation to understand the spectral dispersions of TTF-TCNQ quantitatively within an extended Hubbard model.

Figure 1

(Color online) (a) Central projection view of the crystal structure of TTF-TCNQ along the $b$ axis. (b) Parallel projection view along $a$ with a hypothetical change in the molecule tilt angles at the surface. For details, see text.

Figure 2

(Color online) Bulk GGA band structure of TTF-TCNQ along the 1D direction for the real (black solid lines) and a hypothetical (pink or gray solid lines) band structure based on enlarged molecule tilt angles. For details, see text.

Figure 3

(Color online) Left panel: N $K$ edge NEXAFS spectra of TTF-TCNQ for various light incidence angles $\theta$. In the lower part, the normal incidence (NI) spectrum is reproduced together with the corresponding fit analysis from which the integral peak intensities are obtained. Right panel: Integrated peak areas for the peaks as in the fit of the left panel as a function of light incidence angle (scattered curves). The solid lines mark fits according to the model as described in the text.

Figure 4

(Color online) Left panel: S $L$ edge NEXAFS spectra of TTF-TCNQ for various light incidence angles $\theta$. Right panel: Integrated area for the pronounced ${\sigma }^{*}$ resonance peak at $164.8\mathrm{eV}$ (dotted curve). The solid line depicts a fit according to the model as described in the text.

Figure 5

(Color online) Left panel: C $K$ edge NEXAFS spectra of TTF-TCNQ for various light incidence angles $\theta$. In the lower part, the spectrum for $\theta =20°$ is reproduced together with the corresponding fit analysis from which the integral peak intensities were obtained. Right panel: Integrated peak area for the peak marked in orange or gray in the left panel as a function of light incidence angle (dotted curve). The solid line depicts a fit according to the model as described in the text.