Are the renormalized band widths in TTF-TCNQ of structural or electronic origin? - An angular dependent NEXAFS study

TL;DR

This study uses angle-dependent NEXAFS measurements to determine that the surface molecular orientation in TTF-TCNQ matches the bulk, suggesting electronic interactions, not structural relaxation, cause the renormalized band widths observed.

Contribution

It provides direct experimental evidence that surface relaxation is not responsible for band width renormalization in TTF-TCNQ, supporting theories involving long-range Coulomb interactions.

Findings

Surface molecules in TTF-TCNQ have the same tilt angles as in the bulk.

Surface relaxation is ruled out as the cause of band width renormalization.

Results support extended Hubbard model with long-range Coulomb interactions.

Abstract

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 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.