One-dimensional versus two-dimensional correlation effects in the oxyhalides TiOCl and TiOBr

We have performed a comparative study of the electronic structures of the spin-Peierls systems TiOCl and TiOBr by means of photoemission spectroscopy and density-functional calculations. While the overall electronic structure of these isostructural compounds is qualitatively similar, the bromide appears to be less one-dimensional. We present a quantitative analysis of the experimental dispersions in terms of exchange constant $J$ and hopping integral $t$, as well as a discussion of the qualitative spectral features. From that, we conclude that despite the one-dimensional physics triggering the ground state in both compounds, a proper description of the normal-state electronic structure has to take into account the anisotropic frustrated interchain interactions on the underlying triangular lattice.