We present results for the momentum-resolved single-particle spectral function of the low-dimensional system TiOCl in the insulating state, obtained by a combination of ab initio Density Functional Theory (DFT) and Variational Cluster (VCA) calculati
ons. This approach allows to combine a realistic band structure and a thorough treatment of the strong correlations. We show that it is important to include a realistic two-dimensional band structure of TiOCl into the effective strongly-correlated models in order to explain the spectral weight behavior seen in angle-resolved photoemission (ARPES) experiments. In particular, we observe that the effect of the interchain couplings is a considerable redistribution of the spectral weight around the Gamma point from higher to lower binding energies as compared to a purely one-dimensional model treatment. Hence, our results support a description of TiOCl as a two-dimensional compound with strong anisotropy and also set a benchmark on the spectral features of correlated coupled-chain systems.
Using a combined local density functional theory (LDA-DFT) and quantum Monte Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of the superconducting transition temperature Tc of several single-layer hole-doped cuprate supe
rconductors with experimentally very different Tcmax is investigated. The parameters of two different three-band Hubbard models are obtained using the LDA and the downfolding Nth-order muffin-tin orbital technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show that the d-wave transition temperature Tc depends sensitively on the parameters. While the N=1 MTO basis set which reproduces all three $pdsigma$ bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA Fermi surface and velocities does not.
