Theoretical investigations of the phase diagram of high-Tc superconductors
One essential new point of the Forschergruppe (FG) is to study the hole- and electron-doped phase diagram of the high-temperature superconductors (HTSC) in a tight-knit effort between experiment and theory. In the first funding period, we have already made use of this unique possibility, namely using input from a variety of different experiments (neutron scattering, photoemission) on one and the same material to construct a phenomenology of the HTSC, i.e. aiming at the key question ``which boson drives the superconducting pairing''. This concerted effort, together with a variational cluster solution of the Hubbard model gave reason to believe that the correct physics of the HTSC is that of ``doping into a Mott insulator'', i.e. the physics of strong electronic correlations. It is the central aim of our present proposal to substantiate this belief employing both T=0 and finite-T cluster techniques, in combination with a new scheme we have developed for extracting two-particle (i.e. spin- and charge-) excitations. Together with our already working schemes (VCA and DCA) for calculating single-particle excitations, this gives us a previously not available complete dynamical information of the HTSC compounds under study. This information will be used to clarify the still unresolved issue of what is ``universal'' and what is ``material specific''. The following topics, which are central for arriving at a ``universal picture'' will be addressed: the (finite-T) phase diagram for Hubbard-type models also with additional orbital degrees of freedom, the physics of the pseudogap and the effects of inhomogeneities (stripes, static and fluctuating, etc.) and defects.