Theoretical investigation of the phase diagram of high-Tc superconductors

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The essential new point of this joint Forschergruppe is to study the global phase diagram of the high-temperature superconducting compounds and the competition between the different phases in a tightly coordinated effort between sample preparation, experiment and theoretical modeling. The different experimental techniques yield all essential information on the thermodynamics and the elementary excitations in these compounds. On the basis of a variety of complementary and partly new theoretical techniques, namely quantum Monte-Carlo (QMC), variational cluster techniques going to the infinite-lattice limit combined with QMC, and new contractor renormalizationgroup concepts (CORE), we will directly use this experimental input to clarify the at present still unresolved issue of what is of "general relevance" and what is more "material specific". This is a necessary precondition to answer, in a subsequent step and again in close connection with experiment, questions that are central for an understanding of the high-Tc phenomenon: Crucial questions to be investigated concern the asymmetry of the phase diagram (n- vs. p -doping), the relevant couplings of charge carriers to bosonic excitations, the role of electron-phonon interaction in the pairing mechanism, the nature of the pseudo-gap "phase", and, quite generally, the connection of the microscopic interactions at "high energy" (from eV to order J ) with the observed phases competing with each other at very low temperatures.