Microscopic approach to high-temperature superconductors: Superconducting phase

Despite the intense theoretical and experimental effort, an understanding of the superconducting pairing mechanism of the high-temperature superconductors, leading to an unprecedented high transition temperature $T_c$, is still lacking. Starting from the $t$-$J$ model, we present a microscopic approach to the physical properties of the superconducting phase at moderate hole-doping in the framework of a novel renormalization scheme, called PRM. Our microscopic approach allows us to explain the experimental findings in the underdoped as well as in the optimal hole doping regime. In good agreement with experiments, we find no superconducting solutions for very small hole doping. In the superconducting phase, the order parameter turns out to have d-wave symmetry with a coherence length of a few lattice constants. The spectral function, obtained from angle-resolved photoemission spectroscopy (ARPES) along the Fermi surface, is also in good agreement with experiment: The spectra display peak-like structures which are caused alone by coherent excitations in a small range around the Fermi energy.