Collective Phenomena in Correlated Electron Systems: High-T Superconductivity and High-T Density Waves

For doped two-dimensional quantum spin systems the competition between insulating states at low carriers concentration and superconductive pairing at higher densities has emerged as a key feature of the high-Tc problem. We study magnetic correlations in low-dimensional quantum systems and the carrier dynamics in doped antiferromagnetic environment with relevance to the phase diagram high-Tc cuprates. Magnetic correlations which give rise to a finite spin gap were predicted to generate an attractive interaction between doped carriers leading to superconductivity with a d-wave like order parameter. Novel ground states with broken translational symmetry in which single holes or hole pairs order in a crystalline pattern were observed.