Low-energy charge excitations in an undoped cuprate beyond the standard pdσ-model

Despite enormous efforts worldwide during the last 16 years, the origin of high-Tc superconductivity (HTSC) in the doped cuprates has still not been settled. From a theoretical and microscopic point of view it is even unclear in the framework of which model the proper answer to the challenging HTSC problem should be searched, apart from all serious many-body difficulties caused by the strong correlation present in these compounds. Shortly after the discovery of HTSC it was widely believed that the so called pdσ-model (see Fig. 1) provides a general appropriate starting point to describe correctly the low-energy physics of undoped and doped cuprates including also superconductivity. The linear-chain compound Sr2CuO3 (see Fig.2a) consists of corner-shared CuO4 plaquettes, being the onedimensional (1D) analogue of the twodimensional (2D)-HTSC-compounds with CuO2 –planes, and provides an Stefan-Ludwig, Drechsler, Martin Knupfer, Jiri Málek, Jörg Fink Low-energy charge excitations in an undoped cuprate beyond the standard pdσ-model