The temperature dependence of the spin susceptibility of high temperature superconductors

The opening of the so called pseudogap or spin-gap in underdoped high temperature superconductors (HTSC) is of crucial importance for the understanding of the phase transition and hence of the mechanism of superconductivity. There is conflicting evidence in literature as to whether the HTSC behave more or less like BCS-type superconductors but, unlike the conventional superconductors, not in the weak coupling but in the strong coupling limit. Or whether the mechanism and maybe even the superconducting pseudo-particles are different from those in the BCS theory. Up to now there is not even agreement in the literature if the pseudogap is directly related to the superconductivity or not. A direct probe of the opening of the gap as seen for instance by NMR is therefore very important. The temperature dependencies of the static (via Knight-shift Ks) and dynamic (via spin-lattice relaxation time T1) spin susceptibility of the conduction electrons give independent probes of this situation. In most cases data are obtainable for various NMR-active nuclei like 170, 63Cu, 89Y etc. and thus superconductivity can be studied on a truly local basis. In this paper the authors summarize the results on Knight shift studies in Y-123, Y-124, La-2212, Bi-2201, Hg-2201, Hg-1223 HTSC with various doping levels. The doping has been achieved either by the variation of the oxygen content of the samples or by doping Zn, Ni, Pb and other ions of different valency into the structure in order to change their hole concentration p