Temperature independent pseudogap of the cuprate superconductors and NMR

NMR is a powerful bulk quantum probe that can give vital information about the electronic properties of materials. Well known examples are the changes of NMR shift and relaxation due to a redistribution of electronic states caused by the opening of a superconducting gap or the pseudogap in the cuprates. In the early days, the interpretation of the NMR data was led astray, to some extent, by the focus on just a few materials. By gathering all literature shift and relaxation data of the cuprates we establish a rather different NMR phenomenology. For example, planar oxygen relaxation and shift show a temperature-independent but doping dependent pseudogap at the Fermi surface. States above the pseudogap are similar for all cuprates and doping levels, and appear to be Fermi liquid-like. All data can be explained with a single spin component. Also, the deduced orbital shifts agree with first principal calculations. The planar Cu shift data shows a similarity with the oxygen data, i.e. there is a high temperature offset in the shifts for both directions of the magnetic field as demanded by a temperature independent pseudogap. However, there is a strong family dependence that is not present in the planar oxygen data. Further details including relaxation will be discussed.