The New NMR Shift and Relaxation Scenario of the Cuprates

Based on experiments and a comprehensive literature analysis of cuprate NMR shift and relaxation data we present the new scenario for the cuprates. It has almost ubiquitous fermionic excitations that govern the nuclear relaxation above Tc and begin to disappear below Tc, i.e., for all materials from slightly underdoped to heavily overdoped. For the highest doping levels, this nuclear relaxation is related to the sample’s spin shift via the Korringa relation for a simple Fermi liquid to which the nuclei couple isotropically. Therefore, it is argued that the spin shifts of the cuprates are typically suppressed, already in the overdoped region, and that there is no enhanced nuclear relaxation compared to that of a Fermi liquid. This means, no spin fluctuations have to be invoked to explain the difference between shift and relaxation. This suppression of the shift above Tc must be related to the pseudogap, which enters nuclear relaxation only through a temperature independent change in the anisotropy of the coupling of the nuclei to the electronic bath. It is argued that two antiferromagnetically coupled spin components that have different orbital origin can explain all the data, even the NMR orbital shift conundrum.