Abstract
Recently, an analysis of all available planar oxygen shift and relaxation data for the cuprate high-temperature superconductors showed that the data can be understood with a simple spin susceptibility from a metallic density of states common to all cuprates. It carries a doping dependent but temperature independent pseudogap at the Fermi surface, which causes the deviations from normal metallic behavior, also in the specific heat. Here, a more coherent, unbiased assessment of all data, including planar Cu, is presented and consequences are discussed, since the planar Cu data were collected and analyzed prior to the O data. The main finding is that the planar Cu shifts for one direction of the external magnetic field largely follow from the same states and pseudogap. This explains the shift suppression stated more recently, which leads to the failure of the Korringa relation in contrast to an enhancement of the relaxation due to antiferromagnetic spin fluctuations originally proposed. However, there is still the need for a second spin component that appears to be associated with the Cu 3d(x2−y2) hole to explain the complex Cu shift anisotropy and family dependence. Furthermore, it is argued that the planar Cu relaxation which was reported recently to be rather ubiquitous for the cuprates, must be related to this universal density of states and the second spin component, while not being affected by the simple pseudogap. Thus, while this universal metallic density of states with a pseudogap is also found in the planar Cu data, there is still need for a more elaborate scenario that eludes planar O.
Highlights
Nuclear magnetic resonance (NMR) played an important role in high-temperature superconductivity [1], in particular in its early days with the focus on the La2− x Srx CuO4 and YBa2 Cu3 O7−δ families of materials
Two very recent publications [6,7] revealed that all planar oxygen NMR relaxation and shift data available from the literature for hole-doped cuprates are in agreement with a simple metallic spin susceptibility from a density of states that has a temperature independent pseudogap at the Fermi surface
Uncovered compelling arguments in favor of a simple metallic density of states with a temperature independent pseudogap set by doping
Summary
Nuclear magnetic resonance (NMR) played an important role in high-temperature superconductivity [1], in particular in its early days with the focus on the La2− x Srx CuO4 and YBa2 Cu3 O7−δ families of materials. Two very recent publications [6,7] revealed that all planar oxygen NMR relaxation and shift data available from the literature for hole-doped cuprates (more than 60 independent data sets in total) are in agreement with a simple metallic spin susceptibility from a density of states that has a temperature independent pseudogap at the Fermi surface. Matter 2022, 7, 21 material and doping, and (ii) a high-temperature, doping dependent offset in the spin shifts, since electronic polarization from the low energy states is still missing even at the highest T in the presence of the pseudogap. If one uses ζ, the doping measured with NMR from the charges at planar Cu and O, with ζ = nCu + 2nO − 1 [10], this is not surprising as optimally doped YBa2 Cu3 O7−δ is found to have ζ ≈ 20%, significantly larger than what is typically assumed This family dependence does not concern the density of states. Thereafter, we turn to the relaxation data and their explanation in the new scenario
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