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(x 2 − y 2 ) 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.

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.

FIG. 1. Anvil cell high-pressure excerted on YBa2Cu3O6+y changes the charges in the CuO2 plane. (a) Schematic of the anvil cell used for NMR; the micro-coil surrounds the single crystal of about 1 nano-L volume, and both are placed in the high pressure chamber with a ruby chip as an optical pressure gauge. (b) Sketch of the crystal structure of YBa2Cu3O6+y with highlighted bonding orbitals in one of the CuO2 planes. The charge content of these bonding orbitals can be measured with Cu and O NMR quadrupole splittings. As indicated in (c) it reflects the corresponding hole contents for Cu (n Cu ) and O (n O ), from which the NMR doping ζ follows (1 + ζ = n Cu + 2n O ).

High-pressure anvil cell nuclear magnetic resonance (NMR) studies of single crystals are challenging, but they can offer much insight into material properties. A microcoil inside the high-pressure region that encloses the crystal offers a good signal-to-noise ratio, but special care has to be taken to warrant hydrostatic conditions or to avoid rupture of the crystal or coil. By introducing precise monitoring of the height and diameter of the pressurized sample chamber, this can be ensured, and the data reveal the behavior of the sample chamber under pressure. While its total volume is given by the compression of the enclosed pressure transmitting fluid, the aspect ratio of the cylindrical chamber changes considerably. 63Cu and 17O NMR of two differently doped single crystals of YBa2Cu3O7−δ at pressures of up to about 4.4 GPa show the function of the cell, and orientation dependent spectra prove the soundness of the arrangement.

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature ( T c ). But the family-specific maximum T c can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO 2 plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances T c above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum T c is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum T c on the Cu–O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum T c points to the same mechanism.