High-pressure charge distribution in YBa2Cu3O6+y by NMR

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We propose to quantitatively determine how the application of external pressure changes the local charge distribution in the CuO2 plane in the cuprate high-temperature superconductor YBa2Cu3O6+y. Pressure, chemically induced as well as externally applied, has a very prominent role in cuprate physics, since it not only induces the highest critical temperatures of superconductivity (Tc) in these materials, but even led to the discovery of new materials. However, it is not clear how pressure affects the changes in the properties of the cuprates. NMR is a very powerful probe in cuprate research, but its use with the necessary high pressures became available only very recently. In a first experiment in 2011, we showed the new capability with 17O NMR of YBa2Cu4O8 where we observed the vanishing of the pseudo gap features at about 7 GPa. Meanwhile, we have become world-leading in high-pressure NMR, having developed our own high-pressure NMR cells and also demonstrated their use with single crystal materials. Moreover, in 2014 we showed that the NMR quadrupole splittings at Cu and O provide a quantitative measure of local charges in the CuO2 plane in all cuprates, and that this distribution sets the maximum achievable Tc. This relates the charge distribution, which is a material chemistry parameter, to the superfluid density, a property measured at very low temperatures in superconducting samples, reproducing Uemura's famous relation. Consequently, we are now in a position to find out what happens to the charge and its distribution in the cuprates at pressures that change Tc significantly. With the proposed research it will be revealed whether indeed a change in doping or an intraplanar charge redistribution between Cu and O, or suppression of charge order that is believed to compete with superconductivity is responsible for the change in Tc. The results will not only clarify the role of pressure in cuprate physics, but also help in understanding what sets the maximum Tc. Furthermore, we will be able to quantitatively investigate a possible formation of static charge density waves in these materials, and their response to external pressure.