Emergence of a new interpretation of NMR of cuprates superconductors
As a fundamental, local, bulk probe nuclear magnetic resonance (NMR) is expected to be at the center of the discussion of the properties of cuprate superconductors. Indeed, in addition to profound insight into chemical structure and bonding, NMR provided vital clues about the electronic spin susceptibility, based on the magnetic hyperfine interaction: the pseudogap (spin gap), single-fluid physics, and spin-singlet pairing were discovered by NMR. However, the data did not appear to contain necessary details for theory. The less often discussed electric hyperfine interaction served a similar chemical purpose, but its understanding in terms of planar charge remained difficult. Here, early on, NMR found that the stoichiometric compounds (Y-1237, Y-1248) appeared to be very homogeneous, as very narrow NMR lines for Cu and O in the CuO$_2$ plane seemed to prove very small spatial variations of charge. However, all the other materials exhibit mostly broad featureless NMR resonances, indicative rather strong electric field variations in the CuO$_2$ plane. This conundrum was often considered as proof that charge ordering - apparently not ubiquitous - must be due to chemical inhomogeneity from doping and other crystal imperfections.
Recently, with a number of experiments on different materials we established that a single spin component is not able to explain the NMR shifts, pointing to a different magnetic hyperfine scenario. Very recently, we compiled all literature NMR shift data for planar Cu and with simple plots it becomes already obvious that the hitherto adopted NMR interpretation is wrong, e.g., the magnetic hyperfine scenario is inappropriate. Also recently, we showed that the charges in the plane can be quantified with NMR, which led to the discovery that the sharing of holes between Cu and O (not the doping) is responsible for various cuprate properties, e.g., the maximum T$_c$. In another set of challenging experiments we just completed a fundamental proof that shows that the above mentioned ‘homogeneous’ materials with sharp NMR lines are in fact highly charge ordered systems, with the order responding to pressure, temperature, and magnetic field. This charge ordering which is ubiquitous to the Y-based systems is likely to be ubiquitous to the CuO$_2$ plane of all cuprates as it would solve the above mentioned conundrum.
Thus, we view our findings - that will be discussed in more detail - as the emergence of a new interpretation of cuprate NMR, which must have fundamental impact on the understanding of these materials.
Cm-Bio 11-2 Haase flyer