Abstract
Nuclear magnetic resonance (NMR) in cuprate research is a prominent bulk local probe of magnetic properties. NMR also, as was shown over the last years, actually provides a quantitative measure of local charges in the CuO 2 plane. This has led to fundamental insights, e.g., that the maximum T c is determined by the sharing of the parent planar hole between Cu and O. Using bonding orbital hole contents on planar Cu and O measured by NMR, instead of the total doping x, the thus defined two-dimensional cuprate phase diagram reveals significant differences between the various cuprate materials. Even more importantly, the reflected differences in material chemistry appear to set a number of electronic properties as we discuss here, for undoped, underdoped and optimally doped cuprates. These relations should advise attempts at a theoretical understanding of cuprate physics as well as inspire material chemists towards new high- T c materials. Probing planar charges, NMR is also sensitive to charge variations or ordering phenomena in the CuO 2 plane. Thereby, local charge order on planar O in optimally doped YBCO could recently be proven. Charge density variations seen by NMR in both planar bonding orbitals with amplitudes between 1% to 5% appear to be omnipresent in the doped CuO 2 plane, i.e., not limited to underdoped cuprates and low temperatures.
Highlights
Nuclear magnetic resonance (NMR) as a bulk, local quantum sensor provides unparalleled insights into material properties [1]
Charge density variations seen by NMR in both planar bonding orbitals with amplitudes between 1% to 5% appear to be omnipresent in the doped CuO2 plane, i.e., not limited to underdoped cuprates and low temperatures
The possibility to gain further insights was widely dismissed apart from few attempts, e.g., [8]. Our understanding of this electric hyperfine interaction has significantly improved so that we know, that NMR provides a quantitative measure of local charges at planar
Summary
Nuclear magnetic resonance (NMR) as a bulk, local quantum sensor provides unparalleled insights into material properties [1]. Despite the rather ubiquitous CuO2 plane, experiments showed, from the very beginning, that planar Cu and O can have vastly different splittings for different materials and that in most materials there are rather strong spatial inhomogeneities in terms of this local charge distribution. Even the effect of chemical doping (x) can be quantified with NMR and the simple relation 1 + x = nd +2np is obtained, again to no surprise, but in terms of np and nd This new insight into the chemistry of the CuO2 plane is addressed in this manuscript and consequences for relations to other cuprate properties will be discussed. YBa2Cu3O6.9, especially designed high pressure NMR experiments have recently identified the origin of this double peak pattern to be commensurate charge density variations on planar O that can order at elevated pressure [12] Consequences of these findings will be discussed, as well
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