The electric hyperfine interaction observable in atomic spectroscopy for O and Cu ions in various configurations is used to analyze the quadrupole splitting of O and Cu nuclear magnetic resonance (NMR) in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ and to determine the hole densities at both sites as a function of doping. It is found that in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ all doped holes (x) reside in the Cu-O plane, but almost exclusively at O. For ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ and $yl0.6$ doped holes are found at planar Cu as well as O. For $yg0.6$ further doping increases the hole content only for planar O. The phase diagram based on NMR data is presented. Further implications from the Cu A and B sites in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and the two planar O sites in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ and consequences for the phase diagram are discussed.

http://arxiv.org/pdf/cond-mat/0307169.pdf

Planar Cu and O hole densities in high-Tc cuprates determined with NMR

The nature of the much debated valence state of an interstitial oxygen atom in oxygen-doped La2CuO4 is the subject of this paper. In model cluster calculations, we studied the position, charge, and spin state of the interstitial oxygen atoms in this superconductor. The models considered allow the interstitial oxygen to move off a symmetrical position, to have varying spin and charge, and to be surrounded by various magnetic environments. UB3LYP calculations show that a model having an interstitial oxygen atom with a total spin of 1 is lowest in energy; the interstitial oxygen atoms here act as stable radicals with a net charge of −1. These results agree with experimental evidence for the paramagnetic behavior for interstitial atoms. The energy associated with a spin flip at a Cu site in our models is lower if interstitial oxygen has a local electron spin density, compared to the case when it does not. We provide a possible explanation for the increase of the doping concentrations of interstitial oxygen wi...

Oxygen interstitials in superconducting La2CuO4: their valence state and role.

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved (“wiped-out”) signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates.

/pdf/cu-nuclear-magnetic-resonance-study-of-charge-and-spin-3y0mfsu7z7.pdf

Cu nuclear magnetic resonance study of charge and spin stripe order in La1.875Ba0.125CuO4

Since the discovery of high-temperature superconductivity in cuprates by Bednorz and Muller, NMR has contributed notably to our view and understanding of cuprate chemistry and physics, of which a brief account is given here. However, recent major insights from NMR that, at least in part, challenge certain views developed in the early years of cuprate research will be our focus: (i) The failure of a single-component spin susceptibility to adequately describe NMR shift data may indicate the presence of more than one electronic fluid. (ii) NMR quadrupole splittings provide a quantitative measure of local charges in the CuO2 plane, and we find unexpectedly large differences between different cuprate families. These local charges, measured with NMR, appear to set the maximum T c , and the superfluid density. A phase diagram using these local Cu and O charges rather than the total doping per CuO2, gives a unified view of all cuprates and further insight can be expected if cuprate properties are discussed in this context. (iii) Based on magnetic field dependent and new pressure dependent NMR experiments, a review of NMR literature data, and the measurability of local charges with NMR, we argue that dynamic charge density variations are likely a universal phenomenon in cuprate superconductors.

NMR of Cuprate Superconductors: Recent Developments

NMR and electronic inhomogeneity in cuprate superconductors

The nuclear quadrupole resonance (NQR) spectrum of strontium-doped La2CuO4 surprisingly resembles the NQR spectrum of La2CuO4 doped with excess oxygen, both spectra being dominated by a main (A) peak and one principal satellite (B) peak at similar frequencies. Using first-principles cluster calculations, this is investigated here by calculating the electric field gradient (EFG) at the central copper site of the cluster after replacing a lanthanum atom in the cluster with a strontium atom or adding an interstitial oxygen to the cluster. In each case the EFG was increased by approximately 10% leading unexpectedly to the explanation that the NQR spectra are only accidentally similar and the origins are quite different.

https://iopscience.iop.org/article/10.1209/epl/i2000-00550-y/pdf

P. F. Meier

Papers

Interpretation of nuclear quadrupole resonance spectra in doped La2CuO4