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The distribution of electrons and holes in the CuO$_2$ plane of the high-temperature superconducting cuprates is determined with nuclear magnetic resonance through the quadrupole splittings of $^{17}$O and $^{63}$Cu. Based on new data for single crystals of electron-doped Pr$_{2-x}$Ce$_x$CuO$_4$(x=0, 0.05, 0.10, 0.15) as well as Nd$_{2-x}$Ce$_x$CuO$_4$ (x=0, 0.13) the changes in hole contents $n_d$ of Cu 3d$(x^2-y^2)$ and $n_p$ of O 2$p_\sigma$ orbitals are determined and they account for the stoichiometrically doped charges, similar to hole-doped \lsco. It emerges that while $n_d+2n_p=1$ in all parent materials as expected, $n_d$ and $n_p$ vary substantially between different groups of materials. Doping holes increases predominantly $n_p$, but also $n_d$. To the contrary, doping electrons predominantly decreases $n_d$ and only slightly $n_p$. However, $n_p$ for the electron doped systems is higher than that in hole doped La$_{1.85}$Sr$_{0.15}$CuO$_4$. Cuprates with the highest maximum $T_{\rm c}$s appear to have a comparably low $n_d$ while, at the same time, $n_p$ is very high. The rather high oxygen hole content of the Pr$_2$CuO$_4$ and Nd$_2$CuO$_4$ with the low $n_d$ seems to make them ideal candidates for hole doping to obtain the highest $T_{\rm c}$., Comment: 5 pages, 5 figures

We report the results from a $^{75}$As NMR study on the overdoped iron pnictide superconductor, CeFeAsO$_{0.8}$F$_{0.2}$. Two peaks are observed in the NMR spectra for temperatures as high as 100 K, which is above the superconducting transition temperature of 39 K and hence they cannot be attributed to the effect of vortices in the superconducting state as previously suggested [A. Ghoshray, et al., Phys. Rev. B 79, 144512 (2009)]. The temperature dependence of the $^{75}$As NMR shifts for the two peaks is consistent with hyperfine coupling from magnetic Ce to As and with different Curie-Weiss temperatures. The appearance of two Curie-Weiss temperatures where one is near zero and the other is $\sim$-18 K suggests that the two peaks arise from ordered and disordered regions where the disorder could be due to regions with significant fluorine site disorder and/or clusters of oxygen vacancies. Similar to some other studies on ReFeAsO$_{1-x}$F$_x$ (Re is a rare earth) we find that there are more than one $^{75}$As spin-lattice and spin-spin relaxation rates that can be attributed to ordered and disordered regions in the ReO layer and suggests that inhomogeneity is a common feature in the ReFeAsO$_{1-x}$F$_x$ superconductors.

A new NMR anvil cell design is used for measuring the influence of high pressure on the electronic properties of the high-temperature superconductor YBa$_2$Cu$_4$O$_8$ above the superconducting transition temperature $T_{\rm c}$. It is found that pressure increases the spin shift at all temperatures in such a way that the pseudo-gap feature has almost disappeared at 63 kbar. This change of the temperature dependent spin susceptibility can be explained by a pressure induced proportional decrease (factor of two) of a temperature dependent component, and an increase (factor of 9) of a temperature independent component, contrary to the effects of increasing doping. The results demonstrate that one can use anvil cell NMR to investigate the tuning of the electronic properties of correlated electronic materials with pressure., 4 pages, 4 figures, accepted for publication in Phys. Rev. B

We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd^{3+} electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the $B\parallel c$ orientation with respect to the $B\perp c$ orientation. The enhancement in an applied field of 15.4 T ($\approx 0.15 H_{c2}$) at 16 K (0.2 $T_c$) is approximately 10 percent of the susceptibility measured at $T_c$. Such a small magnitude suggests that the second critical field of superconductivity, $H_{c2}\approx 100 T$, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high $T_c$ superconductors., 5 pages incl. 5 EPS figures, REVTeX, accepted for publication in Phys. Rev. Lett