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Register a new userPhase diagram of hole-doped cuprates based on $^{17}$O and $^{63}$Cu NMR quadrupole splittings
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The phase diagram of the superconducting cuprates is often used to show how their electronic properties change as a function of the mean doping level, i.e., the average hole content of the CuO$_2$ plane. In Nuclear Magnetic Resonance (NMR) experiments average doping, as well as the distribution of these holes between planar Cu and O reveals itself through the quadrupole splittings of the $^{63,65}$Cu and $^{17}$O NMR. Here we argue based on all published NMR data available to us in favor a new type of phase diagram that has the planar oxygen quadrupole splitting and with it the planar oxygen hole content as abscissa rather than the average hole content of the CuO$_2$ plane. In such a plot the superconducting domes of the different cuprate families are shifted horizontally according to their maximum critical temperature $T_{rm c,max}$ set by the chemistry of the parent material, which determines its oxygen hole content. The higher the O hole content the higher $T_{rm c,max}$ that can be achieved by actual doping. These findings also offer a strategy for finding cuprates with higher $T_{rm c,max}$.
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We have grown and characterized 30 nm thick YBa$_2$Cu$_3$O$_{7-delta}$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO$_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{mathrm{c}}$ and the pseudogap temperature $T^*$) similar to those of the as-grown films and carry critical current densities $J_{mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.
We report $^{63}$Cu- and $^{205}$Tl-NMR studies on six-layered ($n$=6) high-$T_c$ superconducting (SC) cuprate TlBa$_2$Ca$_5$Cu$_6$O$_{14+delta}$ (Tl1256) with $T_csim$100 K, which reveal that antiferromagnetic (AFM) order takes place below $T_{rm N}sim$170 K. In this compound, four underdoped inner CuO$_2$ planes ($n$(IP)=4) sandwiched by two outer planes (OPs) are responsible for the onset of AFM order, whereas the nearly optimally-doped OPs responsible for the onset of bulk SC. It is pointed out that an increase in the out-of-plane magnetic interaction within an intra-unit-cell causes $T_{rm N}sim$ 45 K for Tl1245 with $n$(IP)=3 to increase to $sim$170 K for Tl1256 with $n$(IP)=4. It is remarkable that the marked increase in $T_{rm N}$ and the AFM moments for the IPs does not bring about any reduction in $T_c$, since $T_csim 100$ K is maintained for both compounds with nearly optimally doped OP. We highlight the fact that the SC order for $nge5$ is mostly dominated by the long-range in-plane SC correlation even in the multilayered structure, which is insensitive to the magnitude of $T_{rm N}$ and the AFM moments at the IPs or the AFM interaction among the IPs. These results demonstrate a novel interplay between the SC and AFM orders when the charge imbalance between the IPs and OP is significantly large.
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 $T_{rm c}$.
Two maxima in transverse relaxation rate of Cu(2) nuclei in YBa$_2$Cu$_3$O$_{7-y}$ are observed, at T = 35 K and T = 47 K. Comparison of the $^{63}$Cu(2) and $^{65}$Cu(2) rates at T = 47 K indicates the magnetic character of relaxation. The enhancement at T = 47 K of fluctuating local magnetic fields perpendicular to the CuO$_2$ planes is connected with the critical fluctuations of orbital currents. Maximum at T = 35 K is connected with the appearance of inhomogeneous supeconducting phase. Together with data published to date, our experimental results allow to suggest a qualitatively new phase diagram of the superconducting phase.
We present $^{75}$As Nuclear Magnetic and Quadrupole Resonance results (NMR, NQR) on a new set of LaFeAsO$_{1-x}$F$_x$ polycrystalline samples. Improved synthesis conditions led to more homogenized samples with better control of the fluorine content. The structural$equiv$nematic, magnetic, and superconducting transition temperatures have been determined by NMR spin-lattice relaxation rate and AC susceptibility measurements. The so-determined phase diagram deviates from the published one especially for low F-doping concentrations. However, if the doping level is determined from the NQR spectra, both phase diagrams can be reconciled. The absence of bulk coexistence of magnetism and superconductivity and a nanoscale separation into low-doping-like and high-doping-like regions have been confirmed. Additional frequency dependent intensity, spin-spin, and spin-lattice relaxation rate measurements on underdoped samples at the boundary of magnetism and superconductivity indicate that orthorhombicity and magnetism originate from the low-doping-like regions, and superconductivity develops at first in the high-doping-like regions.
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