Evidence for intrinsic impurities in the high-temperature superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ from ${}^{17}\mathrm{O}$ nuclear magnetic resonance

We have found that high quality crystals of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212) have intrinsic magnetic defects that depend on oxygen doping. Our ${}^{17}\mathrm{O}$ nuclear magnetic resonance spectra provide evidence that local moments form in the $\mathrm{Cu}{\mathrm{O}}_2$ plane in both normal and superconducting states. We suggest that these magnetic impurities are related to the electronic disorder that scanning tunneling microscopy experiments identify with the oxygen dopant atoms.

Figure 1

(Color online) The central transition of ${}^{17}\mathrm{O}$ NMR spectra of two overdoped Bi-2212 single crystals. The left plot is for $T_C=75\mathrm{K}$ and the right is for $T_C=85\mathrm{K}$. The narrow line is for oxygen in the SrO plane O(2) and the broader line is for oxygen in the $\mathrm{Cu}{\mathrm{O}}_2$ plane. The magnetic field $H=8\mathrm{T}$.

Figure 2

(Color online) Temperature dependence of the Knight shift defined to be the first moment of the ${}^{17}\mathrm{O}\left(1\right)$ central transition of each of three crystals.

Figure 3

(Color online) The linewidth of the ${}^{17}\mathrm{O}\left(1\right)$ central transition calculated as the square root of the second moment as a function of temperature. The solid curves are from Eq. (1). The sharp upturn in data near $30\mathrm{K}$ marks the vortex freezing transition (Ref. 16).

Figure 4

(Color online) The Knight shift and inverse Curie constant as a function of hole concentration. The hole concentration in the $\mathrm{Cu}{\mathrm{O}}_2$ plane is related to $T_C$ by Ref. 20 ${\mathrm{T}}_C∕{\mathrm{T}}_{C\mathit{max}}=1-82.6$ ${(p-0.16)}^2$. The nonstoichiometric oxygen content $\delta$ was determined independently from annealing conditions (Ref. 27) giving the values of $\delta$ close to the hole concentration.

Figure 5

(Color online) The Knight shift (first moment of the spectrum) divided by the linewidth (defined as the square root of the second moment), $K∕(\Delta \nu -\Delta {\nu }_0)$, is plotted against temperature. A straight line through the origin indicates a Curie law with slope of the inverse Curie constant.