Nuclear-Magnetic-Resonance Evidence for Charge Inhomogeneity in Stripe Ordered ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$

We report ${}^{17}\mathrm{O}$ nuclear-magnetic-resonance (NMR) results in the stripe ordered ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ system. Below a temperature $T_q\sim 80\mathrm{K}$, the local electric field gradient and the absolute intensity of the NMR signal of the planar O site exhibit a dramatic decrease. We interpret these results as microscopic evidence for a spatially inhomogeneous charge distribution, where the NMR signal from O sites in the domain walls of the spin density modulation are wiped out due to large hyperfine fields, and the remaining signal arises from the intervening Mott insulating regions.

Figure 1

NMR field-swept spectra of the planar oxygen in ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ for $x=0.13$ (left) and $x=0.2$ (right) at 43 MHz. The solid lines are fits as described in the text. All spectra are normalized to equal heights for comparison.

Figure 2

(a) The quadrupolar splitting ${\nu }_c$, for several doping levels (see legend) versus temperature. Solid lines are guides to the eye. (b) The doping dependence of ${\nu }_c$ in ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (squares) and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (circles) versus $x$ at 120 K. The solid line is given by $a+bx$, with $a=0.142\mathrm{MHz}$, and $b=0.538\mathrm{MHz}$ [34]. (c) The effective change in hole doping $\delta n_p(x,T)$, as discussed in the text. The colors and symbols are identical to those in panel (a), and the solid lines are guides to the eye.

Figure 3

The number of visible O nuclei versus temperature, $N_0\sim (I\times T)e^{2\tau /T_2}$, where $I$ is the integrated spectral intensity, $T$ is the temperature, $\tau$ is the pulse spacing in the NMR echo sequence, and $T_2$ is the spin echo decay constant, for $x=0.13$ (●) and $x=0.20$ (△). The data have been normalized to unity for high temperatures. Inset: The number of visible La nuclei versus temperature for the $x=0.13$ sample.

Figure 4

(a) Temperature dependence of the planar hole doping distribution. As temperature decreases, the width of the distribution increases monotonically, but the mean remains temperature independent. For $T<T_q$, a fraction of the sites at the upper end of this distribution are wiped out (shaded regions), so the observed doping decreases. (b) Schematic diagram of a stripe: the black circles and arrows represent the Cu sites and spins, and the red circles represent the oxygens. The diameter of the circle is proportional to the local hole density. The blue arrows at the oxygen sites are the local hyperfine field, and the green line represents $\mathbf{S}(\mathbf{r})$.