${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ NMR investigation of charge order in ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ ($T_c=42$ K)

We report ${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ NMR investigation of the successive charge order, spin order, and superconducting transitions in superoxygenated ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ single crystal with stage-4 excess oxygen order at $T_{\text{stage}}\simeq 290$ K. We show that the stage-4 order induces tilting of ${\mathrm{CuO}}_6$ octahedra below $T_{\text{stage}}$, which in turn causes ${}^{139}\mathrm{La}$ NMR line broadening. The structural distortion continues to develop far below $T_{\text{stage}}$, and completes at $T_{\text{charge}}\simeq 60$ K, where charge order sets in. This sequence is reminiscent of the the charge-order transition in Nd codoped ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ that sets in once the low-temperature tetragonal phase is established. We also show that the paramagnetic ${}^{63}\mathrm{Cu}$ NMR signals are progressively wiped out below $T_{\text{charge}}$ due to enhanced low-frequency spin fluctuations in charge-ordered domains, but the residual ${}^{63}\mathrm{Cu}$ NMR signals continue to exhibit the characteristics expected for optimally doped superconducting ${\mathrm{CuO}}_2$ planes. This indicates that charge order in ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ does not take place uniformly in space. In addition, unlike the typical second-order magnetic phase transitions, low-frequency Cu spin fluctuations as probed by ${}^{139}\mathrm{La}$ nuclear spin-lattice relaxation rate do not exhibit critical divergence at $T_{\text{spin}}$($\simeq T_c$) $=42$ K. These findings, including the spatially inhomogeneous nature of the charge-ordered state, are qualitatively similar to the case of ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ [Imai et al., Phys. Rev. B 96, 224508 (2017) and Arsenault et al., Phys. Rev. B 97, 064511 (2018)], but both charge and spin order take place more sharply in the present case.

Figure 1

Representative ${}^{139}\mathrm{La}$ NMR line shapes in $B_{\text{ext}}=8.7521$ T applied along the $c$ axis. For clarity, the vertical origin is shifted at different temperatures. The gray vertical dashed line marks the peak frequency ${}^{139}f_o=52.65$ MHz. Asymmetrical line broadening becomes significant below $\sim 220$ K. Inset: The very sharp, needle-shaped peak at ${}^{139}f_o=52.65$ MHz, which arises from ${}^{139}\mathrm{La}$ sites neighboring to untilted ${\mathrm{CuO}}_6$ octahedra, remains observable down to $\sim 60$ K.

Figure 2

Temperature dependence of ${}^{63}(1/T_{1}T)_c$, the ${}^{63}\mathrm{Cu}$ nuclear spin-lattice relaxation rate $1/T_1$ divided by temperature $T$, measured in $B_{\text{ext}}=9$ T applied along the $c$ axis. We also show ${}^{63}(1/T_{1}T)_a$ and ${}^{63}(1/T_{2}T)_a$ measured with $B_{\text{ext}}\left|\right|a$ axis, by normalizing their magnitude by multiplying a factor 1/3 and 1/15, respectively. The red solid curve represents the best Curie-Weiss fit with ${\theta }_{\text{CW}}=60$ K between $T_c=T_{\text{spin}}=42$ K and $T_{\text{mobility}}\simeq 200$ K, which slightly underestimates the experimental results above $T_{\text{mobility}}$. Also shown using the right axis is ${}^{139}(1/T_{1}T)$ measured at the ${}^{139}\mathrm{La}$ sites in $B_{\text{ext}}=8.75$ T applied along the $c$ axis.

Figure 3

Representative ${}^{63}\mathrm{Cu}$ NMR line shapes measured in $B_{\text{ext}}=9$ T applied along the crystal (a) $c$ axis and (b) $a$ axis at a constant delay time $\tau =15\mu \mathrm{s}$. Also shown in (c) and (d) are the spin-echo decay curves in $B_{\text{ext}}\left|\right|c$ and $B_{\text{ext}}\left|\right|a$, respectively. The magnitude of the spin echo is normalized at $\tau =15\mu \mathrm{s}$, using the integral of the line shapes in (a) and (b). Solid lines are the best Gaussian-Lorentzian fit in (c) and Lorentzian fit in (d). The extrapolation of the fit to $\tau =0$ therefore yields the integrated intensity of the line shape in the limit of $\tau =0$. For example, (c) shows that the intensity at $2\tau =30\mu \mathrm{s}$ observed at 60 K appears much smaller than at 100 K and 150 K; but the intensity in the limit of $2\tau =0$ is conserved above $T_{\text{charge}}$.

Figure 4

Temperature dependence of the integrated intensity of ${}^{63}\mathrm{Cu}$ NMR line shapes, ${}^{63}I$, corrected for the Boltzmann factor and the transverse relaxation effects based on the results in Figs. 3 and 3. For $B_{\text{ext}}\left|\right|a$, the large superconducting critical field makes the spin-echo signal nearly unobservable due to the Meissner effects below $T_c\simeq T_{\mathrm{spin}}\simeq 42K$.