Structural transitions in a doped lanthanum cuprate

${}^{139}$La NMR and relaxation measurements have been performed on La${}_{1.8-x}$Eu${}_{0.2}$Sr${}_x$CuO${}_4$ ($x=0.13$ and 0.2) single crystals. The temperature dependence of the ${}^{139}$La NMR spectra in all the structural phases [high-temperature tetragonal (HTT) $\to$ low-temperature orthorhombic (LTO) $\to$ low-temperature tetragonal (LTT)] reveals the nonvanishing tilt angle of the CuO${}_6$ octahedra in the HTT phase, opposed to the case of La${}_{2-x}$Sr${}_x$CuO${}_4$ where the tilt angle disappears immediately above the transition. Since ${}^{139}$La relaxation data provide evidence of the thermodynamic critical fluctuations associated with the structural phase transitions, HTT $\to$ LTO and LTO $\to$ LTT, we conclude that the structural transitions in Eu-doped La${}_{2-x}$Sr${}_x$CuO${}_4$ should be of the order-disorder type rather than of the displacive type observed in La${}_{2-x}$Sr${}_x$CuO${}_4$. The change of the nature of the structural transitions caused by doping with Eu appears to be consistent with the LTO $\to$ LTT transition that is absent in La${}_{2-x}$Sr${}_x$CuO${}_4$.

Figure 1

${}^{139}$La NMR spectra in a La${}_{1.67}$Eu${}_{0.2}$Sr${}_{0.13}$CuO${}_4$ single crystal measured at ${\nu }_0=29$ MHz and 4 K (LTT phase) for two different orientations of the applied field H with respect to the crystal $c$ axis. Vertical dotted line indicates the zero Knight shift position. The lines under the spectra are the marks of the positions of the satellites calculated for $\alpha ={11}^{\circ }$ (thick line) and ${17}^{\circ }$ (thin line) by assuming ${\nu }_Q=6$ MHz and $\eta =0$. Spectrum measured in the LTO phase (160 K) for $H\parallel c$ (dotted line) shows a similar asymmetrical pattern with that at 4 K, indicating that the distribution of the local tilt angle $\alpha$ is only slightly reduced above the LTO $\to$ LTT transition.

Figure 2

Temperature dependence of ${}^{139}$La NMR spectrum at the central transition for La${}_{1.6}$Eu${}_{0.2}$Sr${}_{0.2}$CuO${}_4$. The external field $H$ has been transformed to $\mu \equiv \cos \alpha$ using the Eq. (1). The arrows are marks for the average value $\langle \mu \rangle$, which was obtained from the center of gravity for each spectrum. Note that $\langle \mu \rangle$ is still finite at 245 K $>T_{\mathrm{HT}}$.

Figure 3

Temperature dependence of the average angle $\langle \alpha \rangle$. $\langle \alpha \rangle$ does not show an anomaly at both $T_{\mathrm{LT}}$ and $T_{\mathrm{HT}}$ (indicated by arrows), persisting even at temperatures that are much higher than $T_{\mathrm{HT}}$ for $\text{Sr}=0.2$. Inset shows that the shift of spectra is mostly of quadrupolar origin with the small Knight shift ($C/{\nu }_0^2=0$) [see Eq. (2)].

Figure 4

${}^{139}$La spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature measured at the central transition for $H\parallel c$ in La${}_{1.8-x}$Eu${}_{0.2}$Sr${}_x$CuO${}_4$ single crystals. Data for $\text{Sr}=0.13$ are identical with those reported in Ref. 18. While the transition at $T_{\mathrm{LT}}$ causes the sharp drop of ${}^{139}{T}_1^{-1}$ indicating first order transition, the $T_1^{-1}$ anomaly at $T_{\mathrm{HT}}$ is clearly continuous, being consistent with second order transition.