${}^{139}$La NMR investigation in underdoped La${}_{1.93}$Sr${}_{0.07}$CuO${}_4$

We report ${}^{139}$La and ${}^{63}$Cu nuclear magnetic and quadrupole resonance (NMR/NQR) studies in an underdoped La${}_{1.93}$Sr${}_{0.07}$CuO${}_4$ single crystal, focusing on the ${}^{139}$La NMR in the normal state. We demonstrate that the local structural distortions in the low-temperature orthorhombic structure cause the tilting of the direction of the electric field gradient (EFG) at the nuclei from the $c$ axis, resulting in two NMR central transition spectra at both the ${}^{139}$La and ${}^{63}$Cu nuclei in an external field. Taking into account the tilt angle of the EFG, the temperature dependence of the ${}^{139}$La spectra allowed us to determine the ${}^{139}$La Knight shift and the structural order parameter. The angle and temperature dependence of the ${}^{139}$La spectrum is in perfect agreement with the macroscopic average structure and proves a displacive transition. The ${}^{139}$La nuclear spin-lattice relaxation rates, $T_1^{-1}$, suggest that La${}_{1.93}$Sr${}_{0.07}$CuO${}_4$ undergoes a gradual change to a temperature-independent paramagnetic regime in the high-temperature region. Both the spectra and $T_1^{-1}$ of the ${}^{139}$La as a function of temperature reveal a sharp anomaly around $T_S=387\left(1\right)$ K, implying a first-order-like structural transition, and a dramatic change below $\sim 70$ K arising from collective glassy spin freezing.

Figure 1

Angle dependence of ${}^{139}$La NMR spectra measured at room temperature and 10.7 T. Solid (dotted) lines are theoretical calculations assuming $8^{\circ }$ ($-8^{\circ }$) tilting of the principal axis of the EFG from the $c$ axis, using $\mathcal{K}=-0.09$% and ${\nu }_Q=6.3$ MHz which were determined experimentally from this study. The anisotropy parameter $\eta =0.19\left(2\right)$ was obtained from the calculations.

Figure 2

${}^{139}$La NQR spectrum measured at the $3{\nu }_Q$ transition at 40 K giving rise to ${\nu }_Q=6.3$ MHz. The NQR spectrum from Julien et al.[18] measured in the sample of $x=0.1$ is compared. The narrower linewidth of our NQR spectrum indicates less chemical inhomogeneity in the single crystal.

Figure 3

Structure of La${}_{2-x}$Sr${}_x$CuO${}_4$ in the LTO phase at 10 K for $x=0.075$, using the structure parameters reported in Ref. 19, viewed along [001] (a) and along [100] (b). The unit cell is depicted as the dotted line. The CuO${}_6$ octahedra is tilted along the [010]${}_{\mathrm{LTO}}$ (or [110]${}_{\mathrm{HTT}}$) direction by $4^{\circ }$. In addition, La(Sr)-La(Sr) along the $c$ axis is tilted by $1.5^{\circ }$ in opposite direction of the distortion of the CuO${}_6$ octahedra, being consistent with a much larger tilted angle of the EFG at the ${}^{139}$La. Note that the tilt direction of both the CuO${}_6$ octahedra and the La-La is maintained along [010]${}_{\mathrm{LTO}}$, whereas, along the diagonal direction, i.e., [110]${}_{\mathrm{LTO}}$, it is rotated alternately by 180${}^{\circ }$, causing the so-called buckling of the CuO${}_2$ plane.

Figure 4

Angle dependence of the ${}^{63}$Cu NMR spectra measured at 300 K and 8.2 T. The solid (dotted) lines are theoretical calculations assuming $2^{\circ }$ ($-2^{\circ }$) tilting of the direction of the EFG at the ${}^{63}$Cu from the $c$ axis, with parameters ${\nu }_Q=34$ MHz, $K=1.18$%, and $\eta =0$.

Figure 5

Temperature dependence of the ${}^{139}$La NMR spectra in an external field tilted by $\theta \sim 8^{\circ }$ from the crystallographic $c$ axis. In this case, p1 corresponds to the line in which the EFG at the ${}^{139}$La is aligned along $H$, giving rise to the Knight shift for $H\parallel c$, since there is no quadrupole shift. At 387 K, the two peaks collapse to a single peak, indicating the LTO-HTT structural transition. At low temperatures, ${}^{139}$La spectrum is significantly broadened, implying the slowing down of the spin fluctuations due to the spin-density wave (SDW) order. Each spectrum was scaled with temperature to remove the Boltzmann contribution to the intensity.

Figure 6

(a) Temperature dependence of resonance frequencies of two NMR peaks in Fig. 5. (b) Knight shift of the ${}^{139}$La is extracted from the p1 line. The separation of the two peaks $\Delta \nu \left(T\right)$ behaves as a structural order parameter. $\Delta \nu$ reveals first-order-like discontinuous transition at $T_S=387$ K, while it exhibits the second-order-like temperature dependence except the region of the transition. Solid line is a guide to eyes. Below 70 K, data deviate due to the onset of the magnetic broadening associated with the incommensurate SDW order.

Figure 7

${}^{139}$La $T_1^{-1}$ as a function of temperature. A very sharp anomaly at $T_S=387$ K was detected with the transition width less than 10 K. Below 70 K, the critical slowing down due to the SDW order causes extremely steep upturn of $T_1^{-1}$. Inset shows ${\left(T_{1}T\right)}^{-1}$ of the ${}^{139}$La versus temperature. Note that ${\left(T_{1}T\right)}^{-1}$ increases more than four decades as the temperature increases from $\sim 100$ K down to 4.2 K.