139La NMR investigation in underdoped La1.93Sr0.07CuO4

We report La-139 and Cu-63 nuclear magnetic and quadrupole resonance (NMR/NQR) studies in an underdoped La1.93Sr0.07CuO4 single crystal, focusing on the La-139 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 La-139 and Cu-63 nuclei in an external field. Taking into account the tilt angle of the EFG, the temperature dependence of the La-139 spectra allowed us to determine the La-139 Knight shift and the structural order parameter. The angle and temperature dependence of the La-139 spectrum is in perfect agreement with themacroscopic average structure and proves a displacive transition. The La-139 nuclear spin-lattice relaxation rates, T-1(-1), suggest that La1.93Sr0.07CuO4 undergoes a gradual change to a temperature-independent paramagnetic regime in the high-temperature region. Both the spectra and T-1(-1) of the La-139 as a function of temperature reveal a sharp anomaly around T-S = 387(1) K, implying a first-order-like structural transition, and a dramatic change below similar to 70 K arising from collective glassy spin freezing.