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

TL;DR

This study uses $^{139}$La NMR/NQR to investigate structural and magnetic properties of underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$, revealing a displacive structural transition and spin freezing phenomena.

Contribution

It provides detailed NMR/NQR analysis of local structural distortions and magnetic behavior in underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$, highlighting a first-order structural transition and spin freezing.

Findings

Identification of two NMR spectra due to EFG tilt caused by structural distortions.

Determination of the structural order parameter and displacive transition.

Observation of a sharp anomaly at 387 K indicating a structural transition and spin freezing below 70 K.

Abstract

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 $T_S=387(1)$ K, implying a first-order-like structural transition, and a dramatic change below ~70 K arising from collective glassy spin freezing.