Interpretation of Nuclear Quadrupole Resonance Spectra in Doped La$_2$CuO$_4$

TL;DR

This study investigates the similarities in NQR spectra of doped La2CuO4 using first-principles calculations, revealing that similar spectral features arise from different underlying causes related to doping mechanisms.

Contribution

The paper demonstrates that similar NQR spectral features in doped La2CuO4 originate from different physical mechanisms, clarified through first-principles EFG calculations and a proposed hole mobility model.

Findings

NQR spectra similarities are coincidental, not due to identical origins.

EFG at copper sites increases by about 10% with doping, affecting spectral peaks.

A hole mobility model explains the doping-dependent frequency shifts.

Abstract

The nuclear quadrupole resonance (NQR) spectrum of strontium doped La$_2$CuO$_4$ surprisingly resembles the NQR spectrum of La$_2$CuO$_4$ doped with excess oxygen, both spectra being dominated by a main peak and one principal satellite peak at similar frequencies. Using first-principles cluster calculations this is investigated here by calculating the electric field gradient (EFG) at the central copper site of the cluster after replacing a lanthanum atom in the cluster with a strontium atom or adding an interstitial oxygen to the cluster. In each case the EFG was increased by approximately 10 % leading unexpectedly to the explanation that the NQR spectra are only accidentally similar and the origins are quite different. Additionally the widths of the peaks in the NQR spectra are explained by the different EFG of copper centres remote from the impurity. A model, based on holes moving rapidly across the planar oxygen atoms, is proposed to explain the observed increase in frequency of both the main and satellite peaks in the NQR spectrum as the doping concentration is increased.