Interpretation of nuclear quadrupole resonance spectra in doped La ₂ CuO ₄

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.Typeset using REVT E X 1

Cu nuclear magnetic resonance study of charge and spin stripe order inLa1.875Ba0.125CuO4

“…Since 214 cuprates are doped by ions close to their copper oxide planes, two distinct lines are always observed in copper NQR spectra. The high-frequency signal -called the B line -is from Cu nuclei close to the dopand ions (in our case Ba), while the low-frequency A line is from all other Cu nuclei 31,32 . Two sets of A and B lines are observed for two Cu isotopes, 63 Cu and 65 Cu.…”

Charge Distribution in La2-xSrxCuO4

“…9 For the case of copper oxides, such localised changes have been studied first by Kamimura and collaborators 10 and by Martin and Hay. 11 Changes in the EFG values of Cu adjacent to a Sr counterion in La 2 CuO 4 have already been reported 12,13 as have changes in the local structure due to holes. 14 These studies pertain to localised holes in the insulating phase.…”

63CuNQR Evidence for Spatial Variation of Hole Concentration inLa2−xSinger1, Hunt2, Imai3 2002Phys. Rev. Lett.165181742View full textAdd to dashboardCite

“…2a is that the B line [8] shows a very similar distribution in 63 1͞T 1 to the A line. The B line itself originates from structural effects [19]. More specifically, a B site corresponds to a Cu nucleus with a Sr 21 ion situated either directly above or below it, as supported in Fig.…”