Wave Vector Difference of Magnetic Bragg Reflections and Low Energy Magnetic Excitations in Charge-stripe Ordered La2NiO4.11

Abstract

When charge-stripe order was discovered in a non-superconducting La-based cuprate, it was compared to the charge-stripe order of insulating non-superconducting La2−xSrxNiO4+δ (LSNO)1. Since this discovery, the La-based cuprates have often been compared and contrasted to issostructural charge ordered materials in order to
gain insights for our understanding of high temperature cuprate superconductivity. The striking discovery of the near universal hourglass shaped magnetic excitation spectrum of the hole doped cuprates however brought into question the relevance of understanding charge-stripe ordered LSNO, whose magnetic excitation spectrum are the typical spin wave cone shape of an antiferromagnet2–4. That was until the recent discovery of a highly similar hourglass shaped magnetic excitation spectrum in the insulating non-superconducting charge ordered cobaltite material La2−xSrxCoO4+δ (LSCoO)5. As LSNO has a single charge-stripe ordered phase below half doping, it is a
simpler material to study than La2−xSrxCoO4+δ (LSCoO) which appears to phase separate into different charge
ordered phases6–9.
Recently, a striking observation was reported on the magnetism in La2CuO4+δ (LCO+O). On going towards zero energy transfer the magnetic excitations were observed to have a different wave vector centring compared to the magnetic Bragg reflections10, an offset. The magnetic excitations are not acting as Goldstone modes of a
symmetry breaking magnetic order, which would have the same wave vector centring at lowest energy, that they were thought to be. As La-based cuprates are known not to fully magnetically order11,12, the offset of the magnetic excitations in LCO+O was attributed to the lowest energy magnetic excitations occurring in unordered magnetic volume of the material, instead of the magnetically ordered volume of the material10. In this report we will show there is a similar offset in wave vector between the lowest energy magnetic excitations and the magnetic Bragg reflections in tetragonal charge-stripe ordered La2NiO4.11, and that these magnetic excitations have to occur in the magnetically ordered phase of the material13,14.