Charge modulations in La2−xSrxNiO4+y: Ordering of polarons

Abstract

Electron diffraction of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{NiO}}_{4+\mathit{x}}$ (0≤x≤1) reveal at low temperatures the presence of sharp, quasi-2D superlattice peaks around the (π,π) point. We show that the modulation wave vector δ measured from the (π,π) point decreases with increasing hole concentration. However, a plateau with δ∼1/3 was observed for 0.075≤x≤0.2 and the superlattice peaks were found to collapse into a single spot at the (π,π) point with a truly 2D ordering for x=0.5 where anomalous change of physical properties has been previously reported. Our results can be understood in terms of the ordering of polarons.