Evidence for Fluctuating Charge Stripes far above the Charge-Ordering Transition in La$_{1.67}$ Sr$_{0.33}$ NiO$_{4}$

TL;DR

This study reveals that fluctuating charge stripes in La$_{1.67}$Sr$_{0.33}$NiO$_{4}$ persist well above the charge-ordering transition, evidenced by structural and optical measurements indicating dynamic correlations up to twice the transition temperature.

Contribution

It provides evidence for fluctuating charge stripes existing far above the charge-ordering transition temperature using neutron diffraction and optical conductivity data.

Findings

Anomalous shrinking of the c/a lattice parameter ratio correlates with charge-ordering temperature.

Persistent non-Debye-like atomic displacements correlate with pseudogap features.

Dynamic charge-stripe correlations survive up to approximately twice the charge-ordering temperature.

Abstract

The temperature evolution of structural effects associated with charge (CO) and spin order (SO) in La$_{1.67}$Sr$_{0.33}$NiO$_{4}$ has been investigated using neutron powder diffraction. We report an anomalous shrinking of the $c/a$ lattice parameter ratio that correlates with $T_{\rm CO}$. The sign of this change can be explained by the change in interlayer Coulomb energy between the static stripe-ordered state and the fluctuating stripe-ordered state or the charge-disordered state. In addition, we identify a contribution to the mean-square displacements of Ni and in-plane O atoms whose width correlates quite well with the size of the pseudogap extracted from the reported optical conductivity, with a non-Debye-like component that persists below and well above $T_{\rm CO}$. We infer that dynamic charge-stripe correlations survive to $T\sim2T_{\rm CO}$