Evidence for rare-region physics in the structural and electronic degrees of freedom of the nickelate La$_{2-x}$Sr$_x$NiO$_4$

Abstract

We present a diffuse neutron and x-ray scattering study of structural, spin- and charge-density-wave fluctuations in the electrical insulator La$_{2-x}$Sr$_x$NiO$_4$. This lamellar nickelate is an isostructural analogue of the high-temperature cuprate superconductor La$_{2-x}$Sr$_x$CuO$_4$, for which recent experiments uncovered evidence for unusual structural and superconducting fluctuations indicative of rare-region physics due to inherent inhomogeneity unrelated to common point disorder effects. We find closely analogous nanoscale orthorhombic fluctuation behavior in La$_{2-x}$Sr$_x$NiO$_4$, including exponential scaling of the diffuse scattering intensity and power-law scaling of the characteristic length with relative temperature. Moreover, our neutron and x-ray scattering data reveal similar behavior for short-range magnetic and charge fluctuations above the respective ordering temperatures. These observations indicate that rare-region effects are a generic feature of perovskite-related structures and lead to universal fluctuations of both structural and electronic degrees of freedom over extended temperature ranges.