Charge disproportionation and nano phase separation in $R$SrNiO$_4$

TL;DR

This study reports the growth of high-quality RSrNiO4 single crystals, revealing charge disproportionation and nano phase separation phenomena through neutron scattering and phonon analysis, challenging the Jahn-Teller effect explanation.

Contribution

The paper demonstrates the existence of charge disproportionation and nano phase separation in RSrNiO4, supported by neutron scattering and phonon dispersion analysis, providing new insights into the material's electronic structure.

Findings

Observation of quarter-integer magnetic peaks below 77 K.

Softening of phonon dispersion at checkerboard modulation vector.

Charge segregation into a checkerboard pattern rather than Jahn-Teller distortion.

Abstract

We have successfully grown centimeter-sized layered $R$SrNiO$_4$ single crystals under high oxygen pressures of 120 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below $\sim$77 K that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni-O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. Together with our spin wave simulations these observations reveal that this Ni$^{3+}$ system exhibits charge disproportionation with charges segregating into a checkerboard pattern within a nano phase separation scenario rather than showing a Jahn-Teller effect.