Giant electron-phonon coupling of the breathing plane oxygen phonons in the dynamic stripe phase of La$_{1.67}$Sr$_{0.33}$NiO$_4$

TL;DR

This study reveals strong electron-phonon interactions in La$_{1.67}$Sr$_{0.33}$NiO$_4$, especially in breathing phonons, which are heavily overdamped in the dynamic stripe phase, indicating potential small polaron formation and similarities with manganites.

Contribution

The paper uncovers giant renormalization and overdamping of specific phonons in LSNO's stripe phase, highlighting unique electron-phonon coupling behavior in nickelates.

Findings

Giant renormalization of Ni-O bond-stretching phonons in stripe phase

Overdamping of breathing phonons suggests small polaron formation

Distinct electron-phonon coupling features compared to cuprates and manganites

Abstract

Doped antiferromagnets host a vast array of physical properties and learning how to control them is one of the biggest challenges of condensed matter physics. La$_{1.7}$Sr$_{0.3}$NiO$_4$ (LSNO) is a classic example of such a material. At low temperatures holes introduced via substitution of La by Sr segregate into lines to form boundaries between magnetically ordered domains in the form of stripes. The stripes become dynamic at high temperatures, but LSNO remains insulating presumably because an interplay between magnetic correlations and electron-phonon coupling localizes charge carriers. Magnetic degrees of freedom have been extensively investigated in this system, but phonons are almost completely unexplored. We searched for electron-phonon anomalies in LSNO by inelastic neutron scattering. Giant renormalization of plane Ni-O bond-stretching modes that modulate the volume around Ni appears on entering the dynamic charge stripe phase. Other phonons are a lot less sensitive to stripe melting. Dramatic overdamping of the breathing modes indicates that dynamic stripe phase may host small polarons. We argue that this feature sets electron-phonon coupling in nickelates apart from that in cuprates where breathing phonons are not overdamped and point out remarkable similarities with the colossal magnetoresistance (CMR) manganites.