Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

TL;DR

This study used high-resolution inelastic x-ray scattering to investigate phonon behavior in Sr2IrO4 and Sr3Ir2O7, finding no temperature-dependent anomalies across magnetic transitions, contrary to previous reports.

Contribution

It provides the first comprehensive momentum-resolved phonon analysis in these iridates, confirming the absence of phonon anomalies at magnetic transitions with ab-initio calculations.

Findings

Phonon energies match non-magnetic calculations.

No phonon renormalization across magnetic transition.

Anomalous softening of a phonon branch in Sr3Ir2O7.

Abstract

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering (IXS). The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab-initio lattice dynamical calculations based on non-magnetic density-functional-perturbation theory (DFPT). Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of Sr2IrO4. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in Sr3Ir2O7, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.