Persistent paramagnons deep in the metallic phase of Sr$_{2-x}$La$_x$IrO$_4$

TL;DR

This study reveals that dispersive paramagnons persist deep into the metallic phase of electron-doped Sr$_{2-x}$La$_x$IrO$_4$, showing robust magnetic excitations despite loss of long-range order, with implications for understanding doped iridates.

Contribution

It demonstrates that short-range magnetic order and dispersive magnons persist in doped Sr$_{2-x}$La$_x$IrO$_4$, modeled effectively by a pseudospin-1/2 Heisenberg framework, even in the metallic regime.

Findings

Magnons persist into the metallic phase with nearly unchanged spectral weight.

Damped and anisotropically softened magnons are well described by a Heisenberg model.

A doping-independent high-energy magnetic continuum is observed.

Abstract

We have studied the magnetic excitations of electron-doped Sr$_{2-x}$La$_x$IrO$_4$ ($0 \leq x \leq 0.10$) using resonant inelastic x-ray scattering (RIXS) at the Ir L$_3$-edge. The long range magnetic order is rapidly lost with increasing $x$, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.