Spin-wave gap collapse in Rh-doped Sr2IrO4

Abstract

We use resonant inelastic x-ray scattering (RIXS) at the Ir L-3 edge to study the effect of hole doping upon the J(eff) = 1/2 aMott-insulating state in Sr2IrO4, via Rh replacement of the Jr site. The spin-wave gap, associated with XY-type spin-exchange anisotropy, collapses with increasing Rh content, prior to the suppression of the Mott-insulating state and in contrast to electron doping via La substitution of the Sr site. At the same time, despite heavy damping, the d-d excitation spectra retain their overall amplitude and dispersion character. A careful study of the spin-wave spectrum reveals that deviations from the J(1)-J(2)-J(3) Heisenberg used to model the pristine system disappear at intermediate doping levels. These findings are interpreted in terms of a modulation of Jr-Ir correlations due to the influence of Rh impurities upon nearby Jr wave functions, even as the single-band J(eff) = 1/2 amodel remains valid up to full carrier delocalization. They underline the importance of the transition metal site symmetry when doping pseudospin systems such as Sr2IrO4.