Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr2 Ir1-x Rux O4

We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr2Ir1-xRuxO4. The maximum energy of the magnetic excitation remains robust up to x=0.77, with a gap opening at low dopings and increasing to over 150 meV at x=0.77. At these higher Ru concentrations, the dispersive magnetic excitations in Sr2IrO4 are rendered essentially momentum independent. Up to a Ru concentration of x=0.77, both experiments and first-principles calculations show the Ir Jeff=1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.