Crossover from coherent to incoherent scattering in spin-orbit dominated Sr2IrO4

Strong spin-orbit interaction in the two-dimensional compound Sr2IrO4 leads to the formation of J(eff) = 1/2 isospins with unprecedented dynamics. In Raman scattering a continuum attributed to two-magnon scattering is observed. With higher excitation energy of the incident laser this signal crosses over to an incoherent signal of large intensity. The characteristic energy scale of this crossover is identical to that of intensity resonance effects in phonon scattering that involves Ir-O modes. This crossover is related to excitonlike orbital excitations that are also evident in resonant x-ray scattering. The crossover and the evolution of incoherent excitations are related to their coupling to spin excitations. This signals a spin-orbit induced entanglement of spin, lattice, and charge degrees of freedom in Sr2IrO4.