Fingerprints of Kitaev physics in the magnetic excitations of honeycomb iridates

In the quest for realizations of quantum spin liquids, the exploration of Kitaev materials (spin-orbit-entangled Mott insulators with strong bond-directional exchanges) has taken center stage. However, in these materials the local spin-orbital j=1/2 moments typically show long-range magnetic order at low temperature, thus defying the formation of a spin-liquid ground state. It is an intriguing question how far the properties of the Kitaev spin liquid survive in these materials despite the presence of long-range magnetic order. Using resonant inelastic x-ray scattering (RIXS), we here report on a proximate spin-liquid regime with distinctive fingerprints of Kitaev physics in the magnetic excitations of the honeycomb iridates alpha-Li2IrO3 and Na2IrO3. We observe a broad continuum of magnetic excitations and prove its magnetic character via the resonance behavior. The continuum is insensitive to temperature up to about 90 K and persists up to at least 300 K, more than an order of magnitude larger than the magnetic ordering temperatures. This striking temperature dependence of the RIXS intensity agrees with the behavior of the nearest-neighbor spin-spin correlations of the Kitaev model. This comparison is motivated by momentum-dependent RIXS measurements of the dynamical structure factor for energies within the continuum which show that dynamical spin-spin correlations are restricted to nearest neighbors, which is a key property of the Kitaev model. Notably, these spectroscopic observations are also present in the magnetically ordered state for excitation energies above the conventional magnon excitations. Phenomenologically, our data agree with inelastic neutron scattering results on the related honeycomb compound alpha-RuCl3, establishing a common ground for a proximate Kitaev spin-liquid regime in these materials.