High-resolution RIXS as a probe of Kitaev Magnetic Excitations in β-Li₂IrO₃

Spin-orbit entangled Mott insulators with strong bond-directional interactions have been proposed as a possible venue for the realization of the Kitaev honeycomb model. Most candidate materials, however, order at low temperatures due to the presence of additional spin coupling interactions. Nonetheless, a wealth of theoretical and experimental work has shown that the Kitaev term is the dominant magnetic exchange.
The 3D Kitaev material β-Li2IrO3 hosts nearly degenerate states below 38K when a magnetic field is applied along a Kitaev direction: an incommensurate spiral (INC) and a commensurate field-induced ‘zig-zag’ phase (FIZZ). Most recently, a high-temperature magnetic anomaly was revealed at 100K using thermodynamics and muon relaxation techniques. In this work, we used a high-resolution RIXS spectrometer (FWHM=10meV) to measure the low-energy spin excitations in β-Li2IrO3 under a 2T applied field. We have identified magnon branches near each q-vector and the closing of a gap at the FIZZ q-position with the applied field. More importantly, we have observed a broad continuum of magnetic excitations centered around 35meV, which is unaffected by the low-temperature ordered states, and whose integrated intensity is constant up to 100K and continuously decreases above that.