High-resolution RIXS with in-situ magnetic field as a probe of magon-spinon dichotomy in β-Li₂IrO₃

The family of tri-coordinated iridates have been identified as potential candidates supporting a Kitaev quantum spin liquid, in which spins fractionalize into emergent Majorana fermions and magnetic flux excitations. These quasiparticles acquire long-range topological entanglement and are ideal for fault-tolerant quantum computers. While the presence of additional interactions usually leads to conventional ordering, a dominant Kitaev exchange leads to QSL behavior after the magnetic order is suppressed by temperature or magnetic field, with signatures appearing in thermodynamic measurements and dynamical probes.
β-Li₂IrO₃ is an intriguing example of the complex interplay between Kitaev coupling and magnetic field. At low temperatures, an applied field suppresses the incommensurate spiral order and drives the system into a uniform zig-zag state, which are strongly intertwined due to Kitaev interactions. Moreover, a magnetic anomaly has been observed at 100K, with onset in magnetization and a crossover in the heat capacity without causing true magnetic order. Although these results have been suggested to emerge from thermal fractionalization of the spins, very little is known about the low-energy magnetic excitations.
In this work, we present a comprehensive picture of the dynamical response of β-Li₂IrO₃ in an applied magnetic field. The spin excitations were measured using a high-resolution RIXS spectrometer, which identified dispersing spin waves reaching a maximum of 16meV, in perfect agreement with semiclassical calculations of the dynamical spin structural factor for the intertwined states. The low-energy magnon is superimposed by a broad continuum of excitations centered around 35meV, which is unaffected by the low-temperature ordered states but sensitive to the high-temperature anomaly. This continuum alludes to the long coherence time of the fractional excitations in the proxime QSL phase.