Intermediate Field Spinon Dynamics in alpha-RuCl3

We present comprehensive inelastic neutron spectroscopic maps of the magnetic field-induced disordered phase of the Kitaev quantum spin liquid candidate material RuCl3. We observe that the spin excitation spectrum at and above a magnetic field of 8~T is gapped. Excitation modes then sharpen for increasing field but are consistently broader than experimental resolution even at 13.5~T. The out-of-plane dispersion diminishes in the 7-10~T regime, signifying enhanced two-dimensional behaviour consistent with expectations of spin liquid behaviour. In this regime, excitations are very broad and largely flat for all accessible energy-momenta. By careful analysis of the lowest energy part of the spectrum, we conclude that these excitations are kinematically at odds with a magnon-decay picture. By contrast, a continuum of fractionalized excitations naturally yields a broad continuum response, accompanied by sharper modes of bound states of fractionalized excitations below the continuum at higher fields. Taken together, our results provide strong evidence for the existence of fractionalized excitations in RuCl3 in a magnetic field. I will conclude my talk with a description of the thermal statistical behaviour of the high-energy section of the 8 T continuum, which also bolsters the same conclusions.