In spin-$\frac{1}{2}$ Mott insulators, nonmagnetic quantum liquid phases are often argued to arise when the system shows no magnetic ordering, but identifying positive signatures of these phases or related spinon quasiparticles can be elusive. Here, we use Raman scattering to provide three signatures for spinons in a possible spin-orbit quantum liquid material $\mathrm{Ba}{}_4\mathrm{Ir}{}_3\mathrm{O}{}_{10}$: (1) a broad hump, which we show can arise from Luttinger liquid spinons in Raman with parallel photon polarizations normal to one-dimensional chains; (2) strong phonon damping from phonon-spin coupling via the spin-orbit interaction; and (3) the absence of (1) and (2) in the magnetically ordered phase that is produced when 2% of Ba is substituted by Sr [($\mathrm{Ba}{}_{0.98}\mathrm{Sr}{}_{0.02}){}_4\mathrm{Ir}{}_3\mathrm{O}{}_{10}$]. The phonon damping via itinerant spinons seen in this quantum liquid insulator suggests a mechanism for enhancing thermoelectricity in strongly correlated conductors, through a neutral quantum liquid that need not affect electronic transport.

• Received 28 October 2021
• Revised 30 May 2022
• Accepted 15 July 2022

DOI:https://doi.org/10.1103/PhysRevB.106.075108