Raman response via 4-spinon continuum in spin-1/2 quantum liquid Ba₄Ir₃O₁₀

Spin-1/2 magnetic insulators can avoid ordered phases by forming a quantum liquid (QL) state. But beyond lack of ordering, positive signatures of QL states are difficult to access. Ba₄Ir₃O₁₀ is a recently discovered S_eff = 1/2 insulator with no magnetic order down to 0.2 K despite strong antiferromagnetic interactions T_CW = 100-700 K. Here, I report on experimental results and our theoretical analysis of inelastic Raman scattering measurements on Ba₄Ir₃O₁₀ which find broadened phonon peaks superposed on a large bandwidth hump feature. Adding disorder via 2% Ba-to-Sr substitution produces magnetic order while also eliminating the hump feature below T_N^Sr = 130 K, suggesting the hump arises from spin excitations in a fragile QL. We employ a model of free 1D spinons to compute the 4-spinon Raman response, finding agreement with experiment across two different mean field limits. For photon polarizations used experimentally, a nonvanishing Raman response requires both frustrated second neighbor couplings and zig-zagged spin chains, as indeed appear in Ba₄Ir₃O₁₀. Together with other signatures such as phonon broadening from spin-orbit-coupled-spinon scattering, the analysis suggests a fragile QL state with gapless spinon excitations in Ba₄Ir₃O₁₀.