Bond-Disordered Spin Liquid and the Honeycomb Iridate H3LiIr2O6: Abundant Low-Energy Density of States from Random Majorana Hopping

The 5d-electron honeycomb compound H3LiIr2O6 [K. Kitagawa et al., Nature (London) 554, 341 (2018)] exhibits an apparent quantum spin liquid state. In this intercalated spin-orbital compound, a remarkable pileup of low-energy states was experimentally observed in specific heat and spin relaxation. We show that a bond-disordered Kitaev model can naturally account for this phenomenon, suggesting that disorder plays an essential role in its theoretical description. In the exactly soluble Kitaev model, we obtain, via spin fractionalization, a random bipartite hopping problem of Majorana fermions in a random flux background. This has a divergent low-energy density of states of the required power-law form N(E) proportional to E-nu with a drifting exponent which takes on the value nu approximate to 1/2 for relatively strong bond disorder. Breaking time-reversal symmetry removes the divergence of the density of states, as does applying a magnetic field in experiment. We discuss the implication of our scenario, both for future experiments and from a broader perspective.