Confinement-Deconfinement Transition as an Indication of Spin-Liquid-Type Behavior in Na${}_2$IrO${}_3$

Abstract

We use ultrafast optical spectroscopy to observe binding of charged single-particle excitations (SE) in the magnetically frustrated Mott insulator Na${}_2$IrO${}_3$. Above the antiferromagnetic ordering temperature ($T_N$) the system response is due to both Hubbard excitons (HE) and their constituent unpaired SE. The SE response becomes strongly suppressed immediately below $T_N$. We argue that this increase in binding energy is due to a unique interplay between the frustrated Kitaev and the weak Heisenberg-type ordering term in the Hamiltonian, mediating an effective interaction between the spin-singlet SE. This interaction grows with distance causing the SE to become trapped in the HE, similar to quark confinement inside hadrons. This binding of charged particles, induced by magnetic ordering, is a result of a confinement-deconfinement transition of spin excitations. This observation provides evidence for spin liquid type behavior which is expected in Na${}_2$IrO${}_3$.

5 pages, 3 figures

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• Stripe-yz magnetic order in the triangular-lattice antiferromagnet KCeS2
• A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator
• Fermi arcs, pseudogap and collective excitations in doped Sr2IrO4: A generalized fluctuation exchange study
• Quasiparticles and charge transfer at the two surfaces of the honeycomb iridate Na${}_2$IrO${}_3$
• Thermal conductivity of local moment models with strong spin-orbit coupling
• Observation of excitons bound by antiferromagnetic correlations