Strongly frustrated triangular spin lattice emerging from triplet dimer formation in honeycomb Li2IrO3

Abstract

In quantum magnetism spin dimers are typically associated with spin-singlet states. To date, $triplet$ $dimerization$ has not been observed in any 3D or quasi-2D material. With this in mind we here discuss the electronic structure of the spin-orbit driven $5d^5$ Mott insulator Li${}_2$IrO${}_3$, a honeycomb-lattice system with two crystallographically inequivalent Ir-Ir bonds. From $ab$ $initio$ many-body calculations we find that, while both Heisenberg and Kitaev couplings are present, the magnetic interactions are dominated by a strong isotropic ferromagnetic exchange on only one set of bonds. This causes the formation of triplet spin dimers effectively placed on a strongly frustrated triangular lattice. The triplet dimers remain protected in a large region of the phase diagram, suggesting that Li${}_2$IrO${}_3$ has a long-range incommensurate magnetic ground state that is pushed by the Kitaev exchange interactions beyond a standard planar helix configuration.

5 pages, 3 figures, revised from the previous version

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