Spin excitations of a proximate Kitaev quantum spin liquid realized in Cu${}_2$IrO${}_3$

Abstract

Magnetic moments arranged at the corners of a honeycomb lattice are predicted to form a novel state of matter, Kitaev quantum spin liquid, under the influence of frustration effects between bond-dependent Ising interactions. Some layered honeycomb iridates and related materials, such as Na${}_2$IrO${}_3$ and $\hat{I}\pm$-RuCl${}_3$, are proximate to Kitaev quantum spin liquid, but bosonic spin-wave excitations associated with undesirable antiferromagnetic long-range order mask the inherent properties of Kitaev Hamiltonian. Here, we use ${}^{63}$Cu nuclear quadrupole resonance to uncover the low energy spin excitations in the nearly ideal honeycomb lattice of effective spin $S=1/2$ at the Ir${}^{4+}$ sites in Cu${}_2$IrO${}_3$. We demonstrate that, unlike Na${}_2$IrO${}_3$, Ir spin fluctuations exhibit no evidence for critical slowing down toward magnetic long range order in zero external magnetic field. Moreover, the low energy spin excitation spectrum is dominated by a mode that has a large excitation gap comparable to the Ising interactions, a signature expected for Majorana fermions of Kitaev quantum spin liquid.

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