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, the 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 α-RuCl_{3}, are proximate to the Kitaev quantum spin liquid, but bosonic spin-wave excitations associated with undesirable antiferromagnetic long-range order mask the inherent properties of the 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 the Kitaev quantum spin liquid.