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

TL;DR

This study uses nuclear quadrupole resonance to reveal low energy spin excitations in Cu$_2$IrO$_3$, providing evidence for Kitaev quantum spin liquid behavior through a gapped excitation spectrum and absence of magnetic ordering.

Contribution

It demonstrates that Cu$_2$IrO$_3$ hosts spin excitations consistent with a proximate Kitaev quantum spin liquid, unlike similar materials with magnetic order.

Findings

Ir spin fluctuations show no critical slowing down.

Low energy excitations have a large gap similar to Ising interactions.

Evidence suggests presence of Majorana fermions in the spin liquid state.

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 $\alpha$-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.