NMR Investigation on Honeycomb Iridate Ag${}_3$LiIr${}_2$O${}_6$

Abstract

We investigate the structural and magnetic properties of a Kitaev spin liquid candidate material Ag${}_3$LiIr${}_2$O${}_6$ based on ${}^7$Li nuclear magnetic resonance line shape, Knight shift and spin-lattice relaxation rate $1/T_1$. The first sample A shows signatures of magnetically ordered spins, and exhibits one sharp ${}^7$Li peak with FWHM increasing significantly below 14~K. $1/T_1^{stretch}$ of this sample displays a broad local maximum at 40~K, followed by a very sharp peak at $T_N=9\pm 1$~K due to critical slowing down of Ir spin fluctuations, a typical signature of magnetic long range order. In order to shed light on the position-by-position variation of $1/T_1$ throughout the sample, we use Inverse Laplace Transform $T_1$ analysis based on Tikhonov regularization to deduce the density distribution function $P(1/T_1)$. We demonstrate that $\sim 60\%$ of Ir spins are statically ordered at the NMR measurement timescale but the rest of the sample volume remains paramagnetic even at 4.2~K, presumably because of structural disorder induced primarily by stacking faults. In order to further investigate the influence of structural disorder, we compare these NMR results with those of a second sample B, which has been shown by transmission electron microscope to have domains with unwanted Ag inclusion at Li and Ir sites within the Ir honeycomb planes. The sample B displays an additional NMR peak with relative intensity of $\sim 17\%$. The small Knight shift and $1/T_1$ of these defect-induced ${}^7$Li sites and the enhancement of bulk susceptibility at low temperatures suggest that these defects generate domains of only weakly magnetic Ir spins accompanied by free spins, leading to a lack of clear signatures of long-range order. The apparent lack of long-range order could be easily misinterpreted as evidence for the realization of a spin liquid ground state in highly disordered Kitaev lattice.

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• Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-barlowite ZnCu${}_3$(OD)${}_6$FBr
• Introducing the monoclinic polymorph of the Kitaev magnet Na${}_2$Co${}_2$TeO${}_6$
• Emergence of the spin polarized domains in the kagome lattice Heisenberg antiferromagnet Zn-barlowite (Zn${}_{0.95}$Cu${}_{0.05}$)Cu${}_3$(OD)${}_6$FBr
• Magnetic properties of a spin-orbit entangled Jeff = 1/2 honeycomb lattice
• Raman fingerprints of fractionalized Majorana excitations in honeycomb iridate Ag${}_3$LiIr${}_2$O${}_6$
• Role of disorder in the electronic and magnetic properties of Ag${}_3$LiIr${}_2$O${}_6$
• Signatures of a Majorana-Fermi surface in the Kitaev magnet Ag${}_3$LiIr${}_2$O${}_6$