Gapped 1/9 Magnetization Plateau in the Anisotropic Kagome Antiferromagnet Y-kapellasite

TL;DR

This study investigates the mysterious 1/9 magnetization plateau in the anisotropic kagome antiferromagnet Y-kapellasite, revealing it is associated with a gapped fractional state and demonstrating its robustness across different materials.

Contribution

The paper provides experimental evidence of the 1/9 plateau's microscopic origin and its relation to a gapped fractional state in Y-kapellasite, a previously unresolved issue.

Findings

Identification of 1/3 and 1/9 magnetization plateaus in Y-kapellasite.

The 1/9 plateau is linked to an ordered spin configuration and a gapped fractional state.

The 1/9 state shows robustness across different kagome antiferromagnets.

Abstract

Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious $1/9$ plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the $S = 1/2$ anisotropic kagome antiferromagnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, using pulsed-field magnetization measurements on single crystals and high-field $^{35}$Cl NMR. We identify a hierarchy of field-induced fractional features, including $1/3$ and $1/9$ plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the $1/9$ plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu$_3$(OH)$_6$Br$_2$[Br$_{1-y}$(OH)$_y$] in which this plateau is observed, implying a surprising robustness of the $1/9$ state to the details of the underlying magnetism.