$\mu$SR Insight into the Impurity Problem in Quantum Kagome Antiferromagnets

TL;DR

This study uses muon spin spectroscopy to explore how impurities affect the magnetic properties of quantum kagome antiferromagnets, revealing a gapless spin-liquid state with unconventional spin dynamics.

Contribution

It demonstrates how muons can probe impurity effects and intrinsic kagome physics, uncovering a Kondo-like coupling and gapless spin-liquid behavior.

Findings

Impurities are highly correlated with kagome spins.

The spin-liquid ground state is gapless.

Spin fluctuations show unconventional spectral density.

Abstract

Impurities, which are inherently present in any real material, may play an important role in the magnetism of frustrated spin systems with spin-liquid ground states. We address the impurity issue in quantum kagome antiferromagnets by investigating ZnCu$_3$(OH)$_6$SO$_4$ (Zn-brochantite) by means of muon spin spectroscopy. We show that muons couple to the impurity magnetism, originating from Cu-Zn intersite disorder, and that the impurities are highly correlated with the kagome spins, allowing us to probe the intrinsic kagome physics via a Kondo-like effect. The low-temperature plateau in local susceptibility identifies the spin-liquid ground state as being gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and an intriguing field dependence.