Abstract
We report a comprehensive muon spectroscopy study of the Zn-barlowite series of S=frac{1}{2} kagomé antiferromagnets, ZnxCu4−x(OH)6FBr, for x = 0.00 to 0.99(1). By combining muon spin relaxation and rotation measurements with state-of-the-art density-functional theory muon-site calculations, we observe the formation of both μ–F and μ–OH complexes in Zn-barlowite. From these stopping sites, implanted muon spins reveal the suppression of long-range magnetic order into a possible quantum spin liquid state upon the increasing concentration of Zn-substitution. In the parent compound (x = 0), static long-range magnetic order below TN = 15 K manifests itself in the form of spontaneous oscillations in the time-dependent muon asymmetry signal consistent with the dipolar fields expected from the calculated muon stopping sites and the previously determined magnetic structure of barlowite. Meanwhile, in the x = 1.0 end-member of the series—in which antiferromagnetic kagomé layers of Cu2+S=frac{1}{2} moments are decoupled by diamagnetic Zn2+ ions—we observe that dynamic magnetic moment fluctuations persist down to at least 50 mK, indicative of a quantum disordered ground state. We demonstrate that this crossover from a static to dynamic magnetic ground state occurs for compositions of Zn-barlowite with x > 0.5, which bears resemblance to the dynamical behaviour of the widely studied Zn-paratacamite series that contains the quantum spin liquid candidate herbertsmithite.
Highlights
Quantum effects play a significant role in the low-temperature physics of magnetic systems in which antiferromagnetic1 2 moments decorate a two-dimensional kagomé array of cornersharing triangles[1]
Heisenberg model on an Despite this promise, the evolution of the magnetic ground state and characteristic magnetic moment correlation dynamics across the Zn-barlowite series are yet to be explored in any great detail using local probe techniques, such as muon spin relaxation and rotation
The development of densityfunctional theory (DFT) to calculate muon stopping sites in crystalline solids is increasingly enabling the determination of these factors to advance the quantitative analysis of experimental μSR data[39]
Summary
1 2 moments decorate a two-dimensional kagomé array of cornersharing triangles[1]. The competing interactions resulting from the geometric frustration in such systems combined with quantum fluctuations may give rise to elusive states of matter, such as quantum spin liquids (QSLs); a possibility that continues to capture the interest of the quantum materials research community[2]. Heisenberg model on an Despite this promise, the evolution of the magnetic ground state and characteristic magnetic moment correlation dynamics across the Zn-barlowite series are yet to be explored in any great detail using local probe techniques, such as muon spin relaxation and rotation (μSR) In these techniques, spin-polarised, positively charged muons are implanted into a sample of interest, in which they thermalise in regions of high electron density. The development of densityfunctional theory (DFT) to calculate muon stopping sites in crystalline solids is increasingly enabling the determination of these factors to advance the quantitative analysis of experimental μSR data[39] We present such a study—combining comprehensive μSR measurements on the Zn-barlowite series with supporting DFT muon-site calculations—to reveal the onset of the possible QSL phase within this family of quantum materials. We take advantage of the formation of both μ − F and μ − OH complexes upon muon implantation in Zn-barlowite which—in combination with our DFT calculations—gives a strong constraint for the accurate determination of muon stopping sites and the local magnetic properties of this series
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