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Crystal and Magnetic Structures of a Family of Quantum Kagome Antiferromagnets

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Materials constructed from kagome layers of antiferromagnetically coupled S = ½ moments are highly prized as they offer a unique opportunity to explore the elusive quantum spin liquid state (QSL) [1]. The mineral herbertsmithite, ZnCu3(OH)6Cl2, for instance, contains such an array of Cu2+ S = ½ ions and consequently, has garnered considerable attention as a QSL candidate [2]. However, the substantial Cu2+/Zn2+ disorder within the crystal structure of herbertsmithite continues to call into question our ability to rationalise its magnetic ground state [3]. More recently, an alternative Cu2+-based mineral known as Zn-doped barlowite, ZnCu3(OH)6FBr, has shown promise as a new materialisation of the QSL state [4], with first-principles studies indicating that the extent of the anti-site disorder is reduced in comparison to herbertsmithite owing to the different stacking of the kagome planes in Zn-barlowite [5]. Despite this interest, the crystal and magnetic structures of the parent material barlowite, Cu4(OH)6FBr, were poorly understood with several conflicting reports in the literature [6-8]. Here, I will introduce these developments in the field of highly frustrated magnetism before presenting our comprehensive powder neutron diffraction study of barlowite. In doing so, I will discuss the intriguing structural phase transition we observe in this material at T = 250 K, and clarify the nature of its magnetic ground below TN = 15 K [9]. Furthermore, I will show that we can tune the magnetic ground state of barlowite from antiferromagnetic order to quantum disorder upon Zn-doping though our magnetometry and muon spectroscopy measurements. Finally, I will discuss our efforts to control the nature of the structural phase transition within a new family compounds through exchange of the halide ions in barlowite. [1] L. Savary and L. Balents, Rep. Prog. Phys. 80, 016502 (2017). [2] M. P. Shores et al., JACS 127 , 13462-13463 (2005). [3] M. Fu et al., Science 350, 655-658 (2015). [4] T.-H. Han, J. Singleton and J. A. Schlueter, Phys. Rev. Lett. 113, 227203 (2014). [5] Z. Lui et al., Phys. Rev. B 92 , 220102® (2015). [6] Z. Feng et al., Phys. Rev. B 98 , 155127 (2018). [7] C. M. Pasco et al., Phys. Rev. Mater. 2, 0444061 (2018). [8] R. W. Smaha et al., J. Solid State Chem. 268, 123-129 (2018). [9] K. Tustain et al., Phys. Rev. Mater. 2, 111405® (2018)

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