Crystal and Magnetic Structures of a Family of Quantum Kagome Antiferromagnets – Dr Lucy Clark, University of Liverpool

April 26, 2019 @ 2:30 pm – 3:30 pm
Small Lecture Theatre
Cavendish Laboratory
J.J. Thomson Avenue
Olivia Matthewson

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(R) (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(R) (2018)