Synthesis and characterization of kagome quantum magnets and novel quantum spin liquid candidates
Keywords: Chemistry.

The search for novel materials that exhibit a quantum spin liquid (QSL) ground state is a highly interdisciplinary problem; motivated by a desire to more fully understand the basic physics of this new state of matter, it requires both synthetic ingenuity as well as extensive characterization of the material's structure and magnetic properties. The QSL can be described as entangled, frustrated spins that continuously fluctuate around a lattice down to T = 0 K without ever freezing into long range magnetic order. Due to the highly entangled nature of the spins, realizing a QSL in a usable material has the potential to revolutionize quantum computing. Few viable candidates currently exist both because many frustrated materials succumb to a structural distortion that breaks the delicate balance of competing magnetic interactions, and because a long series of experimental characterization is required to rule out all other possibilities for the ground state of a novel material. The most promising candidate is a mineral called herbertsmithite (Cu3Zn(OH)6Cl2) with a kagome lattice of S = 1/2 Cu2+ ions, although magnetic defects between the kagome layers complicate precise measurements of its QSL ground state at low temperature. The paucity of experimental systems has long motivated the search for new materials to validate the extensive theoretical work performed on various QSL models, and along the way this undertaking has uncovered many fascinating materials and expanded our understanding of magnetic frustration, related ground states, and the varied effects of structural disorders. In this dissertation, I report for the first time two distinct synthetic routes to a new mineral called barlowite (Cu4(OH)6FBr) that has a Cu2+ kagome lattice. While barlowite itself is not a viable QSL candidate because it orders magnetically, substituting Zn2+ suppresses magnetic coupling between the kagome layers and pushes the system towards a QSL ground state. The two routes yield crystals with different morphologies and low-temperature magnetism albeit the same crystal structure at room temperature. An array of X-ray and neutron diffraction measurements reveals that they have structural phase transitions to different low-temperature structures. One variant becomes orthorhombic and is a simple ordered magnet, while the second variant has only a slight symmetry lowering. In this novel compound, the kagome lattice is subtly modulated with a periodic pattern of distortions, for which numerical simulations predict a pinwheel valence bond crystal ground state instead of a QSL. Using neutron scattering, we find a pinwheel q=0 magnetic structure is induced below TN = 6 K. A clear relationship between the degree of distortion of the kagome lattice and the magnetic properties is evident in these two variants, and the hexagonal one is much closer to a QSL. I synthesize two versions of Zn-substituted barlowite (Cu3ZnxCu1-x(OH)6FBr) with no magnetic order: a polycrystalline version with nearly a full Zn2+ (x = 0.95) and the first single crystals ever reported, with approximately x = 0.56. Using an array of synchrotron X-ray and neutron scattering techniques, I demonstrate that both have an ideal kagome lattice and find no evidence that Zn2+ substitutes onto the kagome layers, which would disturb the QSL. Indeed, first-principles calculations show this antisite disorder is not energetically favorable. Instead, Zn2+ substitutes onto the interlayer; in Zn-substituted barlowite, site-specific X-ray diffraction reveals that Zn2+ and Cu2+ selectively occupy distinct sites, consistent with their different Jahn-Teller activity but in contrast to herbertsmithite, in which both metals occupy the same interlayer site. I uncover an experimental handle in Zn L-edge inelastic X-ray absorption spectroscopy correlated with the loss of inversion symmetry from pseudo-octahedral to trigonal prismatic coordination. Magnetic measurements indicate that both versions of Zn-substituted barlowite are QSL candidates. That the x = 0.56 crystals do not order magnetically even with 0.44 interlayer Cu2+s indicates a surprising robustness of the QSL against interlayer impurities. The two versions display remarkably similar magnetism to each other and to herbertsmithite, indicating universal magnetic behavior of the Cu2+ kagome lattice. In terms of suitability as an ideal QSL candidate, Zn-substituted barlowite has structural advantages over herbertsmithite: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution