Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states

Abstract

When the bonds of a quantum magnet are modulated with a periodic pattern, exotic quantum ground states may emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin-1 2 kagome lattice. Our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a quantum spin liquid (QSL). The presence of interlayer spins eventually leads to novel pinwheel q=0 magnetic order. Partially Zn-substituted barlowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, demonstrating the robustness of the QSL against local impurities. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.Identifying the ground state for interacting quantum spins on the kagome lattice is an important unresolved question in condensed matter physics owing to the great difficulty in selecting amongst competing states that are very close in energy. Antiferromagnetic (AF) spins on this lattice are highly frustrated, and for spin-1 2 the ground state does not achieve magnetic order and is believed to be a quantum spin liquid (QSL). (1-9) The QSL is an unusual magnetic ground state, characterized by long-range quantum entanglement of the spins with the absence of long-range magnetic order. (10-13) The recent identification of herbertsmithite (Cu 3 Zn(OH) 6 Cl 2 ) (14-17) as a leading candidate QSL material has ignited intense interest in further understanding similar kagome materials.Often, real kagome materials have interactions that relieve the frustration and drive the moments to magnetically order. (18,19) In contrast, for the ideal S = 1 2 kagome Heisen-

From magnetic order to quantum disorder in the Zn-barlowite series of S = 1/2 kagomé antiferromagnets

“…In either case, we conclude that x ≈ 0.5 marks a critical threshold in the Zn-barlowite series, above which dynamic magnetic moment correlations characterise the lowtemperature magnetic behaviour. We note that this is in agreement with a recent study which proposes that magnetic moments in single crystals of Zn-barlowite with x = 0.56 remain dynamic to low-temperatures via bulk magnetometry measurements 50 .…”

“…2b). Our observations are thus in support of the recent proposals of a QSL ground state for fully subsituted ZnCu 3 (OH) 6 FBr from both experiment and theory 30,50 . Another important result from our present study, however, is the striking similarity of the temperature dependence of λ norm for x = 0.66(1) and 0.99(1) samples, as seen in Fig.…”

“…We find that incorporating as little as x = 0.16 of Zn 2+ into the crystal structure of barlowite leads to a suppression of its ordered, frozen fraction, while for x > 0.66 the electronic magnetic moments remain dynamically fluctuating at all measurable temperatures. Whether the magnetic ground states of such partially substituted compositions of Zn-barlowite correspond to a QSL state-as proposed for the x = 1.0 end-member 21,29,30,50 -awaits further investigation, for instance, by inelastic neutron scattering.…”

“…However, an additional consideration here is the potential sample dependence on the magnetic properties of barlowite. Whilst the subtle structural differences depending on synthetic route are unlikely to have any significant effect on the muon stopping sites, some studies report several magnetic transitions in their samples synthesised by alternative routes to us which may eventually be insightful to explore using μSR 20,22,24,50 .…”

Expanding the Averievite Family, (MX)Cu5O2(T5+O4)2 (T5+ = P, V; M = K, Rb, Cs, Cu; X = Cl, Br): Synthesis and Single-Crystal X-ray Diffraction Study

“…Of particular interest are minerals containing transition metal cations such as Cu 2+ that form various cation arrays with interesting magnetic behavior. The kagome lattice consisting of cornersharing Cu 3 triangles forming a 2D net with regular hexagonal rings is probably the most famous example-such an array is geometrically frustrated, and this feature is responsible for the spin liquid behavior first demonstrated for synthetic herbertsmithite and later observed for other synthetic mineral analogues [4][5][6][7][8][9][10][11]. In order to exhibit a spin liquid property, the kagome array should possess an ideal trigonal (or hexagonal) symmetry, Molecules 2021, 26, 1833 2 of 12 which is absent in most minerals with the kagome arrangement of Cu 2+ magnetic ions.…”