Tensor network analysis of the maple-leaf antiferromagnet spangolite

Abstract: Spangolite (Cu$6$Al(SO$_4$)(OH)${12}$Cl$\cdot$3H$_2$O) is a hydrated layered copper sulfate mineral whose crystal structure is well described by a depleted triangular lattice of Cu$^{2+}$ ions in each layer. Experimental measurements reveal a non-magnetic ground state at $T \sim 8\, \text{K}$ with magnetization properties dominated by dimerisation. We propose a concrete model of Cu$^{2+}$ spin-$1/2$ degrees of freedom on this geometrically frustrated and effectively two-dimensional maple-leaf lattice. The distorted geometry of the depleted triangular lattice layers results in a spatially anisotropic Heisenberg model featuring five symmetry inequivalent couplings with ferromagnetic bonds on hexagons and antiferromagnetic triangular bonds. The validity of the proposed Hamiltonian is demonstrated by state-of-the-art tensor network calculations which can assess both the nature of the ground state as well as low-temperature thermodynamics, including effects of a magnetic field. We provide theoretical support for a dimerized ground state by calculating the static spin structure factor as well as the magnetic susceptibility, the latter is shown to be in good agreement with experiment. We further predict the emergence of magnetisation plateaus at high values of an external magnetic field and study their melting with increasing temperature.