Sprecher
Beschreibung
The spin-$1/2$ Heisenberg model on the antiferromagnetic kagome lattice is one of the fundamental models in frustrated quantum magnetism with a predicted quantum spin liquid (QSL) ground state, spinon excitations and a complex sequence of magnetization plateaus in applied magnetic fields [1-3]. From an experimental viewpoint, the mineral herbertsmithite with uniform couplings in the kagome layer stands out as candidate material featuring a QSL ground state [4]. Recent advances in quantum magnetism also explicitly cover deformed kagome lattices leading to many different motifs of non-uniform exchange couplings containing novel physics (see, for instance, Refs. [5,6]).
Here, we present a combined experimental and theoretical study on clinoatacamite, Cu$_2$Cl(OH)$_3$ [7], a mineral which is closely related to herbertsmithite. By means of density-functional theory we have derived the dominant magnetic exchange paths in this material forming non-uniform antiferromagnetic kagome layers of Cu sites with weak ferromagnetic coupling to the interlayer Cu site. Experimentally, we have investigated the zero-field magnetic phase diagram of clinoatacamite by means of thermodynamic measurement techniques as well as neutron diffraction using for the first time single-crystalline material. In agreement with earlier studies, we have found a transition of little entropic change at $T_\mathrm{N} = 18.1~\mathrm{K}$ (with an order parameter developing below this temperature) [8]. Further, we have resolved for the first time a sequence of two close-lying transition anomalies at $6.2$ and $6.4~\mathrm{K}$, which leads to a large entropy change in the material. We have refined the magnetic structure at $1.7~\mathrm{K}$ based on single-crystal neutron diffraction data and present inelastic neutron scattering results revealing the low-energy spin excitations in the same temperature region.
[1] C. Broholm et al., Science 367, eaay0668 (2020).
[2] L. Balents, Nature 464, 199 (2010).
[3] S. Nishimoto et al., Nat. Commun. 4, 2287 (2013).
[4] P. Khuntia et al., Nat. Phys. 16, 469 (2020).
[5] O. Janson et al., Phys. Rev. Lett. 117, 037206 (2016).
[6] K. Matan et al., Nat. Phys. 6, 865 (2010).
[7] J. D. Grice et al., Can. Mineral. 34, 73 (1996).
[8] X. G. Zheng et al., Phys. Rev. Lett. 95, 057201 (2005).
