With potential roles in quantum computation, high-temperature superconductivity and a range of exotic anyonic states, it’s no wonder that quantum spin liquids (QSLs) are attracting so much research interest. However, a detailed understanding of the kagome lattice materials that might harbour QSL states has proved harder to come by. Reporting in Chinese Physics Letters, researchers in China and Japan have now identified QSL characteristics in a new candidate kagome lattice material. In addition, their investigations suggest possible pathways for the transition between QSL and magnetically ordered states, as well as insights into some of the anyonic excitations and other exotic properties predicted by theory.

Born from frustration

Nobel laureate Philip Warren Anderson first proposed the existence of QSLs in 1973, when he was studying the ground state of antiferromagnetically interacting spins on a triangular lattice. While the magnetic spins in more conventional ferromagnetic and antiferromagnetic materials align in parallel or antiparallel conformations at low temperatures, QSLs retain a type of magnetic spin disorder – analogous to the disorder of liquid atoms or molecules as compared with a crystalline solid.

With their “frustrated” spins, ground-state honeycomb, triangular or kagome lattice structures remain the primary hunting grounds for new candidate QSLs. And even in these structures, the interactions between layers must be minimal to prevent the onset of order between adjacent parts of the lattice. Kagome lattice materials with known QSL characteristics include Herbertsmithite ZnCu 3 (OH) 6 Cl 2 and Zn-doped Barlowite Cu 3 Zn(OH) 6 FBr, but many other materials with similar structures have exhibited magnetic order at low temperatures, often alongside lattice distortion to accommodate spin alignment.

Bred from claringbullite

In this latest work Shiliang Li, Zi Yang Meng, and Youguo Shi at the Institute of Physics, Chinese Academy of Sciences in Beijing, China, and their colleagues synthesized Cu 4 (OH) 6 FCl, also known as claringbullite, and Cu 3 Zn(OH) 6 FCl, a similar kagome lattice material where most of the interlayer copper atoms are replaced with zinc. They then investigated the structure and temperature-dependent magnetic properties.

The researchers found that a perfect kagome lattice is preserved in Cu 3 Zn(OH) 6 FCl with magnetic order completely suppressed at temperatures as low as 0.8 K, whereas in claringbullite magnetic order sets in at temperatures below 17 K. They point out that recent neutron scattering experiments have revealed lattice distortions accompanying emerging magnetic order in Cu 4 (OH) 6 FBr, and further experiments are needed to determine whether claringbulliteis subject to a similar process.

The studies highlight similarities between Cu 3 Zn(OH) 6 FCl and previously studied herbertsmithite Cu 3 Zn(OH) 6 Cl 2 and Zn-doped Barlowite Cu 3 Zn(OH) 6 FBr. “Looking into the future, the pathway from Cu 4 (OH) 6 FCl to Cu 3 Zn(OH) 6 FCl offers the opportunity to investigate the transition between magnetically ordered systems to QSL states,” conclude Li, Meng, Shi and colleagues in their report. They add that further neutron scattering experiments could also reveal theoretically predicted fractionalized anyonic excitations in the QSL ground state and “encourage further theoretical and experimental developments of the new paradigms of quantum matter”.

“Our materials have the advantage of continuously varying the Zn composition such that we are able to investigate the quantum phase transition from undoped parent compounds Barlowite (Cu 4 (OH) 6 FBr) and claringbullite (Cu 4 (OH) 6 FCl) to their corresponding quantum spin liquid children Cu 3 Zn(OH) 6 FBr and Cu 3 Zn(OH) 6 FCl,” Meng tells Physics World. “Not only the spin liquids themselves are important, but also the quantum phase transitions from magnetically ordered parent compounds to the magnetically disordered spin liquids have great theoretical significance in terms of fundamental theory at the frontiers of condensed matter physics – this is basically the phase transition from symmetry breaking phases to phases with topological order. Such phase transitions are beyond the conventional paradigm of Landau-Ginzburg, which is the cornerstone of our current understanding of phases of matter, and are therefore still under development and will be the new paradigm of quantum matter.”

Full details are reported in Chinese Physics Letters.

Edited 7th January 2019