A nickel-based material belonging to the langbeinite family could be a new three-dimensional quantum spin liquid candidate, according to new experiments at the ISIS Neutron and Muon Source in the UK. The work, performed by researchers from the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, the Helmholtz-Zentrum Berlin (HZB) in Germany and Okayama University in Japan, is at the fundamental research stage for the moment.
Quantum spin liquids (QSLs) are magnetic materials that cannot arrange their magnetic moments (or spins) into a regular and stable pattern. This “frustrated” behaviour is very different from that of ordinary ferromagnets or antiferromagnets, which have spins that point in the same or alternating directions, respectively. Instead, the spins in QSLs constantly change direction as if they were in a fluid, producing an entangled ensemble of spin-ups and spin-downs even at ultracold temperatures, where the spins of most materials freeze solid.
So far, only a few real-world QSL materials have been observed, mostly in quasi-one-dimensional chain-like magnets and a handful of two-dimensional materials. The new candidate material – K2Ni2(SO4)3 – is a langbeinite, a family of sulphate minerals rarely found in nature whose chemical compositions can be changed by replacing one or two of the elements in the compound. K2Ni2(SO4)3 is composed of a three-dimensional network of corner-sharing triangles forming two trillium lattices made from the nickel ions. The magnetic network of langbeinite shares some similarities with the QSL pyrochlore lattice, which researchers have been studying for the last 30 years, but is also quite different in many ways.
A strongly correlated ground state at up to 20 K
The researchers, led by Ivica Živković at the EPFL, fabricated the new material especially for their study. In their previous work, which was practically the first investigation of the magnetic properties of langbeinites, they showed that the compound has a strongly correlated ground state at temperatures of up to at least 20 K.
In their latest work, they used a technique called inelastic neutron scattering, which can measure magnetic excitations, at the ISIS Neutron and Muon Source of the STFC Rutherford Appleton Laboratory to directly observe this correlation.
Theoretical calculations by Okayama University’s Harald Jeschke, which included density functional theory-based energy mappings, and classical Monte Carlo and pseudo-fermion functional renormalization group (PFFRG) calculations, performed by Johannes Reuther at the HZB to model the behaviour of K2Ni2(SO4)3, agreed exceptionally well with the experimental measurements. In particular, the phase diagram of the material revealed a “centre of liquidity” that corresponds to the trillium lattice in which each triangle is turned into a tetrahedron.
Particular set of interactions supports spin-liquid behaviour
The researchers say that they undertook the new study to better understand why the ground state of this material was so dynamic. Once they had performed their theoretical calculations and could model the material’s behaviour, the challenge was to identify the type of geometric frustration that was at play. “K2Ni2(SO4)3 is described by five magnetic interactions (J1, J2, J3, J4 and J5), but the highly frustrated tetra-trillium lattice has only one non-zero J,” explains Živković. “It took us some time to first find this particular set of interactions and then to prove that it supports spin-liquid behaviour.”
Now that we know where the highly frustrated behaviour comes from, the question is whether some exotic quasiparticles can be associated with this new spin arrangement, he tells Physics World.
Živković says the research, which is detailed in Nature Communications, remains in the realm of fundamental research for the moment and that it is too early to talk about any real-world applications.
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