The perovskite structure is widely studied for a range of technological applications as manipulation of the structure, composition, and electron count access a range of fascinating phenomena.  The structure can be further modified by cation-ordering to arrange paramagnetic cations onto 50% of the octahedral sites, leading to a lattice of magnetic ions forming an array of equilateral triangles linked by shared edges. This combination of crystallographic ordering and magnetic spins can lead to unusual ground states and the emergence of quantum magnetic phenomena. These give us an insight into the weaker magnetic interactions that crucially inform design for quantum computing.

The unique behaviour comes from the highly degenerate, antiferromagnetic interactions coupling the single electrons in the d-orbitals. The geometric frustration of the interactions leads to net cancellation of the strongest magnetic interactions and allows weaker quantum effects to come to the fore. One example is the compound Ba₂YMoO₆, which has been shown to exhibit a valence bond glass state, but with some freezing of orphan spins.

This study, published in npj Quantum Materials, focusses on Ba₂LuMoO₆, and uses a comprehensive collection of techniques to fully characterise its ground state.  This included muon spectroscopy on MuSR and inelastic neutron scattering (INS) on Merlin, as well as using the Materials Characterisation Lab at ISIS for magnetometry and heat capacity measurements.

The researchers found that there is an absence of magnetic order or spin freezing down to 60 mK, suggesting that the only options for the ground state are a spin liquid or a valence bond glass state. Their INS measurements show a singlet-triplet excitation and weak in-gap scattering, supporting the valence bond glass state, although a quantum spin liquid state could not be ruled out.

Comparing their results on Ba₂LuMoO₆ to those of Ba₂YMoO₆, a key difference was in the MuSR measurements. In Ba₂YMoO₆, the orphan spins form a dilute spin glass, and the muon spin relaxation rate is much lower than in Ba₂LuMoO₆. The lack of this spin glass transition makes Ba₂LuMoO₆ a promising system for investigating exotic ground states of the d¹ double perovskites. 

Further information

The full paper can be found at DOI: 10.1038/s41535-022-00480-4​