Valence bond glass state in the 4$d^1$ fcc antiferromagnet Ba${}_2$LuMoO${}_6$

Abstract

$B$-site ordered 4$d^1$ and 5$d^1$ double perovskites have a number of potential novel ground states including multipolar order, quantum spin liquids and valence bond glass states. These arise from the complex interactions of spin-orbital entangled $J_{eff}$ = 3/2 pseudospins on the geometrically frustrated fcc lattice. The 4$d^1$ Mo${}^{5+}$ perovskite Ba${}_2$YMoO${}_6$ has been suggested to have a valence bond glass ground state. Here we report on the low temperature properties of powder samples of isostructural Ba${}_2$LuMoO${}_6$: the only other known cubic 4$d^1$ perovskite with one magnetic cation. Our muon spectroscopy experiments show that magnetism in this material remains dynamic down to 60 mK without any spin freezing or magnetic order. A singlet-triplet excitation with a gap of $\Delta$ = 28 meV is observed in inelastic neutron scattering. These results are interpreted as a disordered valence bond glass ground state similar to Ba${}_2$YMoO${}_6$. Our results highlight the differences of the 4$d^1$ double perovskites in comparison to cubic 5$d^1$ analogues, which have both magnetic and multipolar order.

23+4 pages, 5+5 figures

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