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

Authors :: Mustonen, O.
Mutch, H.
Walker, H. C.
Baker, P. J.
Coomer, F. C.
Perry, R. S.
Pughe, C.
Stenning, G. B. G.
Liu, C.
Dutton, S. E.
Cussen, E. J.
Source :: npj Quantum Mater. 7, 74 (2022)
Publication Year :: 2021
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.<br />Comment: 23+4 pages, 5+5 figures