Valence bond glass on an fcc lattice in the double perovskite Ba2YMoO6

TL;DR

This study uncovers a valence bond glass state in Ba2YMoO6, where spins form a disordered singlet pattern due to geometric frustration, without magnetic order down to very low temperatures.

Contribution

It provides experimental evidence of a valence bond glass in a cubic double perovskite, highlighting a new state of matter driven by geometric frustration and orbital degeneracy.

Findings

No magnetic order observed down to 2 K

Spins gradually freeze into a disordered singlet pattern

Orbital degeneracy is quenched while cubic symmetry remains

Abstract

We report on the unconventional magnetism in the cubic B-site ordered double perovskite Ba2YMoO6, using AC and DC magnetic susceptibility, heat capacity and muSR. No magnetic order is observed down to 2 K while the Weiss temperature is ~ -160 K. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate Mo5+ s=1/2 spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin-singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare example of a valence bond glass.