Experimental evidence for a valence-bond glass in the 5d1 double perovskite Ba2YWO6

We report the magnetic susceptibility, specific heat, and muon spin relaxation results of the 5d1 double perovskite Ba2YWO6. The dc magnetic susceptibility shows two distinct Curie-Weiss regimes and a sub-Curie-Weiss increase T-n with n=0.75(1) for T=3-100 K, alluding to the presence of random magnetism. The ac magnetic susceptibility reveals a spin freezing at Tf∼0.3 K with the activation energy of Δ/kB=27.6 K. The specific heat data exhibit the pseudogap behavior at 25 K and the subsequent power-law dependence with decreasing temperature, indicating the gradual spin freezing with the quenching of orbital fluctuations. However, the muon spin relaxation data display only a weak muon spin depolarization with lacking long-range magnetic ordering down to 26 mK. Taken together, our results suggest that Ba2YWO6 is the proximate realization of a random spin-orbit dimer state.