We report the crystal structure, magnetization and neutron scattering measurements on the double perovskite Ba2YOsO6. The F m3m space group is found both at 290 K and 3.5 K with cell constants a0 = 8.3541(4)Å and 8.3435(4)Å, respectively. Os 5+ (5d 3 ) ions occupy a non-distorted, geometrically frustrated face-centered-cubic (FCC) lattice. A Curie-Weiss temperature θ = −772 K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long range antiferromagnetic order, consistent with a Type I FCC state below TN ∼ 69 K, is revealed by magnetization, Fisher heat capacity and elastic neutron scattering, with an ordered moment of 1.65(6) µB on Os 5+ . The ordered moment is much reduced from either the expected spin only value of ∼ 3µB or the value appropriate to 4d 3 Ru 5+ in isostructural Ba2YRuO6 of 2.2(1) µB, suggesting a role for spin orbit coupling (SOC). Triple axis neutron scattering measurements of the order parameter suggest an additional first-order transition at T = 67.45 K, and the existence of a second ordered state. Time-of-flight inelastic neutron results reveal a large spin gap ∆ ∼ 17 meV, unexpected for an orbitally quenched, d3 electronic configuration. We discuss this in the context of the ∼ 5 meV spin gap observed in the related Ru 5+ , 4d 3 cubic double perovskite Ba2YRuO6, and attribute the ∼ 3 times larger gap to stronger SOC present in this heavier, 5d, osmate system.

Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the J = 7/2 Yb 3+ ion in the candidate quantum spin ice pyrochlore magnet Yb2Ti2O7. The precise ground state (GS) of this exotic, geometrically-frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak "stuffing" wherein a small proportion, ≈ 2%, of the non-magnetic Ti 4+ sites are occupied by excess Yb 3+ . We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of Yb2Ti2O7, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb2+xTi2−xO7+y with x = 0.046. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of mJ = ±1/2, as expected. However,"stuffing" at low temperatures in Yb2+xTi2−xO7+y induces a similar finite CEF lifetime as is induced in stoichiometric Yb2Ti2O7 by elevated temperature. We conclude that an extended strain field exists about each local "stuffed" site, which produces a distribution of random CEF environments in the lightly stuffed Yb2+xTi2−xO7+y, in addition to producing a small fraction of Yb-ions in defective environments with grossly different CEF eigenvalues and eigenvectors.

We report time-of-flight neutron spectroscopic and diffraction studies of the 5d 2 cubic double pervoskite magnets, Ba2M OsO6 (M = Zn, Mg, Ca). These cubic materials are all described by antiferromagnetically-coupled 5d 2 Os 6+ ions decorating a face-centred cubic (FCC) lattice. They all exhibit thermodynamic anomalies consistent with phase transitions at a temperature T * , and exhibit a gapped magnetic excitation spectrum with spectral weight concentrated at wavevectors typical of type I antiferromagnetic orders. While muon spin resonance experiments show clear evidence for time reversal symmetry breaking, no corresponding magnetic Bragg scattering is observed at low temperatures. These results are argued to be consistent with low temperature octupolar order, and are discussed in the context of other 5d DP magnets, and theories for d 2 ions on a FCC lattice which predict exotic orders driven by multipolar interactions.

Spin liquid ground states are predicted to arise within several distinct scenarios in condensed matter physics. The observation of these disordered magnetic states is particularly pervasive amongst a class of materials known as frustrated magnets, in which the competition between various magnetic exchange interactions prevents the system from adopting long-range magnetic order at low temperatures. Spin liquids continue to be of great interest due to their exotic nature and the possibility that they may support fractionalised excitations, such as Majorana fermions. Systems that allow for such phenomena are not only fascinating from a fundamental perspective but may also be practically significant in future technologies based on quantum computation. Here we show that the underlying antiferromagnetic sublattice in TbInO3 undergoes a crystal field induced triangular-to-honeycomb dilution at low temperatures. The absence of a conventional magnetic ordering transition at the lowest measurable temperatures indicates that another critical mechanism must govern in the ground state selection of TbInO3. We propose that anisotropic exchange interactionsmediated through strong spin-orbit coupling on the emergent honeycomb lattice of TbInO3give rise to a highly frustrated spin liquid.One notable example of a spin liquid 1,2 is that of the S = ½ Heisenberg antiferromagnet on a two-dimensional kagome lattice, a frustrated network of corner-sharing triangles. It is now widely considered that this magnetic system displays a quantum spin liquid ground state 3 and there is recent experimental evidence to suggest that a gapped quantum spin liquid state is likely realised in the Cu 2+ -based kagome antiferromagnet, herbertsmithite. 4 The two-dimensional honeycomb net, on the other hand, is a bipartite lattice and, therefore, does not give rise to frustrated ground states in the presence of conventional nearest-

Motivated by experimental and theoretical interest in realizing multipolar orders in d-orbital materials, we discuss the quantum magnetism of J = 2 ions on the face-centered cubic lattice which can be realized in spin-orbit coupled oxides with 5d 2 transition metal ions. Based on the crystal field environment, we argue for a splitting of the J = 2 multiplet, leading to a low lying non-Kramers doublet which hosts quadrupolar and octupolar moments. We discuss a microscopic mechanism whereby the combined perturbative effects of orbital repulsion and antiferromagnetic Heisenberg spin interactions leads to ferro-octupolar coupling between neighboring sites, and stabilizes ferrooctupolar order. This same mechanism is also shown to disfavor quadrupolar ordering. We study spin dynamics in the ferro-octupolar state using a slave-boson approach, uncovering a gapped and dispersive magnetic exciton. For sufficiently strong magnetic exchange, the dispersive exciton can condense, leading to conventional type-I antiferromagnetic order which can preempt octupolar order. Our proposal for ferrooctupolar order, with specific results in the context of a model Hamiltonian, provides a comprehensive understanding of thermodynamics, µSR, X-ray diffraction, and inelastic neutron scattering measurements on a range of cubic 5d 2 double perovskite materials including Ba2ZnOsO6, Ba2CaOsO6, and Ba2MgOsO6. Our proposal for exciton condensation leading to type-I magnetic ordering is argued to be relevant to materials such as Sr2MgOsO6.PACS numbers: 75.25.aj, 75.40.Gb, 75.70.Tj Multipolar symmetry-breaking orders have been extensively discussed in f -orbital based lanthanide and actinide compounds, which host ions where spin-orbit coupling (SOC) is a dominant energy scale [1]. For instance, the "hidden order" state of URu 2 Si 2 has been proposed to host hexadecapolar symmetry breaking [2]. Another well-known example is cubic NpO 2 [3-6], where a large body of experiments have been reconciled in terms of a primary antiferro-triakontadipolar (rank-5 magnetic multipolar) symmetry breaking which drives secondary antiferro-quadrupolar order. In certain pyrochlore magnets, all-in all-out magnetic order has been proposed to lead to "effective octupoles" on tetrahedra [7]. Ongoing experimental [8-10] and theoretical investigations [11][12][13][14] of PrTi 2 Al 20 and PrV 2 Al 20 have also uncovered quadrupolar and ferro-octupolar orders.Recently, unconventional multipolar orders have also been proposed in d-orbital metals to occur as Pomeranchuk instabilities of spin-orbit coupled Fermi surfaces [15]. Specifically, metallic oxides which have d-orbital ions with large SOC, such as LiOsO 3 and Cd 2 Re 2 O 7 , have been proposed as potential candidates to realize this physics [15]. Experiments have indeed discovered an oddparity nematic metal in Cd 2 Re 2 O 7 below T c ∼ 200 K via optical second-harmonic generation [16]. Other proposed materials for hosting multipolar orders include A 2 OsO 4 (with A = K,Rb,Cs) [17].However, to the best of our knowledg...