In spin ice research, small variations in structure or interactions drive a multitude of different behaviors, yet the collection of known materials relies heavily on the '227' pyrochlore structure. Here, we present thermodynamic, structural and inelastic neutron scattering data on a new spin-ice material, MgEr2Se4, which contributes to the relatively under-explored family of rare-earth spinel chalcogenides. X-ray and neutron diffraction confirm a normal spinel structure, and places Er 3+ moments on an ideal pyrochlore sublattice. Measurement of crystal electric field excitations with inelastic neutron scattering confirms that the moments have perfect Ising character, and further identifies the ground state Kramers doublet as having dipolar-octupolar form with a significant multipolar character. Heat capacity and magnetic neutron diffuse scattering have ice-like features, but are inconsistent with Monte Carlo simulations of the nearest-neighbor and next-nearest-neighbor dipolar spin-ice (DSI) models. A significant remnant entropy is observed as T → 0 K, but again falls short of the full Pauling expectation for DSI, unless significant disorder is added. We show that these observations are fully in-line with what is recently reported for CdEr2Se4, and point to the importance of quantum fluctuations in these materials.arXiv:1703.04267v3 [cond-mat.dis-nn]

The symmetry of local moments plays a defining role in the nature of exotic grounds states stabilized in frustrated magnetic materials. We present inelastic neutron scattering (INS) measurements of the crystal electric field (CEF) excitations in the family of compounds MgRE2Se4 (RE ∈ {Ho, Tm, Er and Yb}). These compounds form in the spinel structure, with the rare earth ions comprising a highly frustrated pyrochlore sublattice. Within the symmetry constraints of this lattice, we fit both the energies and intensities of observed modes in the INS spectra to determine the most likely CEF Hamiltonian for each material and comment on the ground state wavefunctions in the local electron picture. In this way, we experimentally confirm MgTm2Se4 has a non-magnetic ground state, and MgYb2Se4 has effective S = 1 2 spins with g = 5.188(79) and g ⊥ = 0.923(85) µB. The spectrum of MgHo2Se4 indicates a ground state doublet containing Ising spins with g = 2.72(46), though low-lying CEF levels are also seen at thermally accessible energies δE = 0.591(36), 0.945(30) and 2.88(7) meV, which can complicate interpretation. These results are used to comment on measured magnetization data of all compounds, and are compared to published results on the material MgEr2Se4. :1906.10767v1 [cond-mat.dis-nn] arXiv

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Ferrite microwave circulators allow one to control the directional flow of microwave signals and noise, and thus play a crucial role in present-day superconducting quantum technology. They are typically viewed as a black-box, and their internal structure is not specified, let alone used as a resource. In this work, we demonstrate a low-loss waveguide circulator constructed with single-crystalline yttrium iron garnet (YIG) in a 3D cavity, and analyze it as a multi-mode hybrid quantum system with coupled photonic and magnonic excitations. We show the coherent coupling of its chiral internal modes with integrated superconducting niobium cavities, and how this enables tunable non-reciprocal interactions between the intra-cavity photons. We also probe experimentally the effective non-Hermitian dynamics of this system and its effective non-reciprocal eigenmodes. The device platform provides a test bed for implementing non-reciprocal interactions in open-system circuit QED.