Magnetization plateaus in quantum magnets—where bosonic quasiparticles crystallize into emergent spin superlattices—are spectacular yet simple examples of collective quantum phenomena escaping classical description. While magnetization plateaus have been observed in a number of spin-1/2 antiferromagnets, the description of their magnetic excitations remains an open theoretical and experimental challenge. Here, we investigate the dynamical properties of the triangular-lattice spin-1/2 antiferromagnet Ba3CoSb2O9 in its one-third magnetization plateau phase using a combination of nonlinear spin-wave theory and neutron scattering measurements. The agreement between our theoretical treatment and the experimental data demonstrates that magnons behave semiclassically in the plateau in spite of the purely quantum origin of the underlying magnetic structure. This allows for a quantitative determination of Ba3CoSb2O9 exchange parameters. We discuss the implication of our results to the deviations from semiclassical behavior observed in zero-field spin dynamics of the same material and conclude they must have an intrinsic origin.

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Antiferromagnetic insulators on the diamond lattice are candidate materials to host exotic magnetic phenomena ranging from spin-orbital entanglement to degenerate spiral ground-states and topological paramagnetism. Compared to other three-dimensional networks of magnetic ions, such as the geometrically frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism in real materials is less mature. In this work, we characterize the magnetic properties of model A-site spinels CoRh2O4 (cobalt rhodite) and CuRh2O4 (copper rhodite) by means of thermo-magnetic and neutron scattering measurements and perform group theory analysis, Rietveld refinement, meanfield theory, and spin wave theory calculations to analyze the experimental results. Our investigation reveals that cubic CoRh2O4 is a canonical S = 3/2 diamond-lattice Heisenberg antiferromagnet with a nearest neighbor exchange J = 0.63 meV and a Néel ordered ground-state below a temperature of 25 K. In tetragonally distorted CuRh2O4, competiting exchange interactions between up to third nearest-neighbor spins lead to the development of an incommensurate spin helix at 24 K with a magnetic propagation vector km = (0, 0, 0.79). Strong reduction of the ordered moment is observed for the S = 1/2 spins in CuRh2O4 and captured by our 1/S corrections to the staggered magnetization. Our work identifies CoRh2O4 and CuRh2O4 as reference materials to guide future work searching for exotic quantum behavior in diamond-lattice antiferromagnets.

We report the discovery of a spin one diamond lattice in NiRh2O4. This spinel undergoes a cubic to tetragonal phase transition at T = 440 K that leaves all nearest neighbor interactions equivalent. In the tetragonal phase, magnetization measurements show a Ni 2+ effective moment of p eff = 3.3(1) and dominant antiferromagnetic interactions with ΘCW = -11.3(7) K. No phase transition to a longrange magnetically ordered state is observed by specific heat measurements down to T = 0.1 K. Inelastic neutron scattering measurements on sub-stoichiometric NiRh2O4 reveal possible valencebond behavior and show no visible signs of magnetic ordering. NiRh2O4 provides a platform on which to explore the previously unknown and potentially rich physics of spin one interacting on the diamond lattice, including the realization of theoretically predicted quantum spin liquid and topological paramagnet states. arXiv:1701.06674v2 [cond-mat.str-el]

In the geometrically frustrated materials with the low-dimensional and small spin moment, the quantum fluctuation could interfere with the complicated interplay of the spin, electron, lattice and orbital interactions, and host exotic ground states such as nematic spin-state and chiral liquid phase. While the quantum phases of the one-dimensional chain and S -½ twodimensional triangular-lattice antiferromagnet (TLAF) had been more thoroughly investigated by both theorists and experimentalists, the work on S = 1 TLAF has been limited. We induced the lattice distortion into the TLAFs, A3NiNb2O9 (A = Ba, Sr, and Ca) with S (Ni 2+ ) = 1, and applied the thermodynamic, magnetic and neutron scattering measurements. Although A3NiNb2O9 kept the non-collinear 120° antiferromagnetic phase as the ground state, the Ni 2+ -lattice changed from the equilateral triangle (A = Ba) into isosceles triangle (A = Sr and Ca). The inelastic neutron scattering data were simulated by the linear spin-wave theory, and the competition between the single-ion anisotropy and the exchange anisotropy from the distorted lattice was discussed. I. INTRODUCTIONIn the geometrically frustrated system, the complicated interaction(s) between electron, phonon, spin and orbital could lead to degenerate ground states, which introduce exotic properties and attracted a lot attention over the past decades [1][2][3]. Meanwhile, those degenerate states would easily be held by the symmetrical inconsistency and be destroyed by significant quantum fluctuations, not only induced by the complicated interactions among low dimensionality, geometrical frustration, small spin and the applied magnetic field, but also modified the classical Heisenberg model [4][5]. The triangular-lattice antiferromagnet (TLAF), one of the simplest frustrated two-dimensional (2D) material, had been suggested to be strongly influenced by the strong quantum spin fluctuations with small effective spin (S = 1/2 or 1) and exhibited a rich variety of interesting physics [6][7][8].A striking example of these quantum phenomena is the transition from a non-collinear 120° spin configuration at 0 T into fractional-magnet lattice under a finite range of applied magnetic field, such as a collinear up-up-down (uud) phase corresponding to a magnetization plateau with one-third of its saturation value [9][10][11][12][13].Recently, the theoretical research indicated the uud state as a commensurate analogue of the incommensurate spin-density-wave which was predicted and observed for frustrated one-dimensional spin-1 chains and S = ½ TLAF [14, 15], and suggested the possibility for exotic magnetic excitations. While there was an enormous theoretical activity in the domain, a full consensus on the origin of the uud state, (even the ground magnetic state, non-collinear 120 o at zero field) was limited. This is true as well for the state-of-the art experimental investigation of its spin excitations due to the lack of a triangular-lattice materials. Although the lattice distortion has been believed to influence t...

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Effective separation of analogues structurally differing in a monounsaturated double bond remains challenging. In this study, hydrophilic ionic liquid (IL)/water mixtures were investigated as extractants for the extractive separation of hydrophobic analogues capsaicin and dihydrocapsaicin. A useful “diluent-swing” effect was introduced into the IL-based extraction and the recovery of solute for recycling the extractant. The extractive separation was performed at high IL concentrations where water behaves as a co-solvent, and back-extraction of the solute from the IL/water mixture was performed at low IL concentrations where water acts as an anti-solvent. The influences of IL structures (cations, anions, and substituent groups) and concentration, as well as the analogues concentration, on the separation efficiency were fully investigated. Selectivities of capsaicin versus dihydrocapsaicin of >2.0 were achieved by a number of IL/water mixtures, mainly because of the enhanced dipolarity/polarizability caused by the introduction of water to the extractant. Interestingly, a synergistic effect on the distribution coefficients and enhanced selectivities was achieved with N-ethylpyridinium bromide/water mixtures. This work sheds light on developing a feasible and effective large-scale separation method for compounds structurally different in their monounsaturated carbon–carbon double bonds.