We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.
Modifications of a recent model by Izumi et al. 1 on diffusion controlled growth of YBa 2 Cu 3 07x (123) superconductors are proposed, taking into account especially the engulfment process of the Y 2 BaCuOs (211) particles into the solidifying 123 interface. The proposed modifications are evidenced by experimental results and applied to explain microstructural features of the 123 superconducting material. In particular, the 1:1 correlation between 123 platelet thickness (planar defect spacing) and 211 particle size as described by Jin et al. 2 is explained by an observed bridge growth resulting in a zipper-like mechanism. By this mechanism the platelets grow together in an oriented way leading to a quasi single crystalline material.
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