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The dispersion of the spin-waves in single crystals of Li2CuO2 has been investigated by means of inelastic neutron scattering. The results yield a single spin-wave branch characterized by a large gap ∆ = 1.4 meV at the zone centre due to easy-axis uniaxial anisotropy. Linear spin-wave theory is used to obtain the exchange integrals in this material. It turns out that the nearest-neighbor exchange interactions in the basal plane are antiferromagnetic leading to frustration between the magnetic moments. Our results show that Li2CuO2 is an S = 1/2 antiferromagnetic insulator with competing interactions.

In 1957, Abrikosov described how quanta of magnetic flux enter the interior of a bulk type II superconductor. It was subsequently predicted that, in an isotropic superconductor, the repulsive forces between the flux lines would cause them to order in two dimensions, forming a hexagonal lattice. Flux-line lattices with different geometry can also be found in conventional (type II) superconductors; however, the ideal hexagonal lattice structure should always occur when the magnetic field is applied along a hexagonal crystal direction. Here we report measurements of the orientation of the flux-line lattice in the heavy-fermion superconductor UPt3, for this special case. As the temperature is increased, the hexagonal lattice, which is initially aligned along the crystal symmetry directions, realigns itself with the anisotropic superconducting gap. The superconductivity in UPt3 is unusual (even compared to unconventional oxide superconductors) because the superconducting gap has a lower rotational symmetry than the crystal structure. This special feature enables our data to demonstrate clearly the link between the microscopic symmetry of the superconductivity and the mesoscopic physics of the flux-line lattice. Moreover, our observations provide a stringent test of the theoretical description of the unconventional superconductivity in UPt3.

The possibility of magnetic-order induced phonon anisotropy in single crystals of MnO and NiO is investigated using inelastic neutron scattering. Below TN both compounds exhibit a splitting in their transverse optical phonon spectra of approximately 10%. This behavior illustrates that, contrary to general assumption, the dynamic properties of MnO and NiO are substantially non-cubic.PACS numbers: 75.30.Gw, 78.70.Nx The failure of ab initio approximations to correctly incorporate many-body effects such as electron exchange and correlation is an issue at the core of contemporary solid-state research. Although these effects are important in almost all solids, they are essential for a proper description of co-operative phenomena such as antiferromagnetism, charge-ordering, superconductivity and colossal magnetoresistance.Despite the status of MnO and NiO as benchmark materials for the study of correlated electron systems and frequent use in first principles electronic structure investigations, many aspects of the physics of the 3d transition metal monoxides requires better theoretical explanation. For example, the basic physical properties predicted for MnO and NiO (e.g. band-gaps, distortion angles and phonon spectra) differ radically depending upon which techniques have been employed [1,2].In a recent publication comparing different ab initio and model band-structure models on MnO (including local spin-density approximation (LSDA) and 'LSDA + model' calculations), Massidda et al. [2] suggest that although the electronic density in MnO is approximately cubic, the lowering of symmetry associated with antiferromagnetic ordering can induce an electronic response that is significantly non-cubic. This results in the dynamical properties of the system, such as transverse optical (TO) phonons, exhibiting substantial "magneticorder induced anisotropy".One of the predicted features from such calculations is that the zone center (ZC) optical phonon modes should split depending on their polarization. A higher energy occurs for a mode polarized along the [111] direction and a lower energy for degenerate modes polarized in the orthogonal ferromagnetic plane, see Table I. Massidda et al. [2] find that this effect arises solely due to the magnetic ordering, and would even occur in the absence of any rhombohedral distortion. By comparing results from four different approximation schemes, lower and upper bounds of 3-10% for the magnitude of the splitting are estimated. Variations in the splitting due to the distortion are calculated to be less than 1%.Although similar theoretical calculations have not yet been performed for NiO, the results of measurements of NiO TO phonons are predicted to be qualitatively similar to those in MnO. Indeed, it has been suggested that the splitting in NiO may be even greater in absolute magnitude than in MnO due to the larger magnetic superexchange [3].MnO and NiO are classic examples of type-II antiferromagnets possessing the cubic rock-salt structure. Below T N , exchange-striction causes contra...

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Abstract:We report on a study of various RVO 3 single-crystal samples (R = La, Nd, Sm, Gd, Er and Y) which show temperature-induced magnetization reversal. For compounds with lighter rare-earths (R = La, Nd and Sm), magnetization reversal can be observed for magnetic field applied in the ab plane and along the c axis, whereas for the heavy rare-earths (R = Gd, Er) magnetization reversal is only observed when the field is applied along the a axis. YVO 3 has a magnetization reversal along all the main crystallographic axes in a modest applied field. We have found that some compounds are very sensitive to small trapped fields present in the superconducting solenoid of the magnetometer during the cooling. Based on the observed results, we argue that inhomogeniety caused by defects in the orbital sector in the quasi onedimensional orbital systems could account for the unusual magnetization reversal.

Two superconducting phases of Re3W have been found with different physical properties. One phase crystallizes in a non-centrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature, Tc, of 7.8 K. The other phase has a hexagonal centrosymmetric structure and is superconducting with a Tc of 9.4 K. Switching between the two phases is possible by annealing the sample or remelting it. The properties of both phases of Re3W have been characterized by powder neutron diffraction, magnetization, and resistivity measurements. The temperature dependence of the lower and the upper critical fields have been measured for both phases. These are used to determine the penetration depths and the coherence lengths for these systems.Comment: 6 pages, to appear in Physical Review

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