Spin-ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice. Their low-temperature magnetic state has been predicted to be a phase that obeys a Gauss' law and supports magnetic monopole excitations: in short, a Coulomb phase. We used polarized neutron scattering to show that the spin-ice material Ho2Ti2O7 exhibits an almost perfect Coulomb phase. Our result proves the existence of such phases in magnetic materials and strongly supports the magnetic monopole theory of spin ice.
We found that the high-pressure-synthesized material LiOsO 3 (see Supplementary Information) shows a structural transition at a temperature T s = 140 K. The room-temperature crystal structure of LiOsO 3 was initially examined using powder X-ray diffraction (XRD). The Goldschmidt diagram predicts that LiOsO 3 crystallizes into a LiNbO 3 -type structure 3,10 , and a preliminary refinement of the structure was carried out in the R-3c space group with Os at the 6b site 0,0,0 and O at the 18e siteTo investigate the position of the Li ion we turned to neutron diffraction, which is much more sensitive to Li than XRD. The neutron diffraction patterns collected above T s could be successfully described in the R-3c space group, in agreement with the XRD refinement, with the Li ion at the 6a position 0,0,1/4. Atomic absorption spectrometry (see Supplementary Information) indicated that the average Li mass was 2.77%, which corresponds to the composition Li 0.98 OsO 3 . We have used the stoichiometric composition throughout the structural analysis. The refinement indicated highly anisotropic thermal displacements of the Li ions with considerable extension along the c-axis (Table 1 and Fig. 1), which might indicate that the Li ions are distributed equally among equivalent 12c sites 0,0,z and 0,0,1/2-z either side of the oxygen layer at z = 1/4, as reported for LiNbO 3 and LiTaO 3 (refs 3, 11).The thermal variation of the structure of LiOsO 3 was studied by neutron diffraction for temperatures between 10 and 300 K. Figure 1a-d shows structural data obtained from refinements in the R-3c space group. The lattice parameters ( Fig. 1a) decrease uniformly from 300 K until T s = 140 K, below which the parameter c increases and a decreases with only a small variation in the unit-cell volume. Just below T s , the non-symmetry-breaking strain components e xx + e yy and e zz vary almost linearly (Fig. 1b). These 4 results show that the phase transition is continuous and the strain components behave like a secondary order parameter coupled to a primary one via a linear-quadratic free energy invariant 12 . The primary order parameter must necessarily be symmetry-breaking according to Landau's theory of second-order phase transitions 12 . Furthermore, the anisotropic thermal parameter 33 , which describes Li displacements along the c-axis, increases markedly below T s (Fig. 1c). This indicates that the primary structural instability involves the position of the Li ions along the c-axis (Fig. 1d).Given that the phase transition involves a change in symmetry, we find from representation theory 13 that there are three isotropy subgroups, R-3, R32 and R3c, which maintain the translational invariance of the R-3c space group and allow the transition to be continuous. These space groups were tested by refinement against the neutron diffraction data at 10 K. Note that R-3 and R32 should generate additional reflections below T s which were not observed in the experiment. The refinement in the non-centrosymmetric R3c space group gave the best de...
Lithium iron arsenide phases with compositions close to LiFeAs exhibit superconductivity at temperatures at least as high as 16 K, demonstrating that superconducting [FeAs](-) anionic layers with the anti-PbO structure type occur in at least three different structure types and with a wide range of As-Fe-As bond angles.
Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.
We report neutron scattering measurements of cooperative spin excitations in antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide superconductor. The data extend up to ∼ 100 meV and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.PACS numbers: 74.25. Ha, 74.70.Dd, 75.30.Ds, 78.70.Nx One of the greatest challenges presented by the recently discovered iron pnictide superconductors 1 is to identify the electron pairing interaction which permits the formation of a superconducting condensate. In conventional superconductors this interaction is provided by the exchange of a phonon. For the iron pnictides, however, theoretical calculations 2,3 indicate that the electron-phonon coupling is too weak to account for the observed high critical temperatures. Attention has therefore turned to other types of bosonic excitations which could mediate the pairing interaction.One such candidate is spin fluctuations 4 . In common with the layered cuprates, superconductivity in the pnictides is found in close proximity to parent phases which exhibit long-range antiferromagnetic order 5,6 . However, unlike the cuprates, whose magnetic properties are governed by strong superexchange interactions between localized spin-1 2 moments in a single Cu 3d x 2 −y 2 orbital, magnetism in the pnictides is more itinerant in character and derives from multiple d orbitals. It may also involve a degree of frustration. In magnetically ordered materials the dominant magnetic excitations are coherent spin waves. Wavevector-resolved measurements of the spinwave spectrum by inelastic neutron scattering provide information on the fundamental magnetic interactions and can also reveal effects due to itinerancy and frustration. Such studies on the magnetically ordered parent phases of unconventional superconductors like the cuprates and iron pnictides are important to establish the characteristic energy scales of the spin fluctuations and also to provide a reference against which changes associated with superconductivity can be identified.Here we present neutron scattering data on the collective spin excitations in antiferromagnetic BaFe 2 As 2 . We find that the spin excitation spectrum has a very steep dispersion within the FeAs layers with a bandwidth in the region of 0.17 eV, not much less than that in the cuprates. Such a high characteristic energy suggests that spin fluctuations are a serious candidate to mediate high temperature superconductivity in the iron pnictides.The parent phase BaFe 2 As 2 becomes superconduct- ing on doping with holes 7 or on application of pressure 8 . At T s = 140 K, BaFe 2 As 2 undergoes a structural transition from tetragonal to orthorhombic and simultaneously develops three-dimensional long-range antiferromagnetic order 9,10...
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