Unconventional magnetic order on the hyperhoneycomb Kitaev lattice in β\beta-Li2IrO3: full solution via magnetic resonant x-ray diffraction

The recently-synthesized iridate $\beta$-Li$_2$IrO$_3$ has been proposed as a candidate to display novel magnetic behavior stabilized by frustration effects from bond-dependent, anisotropic interactions (Kitaev model) on a three-dimensional "hyperhoneycomb" lattice. Here we report a combined study using neutron powder diffraction and magnetic resonant x-ray diffraction to solve the complete magnetic structure. We find a complex, incommensurate magnetic order with non-coplanar and counter-rotating Ir moments, which surprisingly shares many of its features with the related structural polytype "stripyhoneycomb" $\gamma$-Li$_2$IrO$_3$, where dominant Kitaev interactions have been invoked to explain the stability of the observed magnetic structure. The similarities of behavior between those two structural polytypes, which have different global lattice topologies but the same local connectivity, is strongly suggestive that the same magnetic interactions and the same underlying mechanism governs the stability of the magnetic order in both materials, indicating that both $\beta$- and $\gamma$-Li$_2$IrO$_3$ are strong candidates to realize dominant Kitaev interactions in a solid state material.Comment: 14 pages, 9 figure

Electrical switching of magnetic polarity in a multiferroic BiFeO3 device at room temperature

We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a (111)-BiFeO3 epitaxial-film device at room temperature by non-resonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed (111)-BiFeO3 epitaxial films consisting of a single ferroelectric domain were found to comprise a sub-micron-scale mosaic of magneto-elastic domains, all sharing a common direction of the magnetic polarity, which was found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magneto-elastic domain population. A real-space polarimetry map of our device clearly distinguished between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in BiFeO3 is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90 degrees at every cycle.Comment: 6 pages, 5 figures; corrected figure

Topology and temperature dependence of the diffuse X-ray scattering in Na0.5Bi0.5TiO3 ferroelectric single crystals

The results of high-resolution measurements of the diffuse X-ray scattering produced by a perovskite-based Na0.5Bi0.5TiO3 ferroelectric single crystal between 40 and 620 K are reported. The study was designed as an attempt to resolve numerous controversies regarding the average structure of Na0.5Bi0.5TiO3, such as the mechanism of the phase transitions between the tetragonal, P4bm, and rhombohedral | monoclinic, R3c | Cc, space groups and the correlation between structural changes and macroscopic physical properties. The starting point was to search for any transformations of structural disorder in the temperature range of thermal depoling (420–480 K), where the average structure is known to remain unchanged. The intensity distribution around the {032} pseudocubic reflection was collected using a PILATUS 100K detector at the I16 beamline of the Diamond Light Source (UK). The data revealed previously unknown features of the diffuse scattering, including a system of dual asymmetric L-shaped diffuse scattering streaks. The topology, temperature dependence, and relationship between Bragg and diffuse intensities suggest the presence of complex microstructure in the low-temperature R3c | Cc phase. This microstructure may be formed by the persistence of the higher-temperature P4bm phase, built into a lower-temperature R3c | Cc matrix, accompanied by the related long-range strain fields. Finally, it is shown that a correlation between the temperature dependence of the X-ray scattering features and the temperature regime of thermal depoling is present

Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

We report resonant X-ray scattering measurements on the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K-edge. The empirically extracted K-edge shift between the crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ~1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.Comment: 5 pages, 4 figure

KINEMATIC AND DYNAMIC ANALYSIS OF SPRINT START

Aims of this study were the tuning of the methods for kinematic and dynamic analysis of sprint start and the assessment of their relevance for field practice. Data collection was performed by means of an Elite Motion Analysis System (two cameras,3D) at 100 Hzi and a Kistler force platform (type 9281b ) at 500 Hz. Three elite athletes participated in the study; each subject performed three trials. statistical analysis ( ONEWAY ANOVA - SPSSPC software package) was performed in order to highlight significant differences ( F>0.05 ) between the subjects. The start movement was divided into three temporal phases: release of the starting block, landing on first supported push off from platform. significant differences were observed with respect to hip horizontal linear velocity within these phases (F > 0.02-0.001-0.0003) between the subjects. In order to give a complete description of the movement pattern four groups of variables were selected: 1)LINEAR DISPLACEMENTS. Eighteen variables were studied ; thirteen of them show significant differences index of different positions on the starting blocks -and different movement pattern during push-of f . 2)ANGULAR DISPLACEMENTS. Thirty-five variables were selected; eighteen show significant differences. 3) LINEAR AND ANGULAR-VEMCITIES Of the 54 variables studied (20 linear var. and 34 angular var.) , 38 (13 linear var. and 15 angular var. ) shows significant differences. 4)DYNAMIC DATA. We studied 62 variables, 26 of which showed significant differences. Exit velocity from starting blocks is influenced by the force produced during the first support phase and by the kinematics of the body segments. The different results obtained by the athletes in hip horizontal velocity, can be explained with the significant differences observed with respect to kinematic and dynamic of the phases of start studied. Main variables that shows significant differences are: hip, knee and ankle peak angular velocities, duration of first support phase and joint moments during push-off from first support phase. These variables seems to be logically related with the index of performance,hip horizontal velocity

Helical spin-waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14

We have investigated the spin fluctuations in the langasite compound Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The low temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously reported by Marty et al. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second order phase transition for a two dimensional triangular antiferromagnet. We will show that the physical properties and energy scales including the ordering wavevector, Curie-Weiss temperature, and the spin-waves can be explained through the use of only symmetric exchange constants without the need for the Dzyaloshinskii-Moriya interaction. This is accomplished through a set of ``helical" exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering which displays a strong vector chirality up to high temperatures well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound.Comment: 16 pages, 16 figures, submitted to Physical Review

Non-coplanar and counter-rotating incommensurate magnetic order stabilized by Kitaev interactions in γ\gamma-Li2IrO3

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here we explore the magnetism of $\gamma$-Li$_2$IrO$_3$, which has the topology of a 3D Kitaev lattice of inter-connected Ir honeycombs. Using resonant magnetic x-ray diffraction we find a complex, yet highly-symmetric incommensurate magnetic structure with non-coplanar and counter-rotating Ir moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally accounts for all key features of the observed magnetic structure. Our results provide strong evidence that $\gamma$-Li$_2$IrO$_3$ realizes a spin Hamiltonian with dominant Kitaev interactions.Comment: 10 pages, 7 figure