Effect of isoelectronic doping on honeycomb lattice iridate A_2IrO_3

We have investigated experimentally and theoretically the series (Na$_{1-x}$Li$_{x}$)$_{2}$IrO$_{3}$. Contrary to what has been believed so far, only for $x\leq0.25$ the system forms uniform solid solutions. For larger Li content, as evidenced by powder X-ray diffraction, scanning electron microscopy and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na$_{3}$LiIr$_2$O$_{6}$ phase with alternating Na$_3$ and LiIr$_2$O$_6$ planes, and a Li-rich phase close to pure Li$_{2}$IrO$_{3}$. For $x\leq 0.25$ we observe (1) an increase of $c/a$ with Li doping up to $x=0.25$, despite the fact that $c/a$ in pure Li$_{2}$IrO$_{3}$ is smaller than in Na$_{2}$IrO$_{3}$, and (2) a gradual reduction of the antiferromagnetic ordering temperature $T_{N}$ and ordered moment. The previously proposed magnetic quantum phase transition at $x\approx 0.7$ may occur in a multiphase region and its nature needs to be re-evaluated.Comment: 8 pages, 9 figures including supplemental informatio

Notas corológicas del macrofitobentos de Andalucía (España). VII

El presente trabajo añade nuevas especies a los catálogos de algas Rhodophyceae (Conde et al., 1996b), Phaeophyceae (Flores- Moya et al., 1995a) y Chlorophyceae (Flores-Moya et al., 1995b) existentes para las costas andaluzas, aumentando de este modo el conocimiento florístico del macrofitobentos de Andalucía. La mayoría de las aportaciones proceden de cubetas intermareales de la Isla de las Palomas (Tarifa, Cádiz), excepto una de Punta Carnero (Cádiz) y una de Marina del Este (Granada). En conjunto se aportan una cita nueva para Andalucía (señalada con un asterisco), cinco citas nuevas y tres confirmaciones para la provincia de Cádiz, y una novedad para el catálogo de la provincia de Granada.El material recolectado fue preservado en formol al 4% en agua de mar, identificado y depositado en el herbario de la Universidad de Málaga (MGC Phyc)

The monoclinic crystal structure of α\alpha-RuCl3_3 and the zigzag antiferromagnetic ground state

The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.Comment: 13 pages, 9 figures, published in Physical Review

Synthesis and characterisation of end-functionalised poly(N-vinylpyrrolidone) additives by reversible addition–fragmentation transfer polymerisation

We describe herein the synthesis of a series of multi-end functionalized poly(N-vinyl pyrrolidone) (PVP) additives bearing two or three C8F17 fluoroalkyl (CF) groups, designed as additives to modify surface properties. The PVP additives were prepared by reversible addition–fragmentation transfer (RAFT) polymerization, with end functionality imparted via the use of CF functionalized chain transfer agents (CTAs). The resulting PVP additives, when used in modest quantities dispersed in thin films of an unmodified PVP matrix significantly reduce the surface energy, rendering their surfaces more hydrophobic and lipophobic. This is achieved by virtue of the low surface energy of the pendant C8F17 end groups which cause the additive to spontaneously surface segregate during the spin coating process. The resulting thin films have been characterized by static contact angle measurements using dodecane as the contact fluid, and the impact of additive molecular weight, matrix molecular weight, the number of CF groups and additive concentration upon surface properties is reported herein. Significant increases in contact angle were observed with increasing additive concentration, up to a critical aggregation concentration (CAC). Increasing the number of CF groups (from 2 to 3); reducing additive molecular weight or increasing the matrix molecular weight, resulted in increased contact angles and hence surface lipophobicity. Rutherford backscattering (RBS) analysis was performed on films containing varying concentrations of additive, in order to quantitatively measure the near-surface fluorine concentration of these films. The results of these experiments were in excellent agreement with those obtained by contact angle analysis, confirming the surface activity and low surface energy of the additives

Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe$_2$As$_2$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 K and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\Gamma$-point and an electron pocket at each of the X-points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation of the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.Comment: 11 pages, 5 figure

Crossing the Gap:Older Adults Do Not Create Less Challenging Stepping Stone Configurations Than Young Adults

Background Despite the known health benefits of physical activity, the number of older adults exercising regularly remains low in many countries. There is a demand for public open space interventions that can safely train balance, muscular strength, and cardiovascular fitness. In this participatory design study, older adults and young adults were to create their own stepping stones configurations. We provided them with six stepping stones, and examined the gap widths that each group of participants created and how they used the configurations. Results The created absolute gap distances by the older adults were smaller than those of the younger adults. Yet, the amount of challenge (in terms of the created gap widths relative to a person's estimated stepping capability) did not significantly differ between the young and older adults. Furthermore, both groups created non-standardized stepping stone configurations in which the number of different gap widths did not significantly differ between the young and older adults. Interestingly, while using their personalized design, older adults made significantly more gap crossings than younger adults over a given timespan. This finding tentatively suggests that personalized design invites physical activity in older adults. Conclusion The present study demonstrated that older adults are not more conservative in designing their own stepping stone configuration than young adults. Especially in light of the public health concern to increase physical activity in seniors, this is a promising outcome. However, field tests are needed to establish whether the older adults' stepping stones designs also invite physical activity in their daily environment