Na 2 IrO 3 , a honeycomb 5d 5 oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal-metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na 2 IrO 3 , the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic-structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant 6 New J. Phys. 16 (2014) 013056 V M Katukuri et al ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin-orbit interactions, this strong dependence on distortions of the Ir 2 O 2 plaquettes singles out the honeycomb 5d 5 oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems. IntroductionThe Heisenberg model of magnetic interactions, J S i · S j between spin moments at sites {i, j}, has been successfully used as an effective minimal model to describe the cooperative magnetic properties of both molecular and solid-state many-electron systems. A less conventional spin model-the Kitaev model [1]-has been recently proposed for honeycomb-lattice materials with 90 • metal-oxygen-metal bonds and strong spin-orbit interactions [2]. It has nontrivial topological phases with elementary excitations exhibiting Majorana statistics, which are relevant and much studied in the context of topological quantum computing [1,[3][4][5][6][7]. Candidate materials proposed to host such physics are the honeycomb oxides Na 2 IrO 3 and Li 2 IrO 3 [2]. The magnetically active sites, the Ir 4+ species, display in these compounds a 5d 5 valence electron configuration, octahedral ligand coordination and bonding of nearest-neighbor (NN) Ir ions through two ligands [8,9]. In the simplest approximation, i.e. for sufficiently large t 2g -e g octahedral crystal-field splittings within the Ir 5d shell and degenerate Ir t 2g levels, the ground-state (GS) electron configuration at each Ir site is a t 5 2g effective j = 1/2 spin-orbit doublet [2,[10][11][12]. The anisotropic, Kitaev type coupling then stems from the particular form the superexchange between the Ir j = 1/2 pseudospins takes for 90 • bond angles on the Ir-O 2 -Ir plaquette [2,13,14].Recent measurements on Na 2 IrO 3 [8,9] indicate significant lattice distortions away from the idealized case of cubic IrO 6 octahedra and 90 • Ir-O-Ir bond angles for which the Kitaev-Heis...

This paper sheds light into the investigation of differential patterns of utilisation of physician services by populations subgroups that is emerging in a number of studies. Using Spanish data from the National Health Survey of 1997 we try to explain the distinct role of the type of insurance on the choice between specialists and GPs and its intertwining with the choice between private and public providers. We estimate a two-stages probit to conclude that differences in insurance access is the main determinant of both, the choice of sector and the kind of physician contacted, giving rise to very different patterns of consumption of GP and specialist visits. People with only public insurance go 2.8 times to the GP per one time that they visit a specialist; individuals with duplicate coverage have a ratio of GP/specialist visits equal to 1.4 (the combination being public GP and private specialist) and people with only private insurance access actually have an 'inverted' pattern of visits: they contact specialists more often than GPs. Age, sex and health and public supply characteristics also have a distinct and interesting impact on these choices. Finally, equity concerns based on the implied assumption that specialists care is superior to general practitioner care are discussed.

AimCurrent pricing of commercial mechanical ventilators in low-/middle-income countries (LMICs) markedly restricts their availability, and consequently a considerable number of patients with acute/chronic respiratory failure cannot be adequately treated. Our aim was to design and test an affordable and easy-to-build noninvasive bilevel pressure ventilator to allow a reduction in the serious shortage of ventilators in LMICs.MethodsThe ventilator was built using off-the-shelf materials available via e-commerce and was based on a high-pressure blower, two pressure transducers and an Arduino Nano controller with a digital display (total retail cost <75 USD), with construction details provided open source for free replication. The ventilator was evaluated, and compared with a commercially available device (Lumis 150 ventilator; Resmed, San Diego, CA, USA): 1) in the bench setting using an actively breathing patient simulator mimicking a range of obstructive/restrictive diseases; and b) in 12 healthy volunteers wearing high airway resistance and thoracic/abdominal bands to mimic obstructive/restrictive patients.ResultsThe designed ventilator provided inspiratory/expiratory pressures up to 20/10 cmH2O, respectively, with no faulty triggering or cycling; both in the bench test and in volunteers. The breathing difficulty score rated (1–10 scale) by the loaded breathing subjects was significantly (p<0.005) decreased from 5.45±1.68 without support to 2.83±1.66 when using the prototype ventilator, which showed no difference with the commercial device (2.80±1.48; p=1.000).ConclusionThe low-cost, easy-to-build noninvasive ventilator performs similarly to a high-quality commercial device, with its open-source hardware description, which will allow for free replication and use in LMICs, facilitating application of this life-saving therapy to patients who otherwise could not be treated.

We present a systematic study of the correlation-induced corrections to the electronic band structure of zinc-blende BN. Our investigation employs an ab initio wave-function-based local Hamiltonian formalism which offers a rigorous approach to the calculation of the polarization and local charge redistribution effects around an extra electron or hole placed into the conduction or valence bands of semiconducting and insulating materials. Moreover, electron correlations beyond relaxation and polarization can be readily incorporated. The electron correlation treatment is performed on finite clusters. In conducting our study, we make use of localized Wannier functions and embedding potentials derived explicitly from prior periodic Hartree-Fock calculations. The on-site and nearest-neighbor charge relaxation bring corrections of several eV to the Hartree-Fock band gap. Additional corrections are caused by long-range polarization effects. In contrast, the dispersion of the Hartree-Fock bands is marginally affected by electron correlations. Our final result for the fundamental gap of zinc-blende BN compares well with that derived from soft x-ray experiments at the B and N K-edges.