Models and Materials for Generalized Kitaev Magnetism

The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by G. Jackeli and G. Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na$_2$IrO$_3$, $\alpha$-Li$_2$IrO$_3$, and $\alpha$-RuCl$_3$), three-dimensional Kitaev materials ($\beta$- and $\gamma$-Li$_2$IrO$_3$), and other potential candidates, completing the review with the list of open questions awaiting new insights.Comment: updated references, published versio

The grapheme-valued Wright-Fisher diffusion with mutation

In [Athreya, den Hollander, R\"ollin; 2021, arXiv:1908.06241] models from population genetics were used to define stochastic dynamics in the space of graphons arising as continuum limits of dense graphs. In the present paper we exhibit an example of a simple neutral population genetics model for which this dynamics is a Markovian diffusion that can be characterised as the solution of a martingale problem. In particular, we consider a Markov chain in the space of finite graphs that resembles a Moran model with resampling and mutation. We encode the finite graphs as graphemes, which can be represented as a triple consisting of a vertex set, an adjacency matrix and a sampling measure. We equip the space of graphons with convergence of sample subgraph densities and show that the grapheme-valued Markov chain converges to a grapheme-valued diffusion as the number of vertices goes to infinity. We show that the grapheme-valued diffusion has a stationary distribution that is linked to the Poisson-Dirichlet distribution. In a companion paper [Greven, den Hollander, Klimovsky, Winter; 2023], we build up a general theory for obtaining grapheme-valued diffusions via genealogies of models in population genetics.Comment: 25 page

Editorial: Radicalization and deradicalization: Processes and contexts

© 2022 Winter, Morrison and van den Bos. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). https://creativecommons.org/licenses/by/4.0/Peer reviewe

Duurzaamheid in de ogen van de consument; Consumentenonderzoek Duurzaam in retail

Consumentenonderzoek waarin op een kwalitatieve manier onderzocht is hoe mainstream-consumenten duurzaamheid in het algemeen en in retail beleven. Dit rapport verschijnt als onderdeel van Duurzaam in retail. A consumer study involving qualitative research into how mainstream consumers perceive sustainability in general and within the retail sector. This report has been published as part of Duurzaam in retail (Sustainability in the retail sector)

The Stellar Composition of the Star Formation Region CMa R1. II. Spectroscopic and Photometric Observations of 9 Young Stars

We present new high and low resolution spectroscopic and photometric data of nine members of the young association CMa R1. All the stars have circumstellar dust at some distance as could be expected from their association with reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars with discs. Our photometric and spectroscopic observations on these stars reveal new characteristics of their variability. We present first interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha stars in terms of a partially eclipsing binary, a magnetic activity cycle and circumstellar dust variations, respectively. The remaining five stars show no clear indications of Halpha emission in their spectra, although their spectral types and ages are comparable with those of HD 52721 and HD 53367. This indicates that the presence of a disc around a star in CMa R1 may depend on the environment of the star. In particular we find that all Halpha emission stars are located at or outside the arc-shaped border of the H II region, which suggests that the stars inside the arc have lost their discs through evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc) supernova, about 1 Myr ago.Comment: 17 pages, 13 figures, accepted by MNRA

Differences in collagen prolyl 4-hydroxylase assembly between two Caenorhabditis nematode species despite high amino acid sequence identity of the enzyme subunits

The collagen prolyl 4-hydroxylases (P4Hs) are essential for proper extracellular matrix formation in multicellular organisms. The vertebrate enzymes are &#945;2&#946;2 tetramers, in which the &#946; subunits are identical to protein disulfide isomerase (PDI). Unique P4H forms have been shown to assemble from the &lt;i&gt;Caenorhabditis&lt;/i&gt; &lt;i&gt;elegans&lt;/i&gt; catalytic &#945; subunit isoforms PHY-1 and PHY-2 and the &#946; subunit PDI-2. A mixed PHY-1/PHY-2/(PDI-2)&lt;sub&gt;2&lt;/sub&gt; tetramer is the major form, while PHY-1/PDI- 2 and PHY-2/PDI-2 dimers are also assembled but less efficiently. Cloning and characterization of the orthologous subunits from the closely related nematode &lt;i&gt;Caenorhabditis&lt;/i&gt; &lt;i&gt;briggsae&lt;/i&gt; revealed distinct differences in the assembly of active P4H forms in spite of the extremely high amino acid sequence identity (92-97%) between the &lt;i&gt;C. briggsae&lt;/i&gt; and &lt;i&gt;C. elegans&lt;/i&gt; subunits. In addition to a PHY-1/PHY-2(PDI-2)&lt;sub&gt;2&lt;/sub&gt; tetramer and a PHY-1/PDI-2 dimer, an active (PHY- 2)&lt;sub&gt;2&lt;/sub&gt;(PDI-2)&lt;sub&gt;2&lt;/sub&gt; tetramer was formed in &lt;i&gt;C. briggsae&lt;/i&gt; instead of a PHY-2/PDI-2 dimer. Site-directed mutagenesis studies and generation of inter-species hybrid polypeptides showed that the N-terminal halves of the &lt;i&gt;Caenorhabditis&lt;/i&gt; PHY-2 polypeptides determine their assembly properties. Genetic disruption of &lt;i&gt;C. briggsae phy-1&lt;/i&gt; (&lt;i&gt;Cb-dpy-18&lt;/i&gt;) via a &lt;i&gt;Mos1&lt;/i&gt; insertion resulted a small (short) phenotype that is less severe than the dumpy (short and fat) phenotype of the corresponding &lt;i&gt;C. elegans&lt;/i&gt; mutants (&lt;i&gt;Ce-dpy-18&lt;/i&gt;). &lt;i&gt;C. briggsae&lt;/i&gt; &lt;i&gt;phy-2&lt;/i&gt; RNA interference produced no visible phenotype in the wild type nematodes but produced a severe dumpy phenotype and larval arrest in &lt;i&gt;phy-1&lt;/i&gt; mutants. Genetic complementation of the &lt;i&gt;C. briggsae&lt;/i&gt; and &lt;i&gt;C. elegans&lt;/i&gt; &lt;i&gt;phy-1&lt;/i&gt; mutants was achieved by injection of a wild type &lt;i&gt;phy-1&lt;/i&gt; gene from either species

A young stellar group associated with HD 199143 (d = 48 pc)

We present new optical and ultraviolet spectroscopy of the anomalous EUV emitter HD 199143 (F8V). High resolution spectra in the Halpha and Na I D wavelength regions show evidence for very rapid (a few hundred km/s) rotation of the stellar photosphere. Using archive IRAS data we also show that the star has excess emission above photospheric levels at 12 micron. IUE data of HD199143 reveal the presence of emission lines of Mg II, C I, C II, C III, C IV, Si IV, He II and N V and show a large variability, both in the continuum and line fluxes. We propose that all available data of HD 199143 can be explained by assuming that is has been spun up by accretion of material from a close T Tauri like companion, responsible for the emission lines, the ultraviolet variability and the excess infrared emission. The bursting or flaring nature of this object, mostly in high energies, could be explained as episodic mass transfer between the star and its close companion. We show that HD 199143 and the Li-rich late-type dwarf BD-17 6128 form a physical pair and suggest that both may be part of a new nearby (48 pc) young (approx. 10^7 yr) stellar association in Capricornius.Comment: 4 pages, 6 figures, to appear in Astronomy & Astrophysics (Letters