Wurtzite vs rock-salt MnSe epitaxy: electronic and altermagnetic properties

Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from the specific symmetry operations that connect the spin sublattices. In this report, we show with ab initio calculations that the semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and the more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism

Bi incorporation and segregation in the MBE-grown GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments

Fine-Scale Skeletal Banding Can Distinguish Symbiotic from Asymbiotic Species among Modern and Fossil Scleractinian Corals.

Understanding the evolution of scleractinian corals on geological timescales is key to predict how modern reef ecosystems will react to changing environmental conditions in the future. Important to such efforts has been the development of several skeleton-based criteria to distinguish between the two major ecological groups of scleractinians: zooxanthellates, which live in symbiosis with dinoflagellate algae, and azooxanthellates, which lack endosymbiotic dinoflagellates. Existing criteria are based on overall skeletal morphology and bio/geo-chemical indicators-none of them being particularly robust. Here we explore another skeletal feature, namely fine-scale growth banding, which differs between these two groups of corals. Using various ultra-structural imaging techniques (e.g., TEM, SEM, and NanoSIMS) we have characterized skeletal growth increments, composed of doublets of optically light and dark bands, in a broad selection of extant symbiotic and asymbiotic corals. Skeletons of zooxanthellate corals are characterized by regular growth banding, whereas in skeletons of azooxanthellate corals the growth banding is irregular. Importantly, the regularity of growth bands can be easily quantified with a coefficient of variation obtained by measuring bandwidths on SEM images of polished and etched skeletal surfaces of septa and/or walls. We find that this coefficient of variation (lower values indicate higher regularity) ranges from ~40 to ~90% in azooxanthellate corals and from ~5 to ~15% in symbiotic species. With more than 90% (28 out of 31) of the studied corals conforming to this microstructural criterion, it represents an easy and robust method to discriminate between zooxanthellate and azooxanthellate corals. This microstructural criterion has been applied to the exceptionally preserved skeleton of the Triassic (Norian, ca. 215 Ma) scleractinian Volzeia sp., which contains the first example of regular, fine-scale banding of thickening deposits in a fossil coral of this age. The regularity of its growth banding strongly suggests that the coral was symbiotic with zooxanthellates

Defect-free SnTe topological crystalline insulator nanowires grown by molecular beam epitaxy on graphene

SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have a cubic rock-salt structure, they grow along the [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with a Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out the necessity of depositing a protective capping layer in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states

Regular growth increments of TDs and some mineralogical features of the skeleton of the Triassic (Lower Norian) <i>Volzeia</i> sp. from Alakir Ҫay, Turkey (ZPAL H.21/26).

<p>Optical microscope images of transverse thin sections of the skeleton (A,B). Yellow rectangle in B indicates area selected for micro-Raman (C-E) and cathodoluminesence microscope (H), black rectangle in B indicates area selected for SEM (F), and red rectangle in B indicates area selected for WDS analysis (G). Regions of TDs with regular growth increments (enlarged in B) may co-occur with those without banding (blue arrow in A). Micro-Raman maps of aragonite (C), calcite (D) and superimposed images of both polymorphs (E) extracted from raw Raman data through a direct classical least squares (DCLS) modelling procedure. SEM micrograph (F) shows presence of regular growth layers (red arrows). WDS map (G) shows distribution of Mg. White color corresponds to high Mg concentrations; black color corresponds to low Mg concentration. Red luminescence on CL image (H) corresponds to regions composed of calcite, whereas the absence of luminescence corresponds to regions composed of aragonite.</p

Statistical analysis of regularity of growth increments in modern and fossil Scleratinia.

<p>Plot illustrates relationship between regularity of increments (expressed as coefficient of variation [%] of dispersion of values of bands thickness obtained from each skeleton) and presence/lack of endosymbiotic algae. Thickness of growth bands was measured along individual fibers (red marks in image in the upper-right corner). Orange dots indicate azooxanthellate corals, green squares are zooxanthellate corals, pink star is exceptionally well preserved fossil (Triassic) specimen from Antalya, Turkey. Database of all measurements is given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0147066#pone.0147066.s004" target="_blank">S1 Table</a>.</p

Structure of banded Thickening Deposits in skeleton of extant zooxanthellate coral <i>Goniastrea stelligera</i> (ZPAL H.25/47).

<p>Transmitted light microscopy (polarized <b>A</b>, normal light <b>D</b>, and corresponding Scanning Electron Microscopy images (SEM; polished and etched section, <b>B, E</b>). TEM bright field (BF) images (<b>C</b>, <b>F</b>, <b>G</b>) of 21.5μm long FIB lamella (yellow rectangle marks approximate position on etched polished section near border of “FIB crater”). Composite image of FIB lamella (G) consists of aligned of 7 individual images. The contrast difference is related to variation of the lamella thickness; image contrast and clarity of image drops with the lamella thickness. Orientation of FIB lamella is approximately parallel to <i>c</i>-axis of aragonite fibers which can be distinguished due to differences in orientation (boundaries of some fibers marked with yellow arrows in C, F); length of most of fibers exceeds 10 μm which is consistent with SEM images. STEM-HAADF image (F) of the thinnest part of lamella (frame in g); camera length was fixed to 360mm and contrast is related to elastically scattered electrons i.e., orientation of the crystals and average local thickness. The black dots correspond to the voids into structure (C, enlarged part of F). Density of voids is not homogenous—dotted-line in F was subjectively drawn to distinguish zone with particularly numerous voids and zone with relatively few voids. Voids (especially in region right to the dotted line) are also present in monocrystalline fibers suggesting encapsulation of organic material during fiber grow. Region rich in voids most likely corresponds to darker zones in optical microscope: light scattering by voids and by smaller-size crystals differs from that of long monocrystalline fibers. Also etching of defective zone is faster that may explain differences in etching relief of darker vs. lighter banding.</p

Growth increments of TDs in extant azooxanthellate corals.

<p>SEM micrographs of polished and etched skeletons of various azooxanthellate corals (photographs are in pairs: upper and lower photo): (A,D) <i>Bathelia candida</i> (ZPAL H.25/59), (B,E) <i>Cyathelia axillaris</i> (ZPAL H.25/61), (C,F) <i>Hoplangia durotrix</i> (ZPAL H.25/65), (G,J) <i>Caryophyllia inornata</i> (ZPAL H.25/60), (H,K) <i>Paracyathus pulchellus</i> (ZPAL H.25/68), (I,L) <i>Desmophyllum dianthus</i> (ZPAL H.25/62), (M,P) <i>Stephanocyathus paliferus</i> (ZPAL H.25/70), (N,R) <i>Leptopsammia pruvoti</i> (ZPAL H.25/66) and (O,S) <i>Gardineria</i> sp. (ZPAL H.25/64). Thickening Deposits form irregular (yellow arrows) and discontinuous layers (e.g., red dotted line in J).</p

Some geochemical and structural features of regularly banded TDs of extant zooxanthellate corals.

<p>(<b>A</b>) The distribution of magnesium (²⁴Mg/⁴⁴Ca) in TDs of <i>L</i>. <i>scutaria</i> (ZPAL H.25/49); NanoSIMS image—mapped regions marked with red rectangles in <b>B</b> (thin-section in transmitted light). Blue colors correspond to relatively low Mg concentrations; red to yellow colors correspond to increasingly high Mg concentrations. (d) NanoSIMS profile of Mg/Ca across banded region of TDs (profile region marked with yellow line in A). (C,E) Transverse section of the skeleton of <i>C</i>. <i>lacrymalis</i> (ZPAL H.25/43) in transmitted light (C), and mapped with Wavelength Dispersive Spectroscopy technique (E) to show distribution of Mg. Consistent with the NanoSIMS maps, layers of the fibrous skeleton (TDs) exhibit micro-scale zoning in Mg concentration. The distribution of Mg in <i>P</i>. <i>damicornis</i> (ZPAL H.25/54) skeleton visualized with NanoSIMS (F), the corresponding skeletal ultrastructure (g, etched polished section of the region imaged in F), and the two images superimposed (H). Enlarged part of H (I, encircled) suggests that regions showing negative etching relief are enriched in Mg (white arrows in I).</p

Regular banding of TDs in extant zooxanthellate corals.

<p>SEM micrographs of polished and etched skeletons of various zooxanthellate corals (photographs are in pairs: upper and lower photo): (A,D) <i>Goniastrea stelligera</i> (ZPAL H.25/47), (B,E) <i>Symphyllia valenciennesii</i> (ZPAL H.25/57), (C,F) <i>Lobophyllia hemprichii</i> (ZPAL H.25/50), (G,J) <i>Pocillopora damicornis</i> (ZPAL H.25/54), (H,K) <i>Porites porites</i> (ZPAL H.25/55), (I,L) <i>Goniastrea retiformis</i> (ZPAL H.25/45), (M,P) <i>Acanthastrea echinata</i> (ZPAL H.25/42), (N,R) <i>Galaxea fascicularis</i> (ZPAL H.25/44) and (O,S) <i>Merulina ampliata</i> (ZPAL H.25/52). Thickening Deposits form regular (red arrows) and continuous layers (e.g., yellow dotted line in H).</p