158 research outputs found
Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator
Spin-polarized band structure of the three-dimensional quantum spin Hall
insulator (x=0.12-0.13) was fully elucidated by
spin-polarized angle-resolved photoemission spectroscopy using a high-yield
spin polarimeter equipped with a high-resolution electron spectrometer. Between
the two time-reversal-invariant points, and , of the
(111) surface Brillouin zone, a spin-up band ( band) was found to
cross the Fermi energy only once, providing unambiguous evidence for the strong
topological insulator phase. The observed spin-polarized band dispersions
determine the "mirror chirality" to be -1, which agrees with the theoretical
prediction based on first-principles calculations
Bulk and surface electronic structure of BiTe from calculations and photoemission experiments
We present a combined theoretical and experimental study of the electronic
structure of stoichiometric BiTe, a natural superlattice of alternating
BiTe quintuple layers and Bi bilayers. In contrast to the related
semiconducting compounds BiTe and BiTe, density functional
theory predicts BiTe to be a semimetal. In this work, we compute the
quasiparticle electronic structure of BiTe in the framework of the
approximation within many-body perturbation theory. The quasiparticle
corrections are found to modify the dispersion of the valence and conduction
bands in the vicinity of the Fermi energy, leading to the opening of a small
indirect band gap. Based on the analysis of the eigenstates, BiTe is
classified as a dual topological insulator with bulk topological invariants
(1;111) and magnetic mirror Chern number . The bulk
results are used to build a Wannier-functions based tight-binding Hamiltonian
that is further applied to study the electronic properties of the (111)
surface. The comparison with our angle-resolved photoemission measurements
shows excellent agreement between the computed and measured surface states and
indicates the dual topological nature of BiTe
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A history of mild shocks experienced by the regolith particles on hydrated asteroid Ryugu
Micrometeorites, a possible major source of Earth’s water, are thought to form from explosive dispersal of hydrated chondritic materials during impact events on their parental asteroids. However, this provenance and formation mechanism have yet to be directly confirmed using asteroid returned samples. Here, we report evidence of mild shock metamorphism in the surface particles of asteroid Ryugu based on electron microscopy. All particles are dominated by phyllosilicates but lack dehydration textures, which are indicative of shock-heating temperatures below ~500 °C. Microfault-like textures associated with extensively shock-deformed framboidal magnetites and a high-pressure polymorph of Fe–Cr–sulfide have been identified. These findings indicate that the average peak pressure was -2 GPa. The vast majority of ejecta formed during impact on Ryugu-like asteroids would be hydrated materials, larger than a millimetre, originating far from the impact point. These characteristics are inconsistent with current micrometeorite production models, and consequently, a new formation mechanism is required
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Oxygen isotope evidence from Ryugu samples for early water delivery to Earth by CI chondrites
The delivery of water to the inner Solar System, including Earth, is still a debated topic. A preferential role for hydrated asteroids in this process is supported by isotopic measurements. Carbonaceous chondrite (CC) meteorites represent our main source of information about these volatile-rich asteroids. However, the destruction of weaker materials during atmospheric entry creates a bias in our CC data. The return of surface materials from the C-type asteroid 162173 Ryugu by the Hayabusa2 spacecraft provides a unique opportunity to study high-porosity, low-density, primitive materials, unrepresented in the meteorite record. We measured the bulk oxygen isotope composition from four Ryugu particles and show that they most closely resemble the rare CI (CC Ivuna-type) chondrites, but with some differences that we attribute to the terrestrial contamination of the CI meteorites. We suggest that CI-related material is widespread among carbonaceous asteroids and a more important source of Earth’s water and other volatiles than its limited presence in our meteoritic collection indicates
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A pristine record of outer Solar System materials from asteroid Ryugu’s returned sample
Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition
Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator
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