More Like this

1. Atomic Transition Probabilities for Transitions of Si i and Si ii and the Silicon Abundances of Several Very Metal-poor Stars ^∗  

   https://doi.org/10.3847/1538-4365/acb642  

   Den Hartog, E. A.; Lawler, J. E.; Sneden, C.; Roederer, I. U.; Cowan, J. J. (March 2023, The Astrophysical Journal Supplement Series)

   Abstract We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Sii) originating in the 3s²3p4s³P^o_1,2,¹P^o₁, and 3s3p³¹D^o_1,2upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the⁴P_1/2–²P^o_1/2,3/2intercombination lines in the spectrum of singly ionized silicon (Siii). The weak spin-forbidden lines of Siiand Siiiprovide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars. 

   more » « less
2. 3D Hybrid Plasmonic Framework with Au Nanopillars Embedded in Nitride Multilayers Integrated on Si

   https://doi.org/10.1002/admi.202000493  

   Huang, Jijie; Wang, Xuejing; Li, Dongfang; Jin, Tiening; Lu, Ping; Zhang, Di; Lin, Pao‐Tai; Chen, Hou‐Tong; Narayan, Jagdish; Zhang, Xinghang; et al (June 2020, Advanced Materials Interfaces)

   Abstract Integration of nanoscale photonic and plasmonic components on Si substrates is a critical step toward Si‐based integrated nanophotonic devices. In this work, a set of unique complex 3D metamaterials with intercalated nanolayered and nanopillar structures with tunable plasmonic and optical properties on Si substrates is designed. More specifically, the 3D metamaterials combine metal (Au) nanopillars and alternating metal‐nitride (Au‐TiN and Au‐TaN) nanolayers, epitaxially grown on Si substrates. The ultrafine Au nanopillars (d≈ 3 nm) continuously grow throughout all the nanolayers with high epitaxial quality. Novel optical properties, such as highly anisotropic optical property, high absorbance covering the entire visible spectrum regime, and hyperbolic property in the visible regime, are demonstrated. Furthermore, a waveguide based on a silicon nitride (Si₃N₄) ridge with a multilayer structure is successfully fabricated. The demonstration of 3D nanoscale metamaterial design integrated on Si opens up a new route toward tunable metamaterials nanostructure designs with versatile material selection for various optical components in Si integrated photonics. 

   more » « less
3. Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd ₃ S ₄ under high pressure

   https://doi.org/10.1073/pnas.2310779120  

   Huyan, Shuyuan; Ryan, Dominic H.; Slade, Tyler J.; Lavina, Barbara; Jose, Greeshma; Wang, Haozhe; Wilde, John M.; Ribeiro, Raquel A.; Zhao, Jiyong; Xie, Weiwei; et al (December 2023, Proceedings of the National Academy of Sciences)

   We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd₃S₄, by electrical transport, X-ray diffraction, time-domain¹⁵¹Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. Electrical transport measurements show that the antiferromagnetic ordering temperature,T_N, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated withT_Nis detected. A pressure-induced first-order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~30 GPa) that corresponds to theT_Nplateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu²⁺:Eu³⁺to fully Eu³⁺at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhancesT_N, most likely via enhanced hybridization between the Eu 4fstates and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state. 

   more » « less
4. The spectral features and detectability of small, cyclic silicon carbide clusters

   https://doi.org/10.3389/fspas.2022.1074879  

   Sehring, Christopher M.; Palmer, C. Zachary; Westbrook, Brent R.; Fortenberry, Ryan C. (December 2022, Frontiers in Astronomy and Space Sciences)

   Rovibrational spectral data for several tetra-atomic silicon carbide clusters (TASCCs) are computed in this work using a CCSD(T)-F12b/cc-pCVTZ-F12 quartic force field. Accurate theoretical spectroscopic data may facilitate the observation of TASCCs in the interstellar medium which may lead to a more complete understanding of how the smallest silicon carbide (SiC) solids are formed. Such processes are essential for understanding SiC dust grain formation. Due to SiC dust prevalence in the interstellar medium, this may also shed light on subsequent planetary formation. Rhomboidal Si₂C₂is shown here to have a notably intense (247 km mol⁻¹) anharmonic vibrational frequency at 988.1 cm⁻¹(10.1 μm) forν₂, falling into one of the spectral emission features typically associated with unknown infrared bands of various astronomical regions. Notable intensities are also present for several of the computed anharmonic vibrational frequencies including the cyclic forms of C₄, SiC₃, Si₃C, and Si₄. These features in the 6–10 μm range are natural targets for infrared observation with theJames Webb Space Telescope(JWST)’s MIRI instrument. Additionally,t-Si₂C₂,d-Si₃C, andr-SiC₃each possess dipole moments of greater than 2.0 D making them interesting targets for radioastronomical searches especially sinced-SiC₃is already known in astrophysical media. 

   more » « less
5. Hydrogen Dissolution Mechanisms in Bridgmanite by First‐Principles Calculations and Infrared Spectroscopy

   https://doi.org/10.1029/2024JB030403  

   Zhang, Yanyao; Tsuchiya, Jun; Li, ChingChien; Ye, Zefang; Yan, Wei; Okuchi, Takuo; Karato, Shun‐ichiro; Kung, Jennifer; Lin, Jung‐Fu (January 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Understanding hydrogen dissolution mechanisms in bridgmanite (Bgm), the most abundant mineral in the lower mantle, is essential for understanding water storage and rheological and transport properties in the region. However, interpretations of O‐H bands in Fourier transform infrared spectroscopy (FTIR) spectra of Bgm crystals remain uncertain. We conducted density functional theory (DFT) calculations on vibrational characteristics of O‐H dipoles and performed polarized FTIR measurements to address this issue. DFT calculations for four substitution models—Mg vacancies, Si vacancies, Al³⁺ + H⁺substitution for Si⁴⁺, and Al substitution with Mg vacancies—reveal distinct O‐H bands with different polarizations. Deconvolution of polarized FTIR spectra on Mg_0.88Fe²⁺_0.035Fe³⁺_0.065Al_0.14Si_0.90O₃and Mg_0.95Fe²⁺_0.033Fe³⁺_0.027Al_0.04Si_0.96O₃crystals shows five major O‐H bands with distinct polarizations along principal crystallographic axes. These experimental and calculated results attribute O‐H bands centered at 3,463–3,480, 2,913–2,924, and 2,452–2,470 cm⁻¹to Mg vacancies, Si vacancies, and Al³⁺ + H⁺substitution for Si⁴⁺, respectively. The total absorbance coefficient of bridgmanite was calculated to be 82,702(6,217) L/mol/cm². Mg and Si vacancies account for 43%–74% of the total water content, making them dominant hydrogen dissolution mechanisms in Bgm. The band frequencies for the Mg and Si vacancies in Bgm are drastically different from those in olivine and ringwoodite, corresponding to the significant changes in O‐H bond strengths and in the Si and Mg coordination environments from upper‐mantle to lower‐mantle minerals. These results highlight the need to incorporate hydrogen dissolution mechanisms in Bgm for understanding electrical conductivity and rheology of the lower mantle. 

   more » « less