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1. Large H 2 O solubility in dense silica and its implications for the interiors of water-rich planets

   https://doi.org/10.1073/pnas.1917448117  

   Nisr, Carole ; Chen, Huawei ; Leinenweber, Kurt ; Chizmeshya, Andrew ; Prakapenka, Vitali B. ; Prescher, Clemens ; Tkachev, Sergey N. ; Meng, Yue ; Liu, Zhenxian ; Shim, Sang-Heon ( April 2020 , Proceedings of the National Academy of Sciences)

   Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in${\mathrm{H}}_2$O-rich setting at high pressures and temperatures ($P-T$) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between${\mathrm{S}\mathrm{i}\mathrm{O}}_2$and${\mathrm{H}}_2$O as archetypal materials for rock and ice, respectively, at high$P-T$. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{l}}_2$-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (${\mathrm{S}\mathrm{i}}_{1-x}{\mathrm{H}}_{4x}$)${\mathrm{O}}_2\hspace{0.17em}\left(x=0.12\right)$. At pressures above 60 GPa,${\mathrm{H}}_2$O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

    

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2. Optical signatures of Dirac nodal lines in NbAs 2

   https://doi.org/10.1073/pnas.1809631115  

   Shao, Yinming ; Sun, Zhiyuan ; Wang, Ying ; Xu, Chenchao ; Sankar, Raman ; Breindel, Alexander J. ; Cao, Chao ; Fogler, Michael M. ; Millis, Andrew J. ; Chou, Fangcheng ; et al ( January 2019 , Proceedings of the National Academy of Sciences)

   Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of${\mathrm{N}\mathrm{b}\mathrm{A}\mathrm{s}}_2$where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra${\sigma }_1\left(\omega \right)$. This behavior is followed by the linear power-law scaling of${\sigma }_1\left(\omega \right)$at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of${\mathrm{N}\mathrm{b}\mathrm{A}\mathrm{s}}_2$.

    

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3. Comparison of machine learning systems trained to detect Alfvén eigenmodes using the CO 2 interferometer on DIII-D

   https://doi.org/10.1088/1741-4326/acfe8b  

   Garcia, Alvin V. ; Jalalvand, Azarakhsh ; Steiner, Peter ; Rothstein, Andy ; Van Zeeland, Michael ; Heidbrink, William W. ; Kolemen, Egemen ( October 2023 , Nuclear Fusion)

   A Machine-Learning (ML) based detection scheme that automatically detects Alfvén Eigenmodes (AE) in a labelled DIII-D database is presented here. Controlling AEs is important for the success of planned burning plasma devices such as ITER, since resonant fast ions can drive AEs unstable and degrade the performance of the plasma or damage the first walls of the machine vessel. Artificial Intelligence could be useful for real-time detection and control of AEs in steady-state plasma scenarios by implementing ML-based models into control algorithms that drive actuators for mitigation of AE impacts. Thus, the objective is to compare differences in performance between using two different recurrent neural network systems (Reservoir Computing Network and Long Short Term Memory Network) and two different representations of the$\mathrm{C}{\mathrm{O}}_2$phase data (simple and crosspower spectrograms). All$\mathrm{C}{\mathrm{O}}_2$interferometer chords are used to train both models, but only one is processed during each training step. The results from the model and data comparison show higher performance for the RCN model (True Positive Rate = 90% and False Positive Rate = 14%), and that using simple magnitude spectrograms is sufficient to detect AEs. Also, the vertical$\mathrm{C}{\mathrm{O}}_2$interferometer chord passing near the center is better for ML-based detection of AEs.

    

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4. The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

   https://doi.org/10.3847/1538-4357/acb153  

   Gburek, Timothy ; Siana, Brian ; Alavi, Anahita ; Emami, Najmeh ; Richard, Johan ; Freeman, William R. ; Stark, Daniel P. ; Snapp-Kolas, Christopher ( May 2023 , The Astrophysical Journal)

   We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (z_mean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of$\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.29}_{-0.43}^{+0.51}$and a median star formation rate of${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha }^{\mathrm{m}\mathrm{e}\mathrm{d}}={2.25}_{-1.26}^{+2.15}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of$12+\log {(\mathrm{O}/\mathrm{H})}_{\mathrm{direct}}={7.88}_{-0.22}^{+0.25}$(${0.15}_{-0.06}^{+0.12}{Z}_{\odot }$). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM_*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories$(\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.92}_{-0.22}^{+0.31})$, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixedM_*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of$n_e=1_{-0}^{+215}{\mathrm{cm}}^{-3}$($n_e=1_{-0}^{+74}{\mathrm{cm}}^{-3}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

    

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5. Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe 2 monolayers

   https://doi.org/10.1088/1361-648X/acbf92  

   Chiatti, Olivio ; Mihov, Klara ; Griffin, Theodor U. ; Grosse, Corinna ; Alemayehu, Matti B. ; Hite, Kyle ; Hamann, Danielle ; Mogilatenko, Anna ; Johnson, David C. ; Fischer, Saskia F. ( March 2023 , Journal of Physics: Condensed Matter)

   The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling between$\text{NbS}{\text{e}}_{\text{2}}$monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties in${\left[{\left(\text{SnSe}\right)}_{1+\delta }\right]}_m{\left[\text{NbS}{\text{e}}_{\text{2}}\right]}_1$tailored multilayers with varying SnSe layer thickness ($m=1-15$). From high-field magnetotransport the critical fields yield Ginzburg–Landau coherence lengths with an increase of$140\mathrm{\%}$cross-plane ($m=1-9$), trending towards two-dimensional superconductivity for$m>9$. We show cross-overs between three regimes: metallic with proximity-enhanced coupling ($m=1-4$), disordered-metallic with intermediate coupling ($m=5-9$) and insulating with Josephson tunneling ($m>9$). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

    

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