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  1. One-dimensional lepidocrocite, 1DL, titania, TiO2, is a recently discovered form of this ubiquitous oxide that is of interest in a variety of applications ranging from photocatalysis to water purification, among others. The fundamental building blocks of these materials are snippets (30 nm long) of individual 1DLs that self-assemble into nanobundle, NB, structures. These NBs can then be driven to self-assemble into quasi-two-dimensional, 2D, sheets, films, or free-flowing mesoscopic particles. Here, we use analytical atomic-resolution scanning transmission electron microscopy (STEM) and first-principles density functional theory (DFT) calculations to demonstrate that the arrangement of the neighboring NFs can be altered through ion exchange with Li, Na, and tetramethylammonium hydroxide (TMA) cations. Moreover, using cryogenic electron energy-loss spectroscopy (EELS), we show that the introduction of different ion species results in a change in the local occupancy of the TiO2 t2g and eg orbitals. Both experimental findings are predicted by ground-state energy simulations of two-dimensional lepidocrocite TiO2. 
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  2. Free, publicly-accessible full text available March 26, 2025
  3. In this work, we performed in situ nanoindentation in TEM to capture the real-time 〈c + a〉 dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of 〈c + a〉 dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both 〈c + a〉 dislocation retraction and detwinning. Moreover, we note that the plastic zone comprised of 〈c + a〉 dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that, in addition to 〈a〉 dislocations, the I1 stacking fault bounded with a 〈1/2c+p〉 Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant 〈c + a〉 dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of 〈c + a〉 dislocation slip and twinning in Mg and alloys. 
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    Free, publicly-accessible full text available December 1, 2024
  4. Abstract Refractory multi-principal element alloys (RMPEAs) are promising materials for high-temperature structural applications. Here, we investigate the role of short-range ordering (SRO) on dislocation glide in the MoNbTi and TaNbTi RMPEAs using a multi-scale modeling approach. Monte carlo/molecular dynamics simulations with a moment tensor potential show that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies (USFEs). From mesoscale phase-field dislocation dynamics simulations, we find that increasing SRO leads to higher mean USFEs and stress required for dislocation glide. The gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations, with higher SRO decreasing the degree of USFE dispersion and hence, amount of hardening. Finally, we show how the morphology of an expanding dislocation loop is affected by the applied stress. 
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    Free, publicly-accessible full text available December 1, 2024
  5. ABSTRACT

    We conduct a systematic search for quasars with periodic variations from the archival photometric data of the Zwicky Transient Facility by cross-matching with the quasar catalogues of the Sloan Digital Sky Survey and Véron-Cetty and Véron. We first select out 184 primitive periodic candidates using the generalized Lomb–Scargle periodogram and autocorrelation function and then estimate their statistical significance of periodicity based on two red-noise models, i.e. damped random walk (DRW) and single power-law (SPL) models. As such, we finally identify 106 (DRW) and 86 (SPL) candidates with the most significant periodic variations out of 143 700 quasars. We further compare DRW and SPL models using Bayes factors, which indicate a relative preference of the SPL model for our primitive sample. We thus adopt the candidates identified with SPL as the final sample and summarize its basic properties. We extend the light curves of the selected candidates by supplying other archival survey data to verify their periodicity. However, only three candidates (with 6–8 cycles of periods) meet the selection criteria. This result clearly implies that, instead of being strictly periodic, the variability must be quasi-periodic or caused by stochastic red-noise. This exerts a challenge to the existing search approaches and calls for developing new effective methods.

     
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  6. Abstract

    Severe lattice distortion is a prominent feature of high-entropy alloys (HEAs) considered a reason for many of those alloys’ properties. Nevertheless, accurate characterizations of lattice distortion are still scarce to only cover a tiny fraction of HEA’s giant composition space due to the expensive experimental or computational costs. Here we present a physics-informed statistical model to efficiently produce high-throughput lattice distortion predictions for refractory non-dilute/high-entropy alloys (RHEAs) in a 10-element composition space. The model offers improved accuracy over conventional methods for fast estimates of lattice distortion by making predictions based on physical properties of interatomic bonding rather than atomic size mismatch of pure elements. The modeling of lattice distortion also implements a predictive model for yield strengths of RHEAs validated by various sets of experimental data. Combining our previous model on intrinsic ductility, a data mining design framework is demonstrated for efficient exploration of strong and ductile single-phase RHEAs.

     
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  7. Abstract This work was inspired by new experimental findings where we discovered a two-dimensional (2D) material comprised of titanium-oxide-based one-dimensional (1D) sub-nanometer filaments. Preliminary results suggest that the 2D material contains considerable amounts of carbon, C, in addition to titanium, Ti, and oxygen, O. The aim of this study is to investigate the low-energy, stable atomic forms of 2D titanium carbo-oxides as a function of C content. Via a combination of first-principles calculations and an effective structure sampling scheme, the stable configurations of C-substitutions are comprehensively searched by templating different 2D TiO 2 polymorphs and considering a two O to one C replacement scheme. Among the searched stable configurations, a structure where the (101) planes of anatase bound the top and bottom surfaces with a chemical formula of Ti C 1 / 4 O 3 / 2 was of particularly low energy. Furthermore, the variations in the electronic band structure and chemical bonding environments caused by the high-content C substitution are investigated via additional calculations using a hybrid exchange-correlation functional. 
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  8. Abstract

    We report first-time reverberation-mapping results for 14 active galactic nuclei (AGNs) from the ongoing Monitoring AGNs with HβAsymmetry campaign (MAHA). These results utilize optical spectra obtained with the Long Slit Spectrograph on the Wyoming Infrared 2.3 m Telescope between 2017 November and 2023 May. MAHA combines long-duration monitoring with high cadence. We report results from multiple observing seasons for nine of the 14 objects. These results include Hβtime lags, supermassive black hole masses, and velocity-resolved time lags. The velocity-resolved lags allow us to investigate the kinematics of the broad-line region.

     
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  9. Abstract

    We performed a rigorous reverberation-mapping analysis of the broad-line region (BLR) in a highly accreting (L/LEdd= 0.74–3.4) active galactic nucleus, Markarian 142 (Mrk 142), for the first time using concurrent observations of the inner accretion disk and the BLR to determine a time lag for the Hβλ4861 emission relative to the ultraviolet (UV) continuum variations. We used continuum data taken with the Niel Gehrels Swift Observatory in theUVW2 band, and the Las Cumbres Observatory, Dan Zowada Memorial Observatory, and Liverpool Telescope in thegband, as part of the broader Mrk 142 multiwavelength monitoring campaign in 2019. We obtained new spectroscopic observations covering the Hβbroad emission line in the optical from the Gemini North Telescope and the Lijiang 2.4 m Telescope for a total of 102 epochs (over a period of 8 months) contemporaneous to the continuum data. Our primary result states a UV-to-Hβtime lag of8.680.72+0.75days in Mrk 142 obtained from light-curve analysis with a Python-based running optimal average algorithm. We placed our new measurements for Mrk 142 on the optical and UV radius–luminosity relations for NGC 5548 to understand the nature of the continuum driver. The positions of Mrk 142 on the scaling relations suggest that UV is closer to the “true” driving continuum than the optical. Furthermore, we obtainlog(M/M)= 6.32 ± 0.29 assuming UV as the primary driving continuum.

     
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