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1. Atomic-scale observation of dynamic grain boundary structural transformation during shear-mediated migration

   https://doi.org/10.1126/sciadv.abn3785  

   Fang, Zhengwu; Xiao, Jianwei; Tan, Susheng; Deng, Chuang; Wang, Guofeng; Mao, Scott X. (November 2022, Science Advances)

   Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of $\left(\overline{1}\overline{1}1\right)/\left(002\right)$-type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of $\left(111\right)/\left(11\overline{1}\right)$-type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration. 

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2. Development of broad modulus profile upon polymer–polymer interface formation between immiscible glassy–rubbery domains

   https://doi.org/10.1073/pnas.2312533120  

   Gagnon, Yannic J; Burton, Justin C; Roth, Connie B (January 2024, Proceedings of the National Academy of Sciences)

   Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ( $T_{\text{g}}$) where previous work has demonstrated a long-range (∼200 nm) profile in local $T_{\text{g}}\left(z\right)$is established between immiscible glassy and rubbery polymer domains when the polymer–polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus $\tilde{G}\left(z\right)$is established when the polymer–polymer interface ( $\approx$5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad $T_{\text{g}}\left(z\right)$profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus $\tilde{G}\left(z\right)$spanning $\approx$180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in $T_{\text{g}}\left(z\right)$and $\tilde{G}\left(z\right)$arise from a coupling of the spectrum of vibrational modes across the polymer–polymer interface as a result of acoustic impedance matching of sound waves with $\lambda \sim 5$nm during interface broadening that can then trigger density fluctuations in the neighboring domain. 

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3. Observation of a pseudoscalar excess at the top quark pair production threshold

   https://doi.org/10.1088/1361-6633/adf7d3  

   Collaboration, The_CMS (August 2025, Reports on Progress in Physics)

   Abstract A search for resonances in top quark pair ( $\mathrm{t}\bar{\mathrm{t}}$) production in final states with two charged leptons and multiple jets is presented, based on proton–proton collision data collected by the CMS experiment at the CERN LHC at $\sqrt{s}=13\text{TeV}$, corresponding to 138 fb⁻¹. The analysis explores the invariant mass of the $\mathrm{t}\bar{\mathrm{t}}$system and two angular observables that provide direct access to the correlation of top quark and antiquark spins. A significant excess of events is observed near the kinematic $\mathrm{t}\bar{\mathrm{t}}$threshold compared to the non-resonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar ( ${}^1{\text{S}}_0^{\left[1\right]}$) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for ${}^1{\text{S}}_0^{\left[1\right]}$toponium, the cross section of the excess above the pQCD prediction is measured to be $8.8{}_{-1.4}^{+1.2}\text{pb}$. 

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4. (Nearly) Model-independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at z ∼ 5–7 Using Dark Pixel Fractions in Lyα and Lyβ Forests

   https://doi.org/10.3847/1538-4357/aca678  

   Jin, Xiangyu; Yang, Jinyi; Fan, Xiaohui; Wang, Feige; Bañados, Eduardo; Bian, Fuyan; Davies, Frederick B.; Eilers, Anna-Christina; Farina, Emanuele Paolo; Hennawi, Joseph F.; et al (January 2023, The Astrophysical Journal)

   Abstract Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral atz> 7 and largely ionized byz∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low ( ${\overline{x}}_{\mathrm{H}\mathrm{I}}\lesssim {10}^{-3}$) or close to unity ( ${\overline{x}}_{\mathrm{H}\mathrm{I}}\sim 1$). In particular, the neutral fraction evolution of the IGM at the critical redshift range ofz= 6–7 is poorly constrained. We present new constraints on ${\overline{x}}_{\mathrm{H}\mathrm{I}}$atz∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 <z< 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyαand Lyβforests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction atz∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.3)<0.79\pm 0.04$(1σ), ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.5)<0.87\pm 0.03$(1σ), and ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.7)<{0.94}_{-0.09}^{+0.06}$(1σ). The dark pixel fractions atz> 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018. 

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5. Computation of the expectation value of the spin operator ${\hat{S}}^2$ for the spin-flip Bethe–Salpeter equation

   https://doi.org/10.1088/2516-1075/ad48ed  

   Barker, B A; Seshappan, A; Strubbe, D A (May 2024, Electronic Structure)

   Abstract Spin-flip (SF) methods applied to excited-state approaches like the Bethe–Salpeter equation allow access to the excitation energies of open-shell systems, such as molecules and defects in solids. The eigenstates of these solutions, however, are generally not eigenstates of the spin operator ${\hat{S}}^2$. Even for simple cases where the excitation vector is expected to be, for example, a triplet state, the value of $⟨{\hat{S}}^2⟩$may be found to differ from 2.00; this difference is called ‘spin contamination’. The expectation values $⟨{\hat{S}}^2⟩$must be computed for each excitation vector, to assist with the characterization of the particular excitation and to determine the amount of spin contamination of the state. Our aim is to provide for the first time in the SF methods literature a comprehensive resource on the derivation of the formulas for $⟨{\hat{S}}^2⟩$as well as its computational implementation. After a brief discussion of the theory of the SF Bethe–Salpeter equation (BSE) and some examples further illustrating the need for calculating $⟨{\hat{S}}^2⟩$, we present the derivation for the general equation for computing $⟨{\hat{S}}^2⟩$with the eigenvectors from an SF-BSE calculation, how it is implemented in a Python script, and timing information on how this calculation scales with the size of the SF-BSE Hamiltonian. 

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