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1. Stabilizing Ti ₃ C ₂ T _x MXene flakes in air by removing confined water

   https://doi.org/10.1073/pnas.2400084121  

   Fang, Hui; Thakur, Anupma; Zahmatkeshsaredorahi, Amirhossein; Fang, Zhenyao; Rad, Vahid; Shamsabadi, Ahmad A; Pereyra, Claudia; Soroush, Masoud; Rappe, Andrew M; Xu, Xiaoji G; et al (July 2024, Proceedings of the National Academy of Sciences)

   MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti ${}_3$ C ${}_2$ T ${}_x$ MXene monoflakes have exceptional thermal stability at temperatures up to 600 ${}^{°}$ C in air, while multiflakes readily oxidize in air at 300 ${}^{°}$ C. Density functional theory calculations indicate that confined water between Ti ${}_3$ C ${}_2$ T ${}_x$ flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti ${}_3$ C ${}_2$ T ${}_x$ films at 600 ${}^{°}$ C, resulting in substantial stability improvement in multiflake films (can withstand 600 ${}^{°}$ C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti ${}_3$ C ${}_2$ T ${}_x$ oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability. 

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2. The Origin of Da Scaling: Suppressed Cooling in Fast-cooling Mixing Layers

   https://doi.org/10.3847/1538-4357/ae908e  

   Lancaster, Lachlan; Fielding, Drummond; Mohapatra, Rajsekhar; Bryan, Greg L (August 2026, The Astrophysical Journal)

   Abstract In numerical experiments simulating turbulent radiative mixing layers (TRMLs), it is observed that as the cooling time in the mixed gas,t_cool, becomes very short compared to the dynamical time of the turbulence,t_eddy/t_cool ≫ 1, there is a change in the scaling behavior of the total energy radiated in the TRML as a function of this ratio, also known as the Damköhler number, Da ≡ t_eddy/t_cool, from ${\dot{E}}_{\mathrm{cool}}\propto {\mathrm{Da}}^{1/2}$ to ${\dot{E}}_{\mathrm{cool}}\propto {\mathrm{Da}}^{1/4}$ . The latter, so-called “fast-cooling,” regime is of particular interest, as many astrophysical mixing layers lie in this regime. We demonstrate that the origin of this change is the suppression of turbulent folding of the surface by the ram-pressure of the inflowing gas, which becomes much greater than the turbulent pressure in this regime. We present an argument that reproduces the ${\dot{E}}_{\mathrm{cool}}\propto {\mathrm{Da}}^{1/4}$ behavior by appealing to the suppression of the fractal structure of the interface by the ram-pressure of the inflowing gas. 

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3. Don’t Follow Reinforcement Learning Blindly: Lower Sample Complexity of Learning Optimal Inventory Control Policies with Fixed Ordering Cost

   https://doi.org/10.1177/10591478251378851  

   Fan, Xiaoyu; Chen, Boxiao; Olsen, Tava; Qin, Hanzhang; Zhou, Zhengyuan (April 2026, Production and Operations Management)

   We study the sample complexity of offline learning for a class of structured Markov decision processes (MDPs) describing the inventory control system with fixed ordering cost/setup cost, a fundamental problem in supply chains. We find that a naive plug-in sampling-based approach applied to the inventory MDPs leads to strictly lower sample complexity bounds compared to the optimal bounds recently obtained for the general MDPs. More specifically, in the infinite-horizon discounted cost setting, we obtain an $\tilde{O}(min\{\frac{(\overline{S}-\underset{\_}{s}{)}^2}{(1-\gamma {)}^2{ϵ}^2},\frac{1}{(1-\gamma {)}^4{ϵ}^2}\})$ sample complexity bound, where ${(\overline{S}-\underset{\_}{s})}^2$ corresponds to the number of state-action pairs in a generic MDP with state space $\mathcal{S}$ and action space $\mathcal{A}$ . As such, $\tilde{O}\left(\frac{(\overline{S}-\underset{\_}{s}{)}^2}{(1-\gamma {)}^2{ϵ}^2}\right)$ improves on the optimal generic reinforcement learning (RL) bound $\tilde{\mathrm{\Theta }}\left(\frac{(\overline{S}-\underset{\_}{s}{)}^2}{(1-\gamma {)}^3{ϵ}^2}\right)$ (when directly applying $\tilde{\mathrm{\Theta }}\left(\frac{|\mathcal{S}||\mathcal{A}|}{(1-\gamma {)}^3{ϵ}^2}\right)$ here) by a factor of $(1-\gamma {)}^{-1}$ , and $\tilde{O}\left(\frac{1}{(1-\gamma {)}^4{ϵ}^2}\right)$ is able to completely remove the dependence on state and action cardinality. In the infinite-horizon average cost setting, we obtain an $\tilde{O}\left(\frac{(\overline{S}-\underset{\_}{s}{)}^2}{{ϵ}^2}\right)$ bound, improving on the generic optimal RL bound $\tilde{\mathrm{\Theta }}\left(\frac{(\overline{S}-\underset{\_}{s}{)}^2{t}_{mix}}{{ϵ}^2}\right)$ (when directly applying $\tilde{\mathrm{\Theta }}\left(\frac{|\mathcal{S}||\mathcal{A}|t_{mix}}{{ϵ}^2}\right)$ here) by a factor of $t_{mix}$ , and hence removing the mixing time dependence. By carefully leveraging the structural properties of the inventory dynamics in various settings, we are able to improve on those “best-possible” bounds developed in the RL literature. Our results demonstrate the drawbacks one could face by blindly following RL algorithms and the necessity of designing sample efficient algorithms that properly incorporate the special structures of the inventory systems. 

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4. Tropical forests are facing increasing risks of exposure to critical temperature thresholds

   https://doi.org/10.1073/pnas.2528622123  

   van_Tiel, Nina; Lenczner, Gaston; Rao, Mukund P; Grossiord, Charlotte; Tuia, Devis (July 2026, Proceedings of the National Academy of Sciences)

   Understanding how close tropical tree species are to critical temperature thresholds that might impede photosynthetic activity is vital in a world where heat waves have become more severe and frequent. Using remotely sensed surface temperature and species distribution maps, we studied the spatiotemporal variation in the thermal safety margins (TSM, i.e., the difference between $T_{\text{crit}}$ , the critical photosynthetic temperature, and the maximum canopy temperature) of $208$ tropical tree species in South America, Southeast Asia, and Central Africa during the period 2001–2020. Despite overall high-temperature tolerance with an average $T_{\text{crit}}$ of $46.1{}^{°}$ C, we observed a consistent decline in the TSM of tropical forests across the globe. The average pantropical TSM decline was $0.4{}^{°}$ C per decade, with the strongest decline in South America ( $0.5{}^{°}$ C per decade). Over the $20$ -y period, areas that experienced canopy temperatures surpassing the average $T_{\text{crit}}$ across reported species increased from $43$ Mha to $57$ Mha in the tropics, representing $4\%$ of the studied area. This number increases to $10\%$ when computing areas where temperatures have surpassed the $T_{\text{crit}}$ of the most vulnerable reported species. When considering future trends, as predicted by Earth System Models under medium-to-high emission scenarios, average $T_{\text{crit}}$ may be exceeded in an area of $83$ Mha by $2050$ and $160$ Mha by $2100$ (over $10\%$ of the studied area), suggesting major feedback to the global carbon cycle and the world’s biodiversity. 

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5. Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton–proton collisions at $\sqrt{s}=13\text{TeV}$

   https://doi.org/10.1088/1361-6633/ae2207  

   Collaboration, The CMS (December 2025, Reports on Progress in Physics)

   Abstract A search for pseudoscalar or scalar bosons decaying to a top quark pair ( $\text{t}\overline{\text{t}}$ ) in final states with one or two charged leptons is presented. The analyzed proton–proton collision data was recorded at $\sqrt{s}=13\text{TeV}$ by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138  ${\text{fb}}^{-1}$ . The invariant mass $m_{\mathrm{t}\overline{\mathrm{t}}}$ of the reconstructed $\text{t}\overline{\text{t}}$ system and variables sensitive to its spin and parity are used to discriminate against the standard model $\text{t}\overline{\text{t}}$ background. Interference between pseudoscalar or scalar boson production and the standard model $\text{t}\overline{\text{t}}$ continuum is included, leading to peak-dip structures in the $m_{\mathrm{t}\overline{\mathrm{t}}}$ distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic $\text{t}\overline{\text{t}}$ production threshold, while good agreement is found for high $m_{\mathrm{t}\overline{\mathrm{t}}}$ . The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet ${}^1{\text{S}}_0^{\left[1\right]}$ $\text{t}\overline{\text{t}}$ quasi-bound state ${\eta }_{\mathrm{t}}$ , inspired by nonrelativistic QCD, is added. Upper limits at 95% confidence level are set on the coupling between the pseudoscalar or scalar bosons and the top quark for boson masses in the range 365–1000 GeV, relative widths between 0.5% and 25%, and two background scenarios with or without ${\eta }_{\mathrm{t}}$ contribution. 

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