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1. Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

   https://doi.org/10.1029/2022JG006932  

   Pérez‐Angel, Lina C.; Sepúlveda, Julio; Montes, Camilo; Smith, Jamila J.; Molnar, Peter; González‐Arango, Catalina; Snell, Kathryn E.; Dildar, Nadia (December 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract We evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ²H_p) and oxygen (δ¹⁸O_p) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ²H_wax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ²H_p, δ¹⁸O_p, as well as the unweighted mean δ²H_waxvalues ofn‐C₂₉,n‐C₃₁, andn‐C₃₃n‐alkanes in the eastern flank show a dependence on elevation (R² = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R² < 0.23), on top of a negligible (p‐value >0.05) correlation between δ²H_waxand δ²H_p. In general, δ²H_waxvalues along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ²H_pnor δ¹⁸O_p, and therefore δ²H_wax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ²H_waxis only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ²H_pand/or δ¹⁸O_pwest of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount. 

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2. Operando Investigation of the Molecular Origins of Dipole Switching in P(VDF‐TrFE‐CFE) Terpolymer for Large Adiabatic Temperature Change

   https://doi.org/10.1002/adfm.202314705  

   Zhu, Yuan; Wu, Hanxiang; Martin, Andrew; Beck, Paige; Allahyarov, Elshad; Wongwirat, Thumawadee; Rui, Guanchun; Zhu, Yingke; Hawthorne, Daniel; Fan, Jiacheng; et al (February 2024, Advanced Functional Materials)

   Abstract Relaxor ferroelectric polymers exhibiting a giant electrocaloric effect (ECE) can potentially be used to create next‐generation solid‐state coolers. Under an electric field, poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) terpolymer goes through a large dipolar entropy change producing a high adiabatic temperature change (ΔT_ECE). This work resolves the molecular origins of the large entropy change behind the electric field‐induced dipole switching. A Fourier transform infrared spectroscopy equipped with a high voltage source is used to operandoly observe the characteristic molecular vibrational modes. A short‐range trans (T) conformation of the CF₂‐CH₂dyads interrupted by a gauche (G) conformation, e.g., TTTG in the terpolymer chain, undergoes a dynamic transformation that leads to a corresponding ΔT_ECEwhenever an electric field is applied. The molecular dynamics simulation also proves that the energy barrier that the transformation from TTTGs into a long T sequence overcomes is smaller than that for all other conformations. A mixed solvent system is used to obtain T3G‐enriched terpolymer films exhibiting a 4.02 K ΔT_ECEat 60 MV m⁻¹and these films are employed to manufacture a 2‐layer‐cascaded cooling device that achieves a 6.7 K temperature lift, the highest reported value for a 2‐layer cascaded device made of fluoropolymers. 

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3. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part II: δ18Op Response

   https://doi.org/10.1175/JCLI-D-22-0289.1  

   Bao, Yuntao; Liu, Zhengyu; He, Chengfei (July 2023, Journal of Climate)

   Abstract Understanding the hydroclimate representations of precipitationδ¹⁸O (δ¹⁸O_p) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall andδ¹⁸O_p, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of suchδ¹⁸O_pdipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to theδ¹⁸O_pdepletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to theδ¹⁸O_penrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship betweenδ¹⁸O_pand rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennialδ¹⁸O_pchange in the central-western Amazon, whileδ¹⁸O_pdoes not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when usingδ¹⁸O_pas a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennialδ¹⁸O_pdipole response and pose a challenge in reconciling the discrepancy. Significance StatementWe want to comprehensively understand the hydroclimate footprints ofδ¹⁸O_pand the mechanisms of the millennial variability ofδ¹⁸O_pover tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennialδ¹⁸O_pchange in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it. 

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4. Resolving an Asteroseismic Catastrophe: Structural Diagnostics from p -mode Phase Functions off the Main Sequence

   https://doi.org/10.3847/1538-4357/ada949  

   Ong_王, J_M_Joel 加冕; Lindsay, Christopher J; Reyes, Claudia; Stello, Dennis; Roxburgh, Ian W (February 2025, The Astrophysical Journal)

   Abstract On the main sequence, the asteroseismic small frequency separationδν₀₂between radial and quadrupolep-modes is customarily interpreted to be a direct diagnostic of internal structure. Such an interpretation is based on a well-known integral estimator relatingδν₀₂to a radially averaged sound-speed gradient. However, this estimator fails, catastrophically, when evaluated on structural models of red giants: their small separations must therefore be interpreted differently. We derive a single expression that both reduces to the classical estimator when applied to main-sequence stellar models and reproduces the qualitative features of the small separation for stellar models of very evolved red giants. This expression indicates that the small separations of red giants scale primarily with their global seismic properties as $\delta {\nu }_{02}\propto \Delta {\nu }^2/{\nu }_{\max }$, rather than being in any way sensitive to their internal structure. Departures from this asymptotic behavior, during the transition from the main-sequence to red giant regimes, have been recently reported in open-cluster Christensen–Dalsgaard (C-D) diagrams from K2 mission data. Investigating them in detail, we demonstrate that they occur when the convective envelope boundary passes a specific acoustic distance—roughly one-third of a wavelength at ${\nu }_{\max }$—from the center of the star, at which point radial modes become maximally sensitive to the position of the boundary. The shape of the corresponding features onϵ_pand C-D (orr₀₂) diagrams may be useful in constraining the nature of convective boundary mixing in the context of undershooting beneath a convective envelope. 

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5. Extraordinarily Large Electrocaloric Strength of Metal‐Free Perovskites

   https://doi.org/10.1002/adma.201906224  

   Wang, Jian‐Jun; Fortino, Daniel; Wang, Bo; Zhao, Xinye; Chen, Long‐Qing (December 2019, Advanced Materials)

   Abstract The availability of materials with high electrocaloric (EC) strengths is critical to enabling EC refrigeration in practical applications. Although large EC entropy changes, ΔS_EC, and temperature changes, ΔT_EC, have been achieved in traditional thin‐film ceramics and polymer ferroelectrics, they require the application of very high electric fields and thus their EC strengths ΔS_EC/ΔEand ΔT_EC/ΔEare too low for practical applications. Here, a fundamental thermodynamic description is developed, and extraordinarily large EC strengths of a metal‐free perovskite ferroelectric [MDABCO](NH₄)I₃(MDABCO) are predicted. The predicted EC strengths: isothermal ΔS_EC/ΔEand adiabatic ΔT_EC/ΔEfor MDABCO are 18 J m kg⁻¹K⁻¹MV⁻¹and 8.06 K m MV⁻¹, respectively, more than three times the largest reported values in BaTiO₃single crystals. These predictions strongly suggest the metal‐free ferroelectric family of materials as the best candidates among existing materials for EC applications. The present work not only presents a general approach to developing thermodynamic potential energy functions for ferroelectric materials but also suggests a family of candidate materials with potentially extremely high EC performance. 

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