Experimental Observations Research Articles

Ab-initio trained machine learning potential for MAX compound Ti2AlC: construction, validation, and study of non linear elasticity

Abstract One of the intriguing features exhibited by the layered MAX phase compounds, is the nonlinear elastic behaviour. Since the experimental observation of this curious behaviour, the underlying micro-mechanism has been discussed to interpret experimental observations. However, the theoretical investigation remained a challenge due to the associated length and time scales of the phenomena. In the present work, we adopt a data driven approach to develop a machine learned interatomic potential for the MAX compound Ti2AlC following the moment tensor potential protocol. The constructed potential is validated in lattice constant, formation energy, elastic constant, and stacking fault energies. Finally, applying machine learned potential in classical molecular dynamics provides a faithful representation of the experimentally observed nonlinear elasticity for Ti2AlC. The generated atomic configurations confirm the proposal of formation of ripplocations which allow atomic layers to glide relative to each other without breaking the in-plane bonds. We find common defects, like Al vacancy, strongly influence the hysteresis properties of the stress–strain curve, paving the route to defect-engineered nonlinear elasticity.

Exactly Solvable Mobility Edges for Phonons in One-Dimensional Quasiperiodic Chains.

Mobility edges, which demarcate the boundary between extended and localized states, are fundamental to understanding the physics of localization in condensed matter systems. Systems exhibiting exact mobility edges are rare, and the localization properties of phonons have received limited prior investigation. In this work, we reveal analytical mobility edges in one-dimensional quasiperiodic-modulated spring-mass chains. The mobility edges are exactly solved and numerically validated through the eigenfrequency spectra, inverse/normalized participation ratios, and lattice wave dynamics. Our research demonstrates the Anderson localization transition in phonon systems, paving the way for experimental observations of phonon localization.

Discretizing the bistability of mode-locked electron spin precession: An Overhauser field hysteresis manifestation

Electron-nuclear spin interactions by pulsed optical pumping have been found to polarize the nuclear spin system, leading to the nuclei building up an intrinsic magnetic field known as the Overhauser field. Studies have indicated an Overhauser field hysteresis effect dependent on the sweep direction of an externally applied magnetic field in negatively detuned periodically pumped Si-doped GaAs. Although predictions of bistable mode-locked electron spin precession frequency modes have been made for systems exhibiting this hysteresis, there have been no reports on the experimental observation of said bistable spin precession modes. This report details the evolution of bistable Overhauser field solutions leading to a hysteretic effect in negatively detuned optical excitation of Si-doped GaAs by magneto-optic pump–probe spectroscopy in the Voigt geometry and investigates the resulting consequence of this hysteresis acting on the electron spin system. One manifestation of the Overhauser field hysteresis acting on the electron spin system leads to the discretization of bistable mode-locked electron spin precession modes within a given band of externally applied magnetic fields. A method for preferentially accessing the two different and stable mode-locked spin precession modes within a given band of externally applied magnetic field is outlined, which may be of interest for communities utilizing electron and nuclear spins for information processing protocols.

Force-Field Benchmark for Polydimethylsiloxane: Density, Heat Capacity, Isothermal Compressibility, Viscosity and Thermal Conductivity.

Polysiloxanes are versatile polymeric materials with widespread applications in industries ranging from electronics to biomedical devices because of their unique thermal and viscoelastic properties. Accurate molecular simulations of polysiloxanes are essential for understanding their broad applications from a microscopic perspective. However, the accuracy of these simulations is highly dependent on the quality of the force fields used. In this work, we present a comprehensive benchmark and development of force fields tailored for polydimethylsiloxane, which is one of the most widely used polysiloxane materials. Our focus is on their performance in predicting key thermophysical properties including density, heat capacities, isothermal compressibility, and transport properties such as viscosity and thermal conductivity. Experimental measurements are performed in parallel to rigorously validate simulation outcomes. Existing force fields for polydimethylsiloxane, including those derived for organic and inorganic systems, are systematically evaluated against experimental data to identify limitations in accuracy and transferability. Simulation results are compared extensively with experimental observations across a range of temperatures and pressures, revealing the strengths and shortcomings of these commonly utilized force fields for polydimethylsiloxane. Discrepancies between force field predictions and experimental measurements are analyzed in detail for thermodynamic and transport properties of polydimethylsiloxane. This benchmark study serves as a critical assessment of current force fields for polydimethylsiloxane and offers guidelines for their further development, enabling more reliable simulations of polysiloxane-based materials for various industrial applications.

Warming Promotes Nitrogen and Carbon Cycles in Global Grassland.

Grasslands, standing as one of Earth's major ecosystems, offer numerous services vital to human well-being. The productivity of grasslands hinges on the availability of soil reactive nitrogen, which is highly sensitive to climatic variations. Using an extensive synthesis of 1242 experimental observations, reinforced by multiple models, we show that warming as a single driver of climate change intensifies nitrogen dynamics in grasslands. This could lead to increases in net primary productivity of 1% to 9% and escalate nitrogen leakage into the environment by 22% to 141%. Under the warming SSP2-4.5 scenario, we foresee an annual boost of 17 million tons per year (Tg yr-1) of nitrogen inputs, predominantly via biological nitrogen fixation, compared to the baseline scenario by 2050. Total nitrogen harvest is projected to climb by 12 Tg yr-1. However, the nitrogen surplus surge is expected to increase by 5 Tg yr-1, potentially intensifying nitrogen pollution. To counter this, adaptation measures must aim at curtailing reactive nitrogen losses while preserving increased nitrogen harvest. This could reduce nitrogen input and surplus by 10 and 20 Tg yr-1, respectively, while boosting nitrogen harvest by 10 Tg yr-1, potentially yielding economic gains of up to 121 billion USD by 2050. In shaping climate change adaptation policies, it is critical to balance the potential benefits and drawbacks of forging effective management approaches.

Effect of competition on emergent phases and phase transitions in competitive systems.

This paper considers Lotka-Volterra competitive systems characterizing laboratory experiment by Hu et al. (Science, 378:85-89, 2022). Using dynamical systems theory and projection method, we give theoretical analysis and numerical simulation on the model with four species by demonstrating equilibrium stability, periodic oscillation and chaotic fluctuation in the systems. It is shown that varying one competition strength could lead to emergent phases and phase transitions between stable full coexistence, stable partial coexistence, stable persistence of a unique species, persistent periodic oscillation, and persistent chaotic fluctuation in a smooth fashion. Here, the stronger the competition is, the less the number of stable coexisting species, or the higher the amplitude of periodic oscillation, or the more irregular the fluctuation. Our results are consistent with experimental observation and provide new insight. This work is important in understanding effect of competition on emergent phases and phase transitions in competitive systems.

Interaction between F/TRC shear strengthening and steel stirrups: Experimental observations on real-scale RC T-beams

The increasing age, degradation, and performance demands on existing structures are driving the construction industry towards the development of high-performance materials for the production of strengthening solutions. When these solutions are used to increase the shear resistance of Reinforced Concrete elements, they tend to exhibit an adverse interaction with the stirrups. An experimental campaign was conducted to investigate this phenomenon on a rather novel material, i.e., Fibre/Textile Reinforced Concrete (F/TRC). The results of this campaign show that there is indeed an interaction which can significantly reduce the contribution of the F/TRC layers, even if the stirrups were past their yielding deformation. Furthermore, it was also observed that the reduction in contribution of the strengthening solutions was accompanied by a reduction of the deformation of the stirrups at the peak load.

Numerical analysis of nucleation effects on cloud cavitation using multiscale Euler–Lagrange model

This study develops a hybrid multiscale Euler–Lagrange model to investigate the unsteady characteristics of cloud cavitation around a hydrofoil under different water qualities. A homogeneous mixture model is implemented for macroscopic cavity simulations, tracking the vapor–liquid interfaces. In the Lagrangian framework, the dynamics and motion of nuclei and bubbles are resolved. By incorporating more physically accurate conversion criteria to couple the two frameworks, the cavitation model is modified to ensure consistency with the assumption that cavitation inception arises from the expansion of nuclei. Numerical results, obtained under different size distributions of nuclei populations, align well with experimental data, validating the capability of the multiscale model to account for the effects of water quality. They also offer detailed insights into the influence of cavitation flows on microscale bubble behavior, particularly highlighting the significant role of reentry jets in bubble generation and motion. The results underscore the critical interplay between small-scale bubble dynamics and macroscopic cavitation flows. In addition, a statistical analysis of the size distribution of microbubbles reveals a distribution law consistent with experimental observations. This study provides a robust framework for investigating the comprehensive effects of water quality on cloud cavitation flows, offering a promising avenue for future research in this domain.

First-principles calculations of quartz–coesite interfaces

Atomistic interface structures compatible with periodic boundary conditions for the strain-induced subsolidus martensitic transition between quartz and coesite have been investigated. We identified layers of atoms that remained unchanged in terms of neighbor interactions throughout the transformation. Our analysis revealed that the orientation relationships between quartz and coesite, namely (1011)Qz||(010)Coe and (1321)Qz||(010)Coe, are consistent with experimental observations. Using density-functional-theory-based tight-binding model calculations, we determined an interface energy of approximately 660 mJ m−2 for these interfaces and strain energies of 196 (6) and 2760 (160) J mol−1 atom−1 for the (1321)Qz||(010)Coe and (1011)Qz||(010)Coe oriented interfaces, respectively. To visualize these interface structures and facilitate their identification in experiments, we simulated high-resolution transmission electron microscopy images and electron diffraction patterns.

Simulating concrete cracking and failure under impact loading using a novel constitutive integration paradigm in non-ordinary state-based Peridynamics

Concrete cracking and failure under external loads are common in infrastructure, particularly under impact conditions where the variability of loads and the complexity of the cracking process present significant challenges. While the Bond-Based Peridynamic (BBPD) and Ordinary State-Based Peridynamic (OSBPD) models are widely used in concrete damage analysis due to their low computational cost, their simple constitutive laws limit their effectiveness in simulating concrete’s response to impact loading. To overcome this limitation, we propose a novel approach that integrates classical constitutive models into the Peridynamics (PD) framework. Specifically, we reformulate the Concrete Damage Plasticity Model 2 (CDPM2) within a non-ordinary state-based PD (NOSBPD) framework, resulting in the CDPM2-PD model. This model incorporates factors such as strain rate effects and strain hardening and introduces a bond damage criterion for both tensile and compressive damage. Through three numerical examples, the CDPM2-PD model demonstrates its ability to accurately capture the cracking and failure process of concrete under impact, showing strong agreement with experimental observations in areas such as failure mode transitions and crack patterns. These results validate the model’s effectiveness and offer a versatile method for integrating classical constitutive models into the NOSBPD framework for complex material analysis.

Three-dimensional modeling of flow-sediment transport and large-scale bank erosion process in the Lower Yangtze River

Bank erosion in the Lower Yangtze River (LYR) develops rapidly within a few hours, resulting in a large-scale embayment. A three-dimensional model is proposed in the current research to simulate the development of such an erosion process, coupling a hydro- and sediment dynamic module with a bank erosion module, and is evaluated against the observations in a laboratory experiment and against the measurements in LYR. Analysis of the model results indicates that: ① the model reproduced the overall expansion process of the erosion embayment and demonstrated the essential role of changing flow structure; ②the calculated local flow structure within the embayment consists of a major planar circulation and several minor planar circulations, and the major circulation is the primary driving force for the embayment expansion, while these minor circulations probably contribute to the cease of bank erosion; ③Varying water level in the LYR accelerates the bank erosion process in the study reach, compared with a constant water level.

Insights into the separation, enantioseparation and recognition mechanisms of methamphetamine isotopologues on achiral and polysaccharide-based chiral columns in high-performance liquid chromatography.

Isotopologues resulting from the labelling of molecules with deuterium have attracted interest due to the isotope effect observed in chemistry and biosciences. Isotope effect may also play out in noncovalent interactions and mechanisms leading to intermolecular recognition. In chromatography, differences in retention time between isotopologues, as well as between isotopomers have been observed resulting in two different elution sequences (isotope effects): the normal isotope effect when heavier isotopologues retain longer than lighter analogues, and the inverse isotope effect featuring the opposite elution order. Although several cases of deuterium isotope effects have been reported so far, the molecular bases of these phenomena remain unclear. We report the separation of isotopologues of methamphetamine (N-methyl-1-phenylpropan-2-amine) (MET), featuring different number and location of deuterium substituents, on an achiral column by high-performance liquid chromatography (HPLC), as well as the simultaneous separation of isotopologues and their enantiomers on some polysaccharide-based chiral columns. The effects of the number and location of deuterium substituents introduced in MET's structure, of the surface chemistry of the adsorbent, and the mobile phase composition and pH on the retention and separation of isotopologues and their enantiomers were examined. In several cases, the effect of temperature was also evaluated, and the thermodynamic quantities associated with isotopologue adsorption, separation, and enantioseparation were also calculated. To elucidate the molecular bases of the experimental observations, quantum mechanics calculations were performed focusing on the vibrational degree of freedom calculated for models of isotopologue-selector complexes. On this basis, zero-point vibrational energies were computed as useful descriptors to differentiate computationally between deuterated isotopologues. Using HPLC as experimental technique with the following dual function: 1) separation of MET isotopologues and their enantiomers; 2) exploring noncovalent interactions underlying their separation. The degree of deuteration, location of deuterium substituents, and mobile phase pH play key roles in isotopologue separations. By integrating experimental and computational analyses, noncovalent interactions underlying normal and inverse isotope effects were deconvoluted, highlighting the contribution of hydrogen bond, hydrophobic and dispersive forces in isotopologue and enantiomer separation.

A Thermal-Induced Prestress Method for Enhancing Magnetostrictive Material Properties

A technique for inducing prestress in magnetostrictive materials through thermal-induced prestressing is presented. A magneto-thermo-mechanical coupling model has been developed based on the nonlinear constitutive model of the magnetostrictive material and classical laminated beam theory, and the influences of curing temperature and substrate thermal expansion coefficient on the properties of magnetostrictive material have been analyzed. The magnetostrictive composite beams have been manufactured, and the experimental observations confirm the congruence with the proposed model. The composite beam, which includes a sapphire crystal substrate and experiences a temperature shift of ∆T=60℃, demonstrates an increase in magnetostriction by approximately 1.43 times and an enhancement in the maximum piezoelectric coefficient by around 1.45 times. This straightforward and efficient technique for creating pre-stress is especially appropriate for tiny applications, including sensors and actuators.

Numerical Investigation Into Mechanical Behavior of Metastable Olivine During Phase Transformation: Implications for Deep‐Focus Earthquakes

AbstractOne hypothesized mechanism that triggers deep‐focus earthquakes in oceanic subducting slabs below ∼300 km depth is transformational faulting due to the olivine‐to‐spinel phase transition. This study uses finite element modeling to investigate phase transformation‐induced stress redistribution and material weakening in olivine. A thermodynamically consistent constitutive model is developed to capture the evolution of phase transformation in olivine under different pressure and temperature conditions. The overall numerical model enables considering multiscale material features, including the polycrystalline structure, mesoscale heterogeneity, and various phases or variants of phases at the microscopic level, and accounts for viscoplastic behaviors with thermo‐mechanical coupling effects. The model is validated with several benchmarks, including a phase diagram of phase transformation from olivine to spinel. The validated model is used to study the interactive behaviors between defects (heterogeneity) and phase transformation. The simulation results reveal that spinel formation under pressure initiates near inclusions and along the grain boundaries, consistent with experimental observations. At lower temperatures, the transformation leads to the formation of thin conjugate bands of spinel diagonal to the compression loading direction. Local stress analysis along these bands also suggests the initiation of faulting. In contrast, the numerical results at higher transformation rates show that significant spinel formation occurs over a larger area at elevated temperatures, leading to ductile behavior, which agrees with experimental findings. Numerical simulation of multiple inclusions under confined pressure also shows the formation of a network of spinel bands resembling phase‐transformation patterns observed in the laboratory experiments. Additionally, stress softening patterns due to phase transformation are similar to experimental observations.

Modeling the Performance of an Anaerobic Moving Bed Biofilm Reactor.

Sub-models representing transformation processes by microorganisms and hydrolases, a one-dimensional (1-D) biofilm, and a bioreactor were integrated to simulate organic-matter fermentation and methane (CH4) production in an anaerobic moving bed biofilm reactor (AnMBBR). The integrated models correctly represented all experimental observations and identified mechanisms underlying how and why AnMBBR performance changed when the volumetric loading rate (VLR) of total chemical oxygen demand (TCOD) increased from 3.9 to 19.5 kg CODT/m3-d. The fractional removal of TCOD and CH4 production decreased as the VLR of TCOD increased, in part, due to an increasing biofilm thickness that filled the protected channels in the interior of the plastic carriers and led to a decrease in biofilm surface area and an increase in the mass-transfer boundary layer. Also, the ~25-day duration for each VLR of TCOD was too brief to allow the biofilm to establish a new quasi-steady state with respect to biofilm thickness. The mechanistic understanding of how biofilm characteristics and process performance respond to increased VLR of TCOD can be applied in engineering practice to improve AnMBBR process design and operation.

Observation of replica symmetry breaking in filamentation and multifilamentation

Abstract We report the experimental observation and characterization of Replica Symmetry Breaking (RSB) manifestation while analyzing the transverse intensity profile of laser pulses in filamentation experiments using sapphire crystal and distilled water, excited by a femtosecond laser centered at 800 nm. The RSB arises from the competition between self-focusing and plasma defocusing, subject to local fluctuations in the nonlinear refractive index generated by plasma via multiphoton excitation, which subsequently promotes frustration among modes. Our results confirm the existence of glassy-like photonic states not only in multifilamentation, as previously reported [W. Ettoumi, J. Kasparian, and J. Wolf, “Spin-glass model governs laser multiple filamentation,” Phys. Rev. Lett., vol. 115, no. 3, pp. 033902, 2015], but also in the generation of a single filament and in filamentation accompanied by conical emission. These findings improve the understanding of statistical nonlinear optics by establishing connections with magnetism and highlighting the glassy-like behavior of light in the context of ultrafast optical phenomena.

On the nature of high-spin forms in the S2 state of the oxygen-evolving complex.

The Mn4CaO x cluster of the oxygen-evolving complex (OEC) in photosystem II, the site of biological water oxidation, adopts different forms as it progresses through the catalytic cycle of S i states (i = 0-4) and within each S i state itself. This has been amply documented by spectroscopy, but the structural basis of spectroscopic polymorphism remains debated. The S2 state is extensively studied by magnetic resonance spectroscopies. In addition to the common type of g ≈ 2 multiline EPR signal attributed to a low-spin (S = 1/2) form of the manganese cluster, other signals at lower fields (g ≥ 4) associated with the S2 state arise from higher-spin forms. Resolving the structural identity of the high-spin species is paramount for a microscopic understanding of the catalytic mechanism. Hypotheses explored by theoretical studies implicate valence isomerism, proton tautomerism, or coordination change with respect to the low-spin form. Here we analyze structure-property correlations for multiple formulations employing a common high-level protocol based on multiscale models that combine a converged quantum mechanics region embedded within a large protein region treated semiempirically with an extended tight-binding method (DFT/xTB), surpassing conventional quantum mechanics/molecular mechanics (QM/MM) approaches. Our results provide a comprehensive comparison of magnetic topologies, spin states and energetics in relation to experimental observations. Crucial predictions are made about 14N hyperfine coupling constants and X-ray absorption Mn K-pre-edge features as criteria for discriminating between different models. This study updates our view on a persistent mystery of biological water oxidation, while providing a refined and transferable computational platform for future theoretical studies of the OEC.

Deciphering Mg-Surface Interactions with Unsaturated Hydrocarbons: An Integrated Experimental-Theoretical Study.

Here, we used a combination of laser-induced experiments and density functional theory (DFT) calculations to study the mechanism of growth of carbonaceous species on the Mg surface. Experimental observations revealed that the carbon deposit forms upon laser illumination on the Mg surface, with the deposit being clearer and better structured in the presence of 1,3-butadiene (C4H6) compared to ethylene (C2H4) gas. DFT thermodynamic and kinetic calculations of C2-C4 hydrocarbons interaction on low-index Mg(0001) were used to explain this experimental observation. Our results on Mg(0001) showed that the cis isomer of C4H6 binds more strongly than its trans isomer via a [4+2] cycloaddition mechanism. We also investigated the adsorption of two units of C2H4 and C4H6 molecules, as well as the subsequent dehydrogenation stages that produce radical species responsible for chain growth mechanisms. The results showed that free energy of dehydrogenation of two units of cis-C4H6 [i. e. cis-C8H12] is lower than the dehydrogenation of trans conformer of C4H6 and C2H4 molecule, indicating that the dehydrogenation of two units of cis-C4H6 facilitates the initiation of growth of carbonaceous species on Mg surfaces. Therefore, the DFT calculations pinpoint the origin of the experimental observation of clearer carbon deposits on the Mg surface.

Self-Assembly of Mycolic Acid in Water: Monolayer or Bilayer.

The enduring pathogenicity of Mycobacterium tuberculosis can be attributed to its lipid-rich cell wall, with mycolic acids (MAs) being a significant constituent. Different MAs' fluidity and structural adaptability within the bacterial cell envelope significantly influence their physicochemical properties, operational capabilities, and pathogenic potential. Therefore, an accurate conformational representation of various MAs in aqueous media can provide insights into their potential role within the intricate structure of the bacterial cell wall. We have carried out MD simulations of MAs in an aqueous solution and shed light on various structural properties such as thickness, order parameters, area-per-MAs, conformational changes, and principle component (PC) in the single-component and mixture MAs monolayer. The different conformational populations in the monolayer were estimated using the distance-based analysis between the function groups represented as W, U, and Z conformations that lead to the fold of the MAs chain in the monolayer. Additionally, we have also simulated the mixture of alpha-MA (α-MA or AMA), methoxy-MA (MMA), and keto-MA (KMA) with 50.90% AMA, 36.36% MMA, and 12.72% KMA composition. The thickness of the MAs monolayer was observed to range from 5 to 7 nm with an average 820 kg/m3 density for α-MA, MMA, and KMA quantitative agreement with experimental results. The mero chain (long chain), consisting of a functional group at the proximal and distal positions, tends to fold and exhibit a more disordered phase than the short chain. The keto-MA showed the greatest WUZ total conformations (35.32%) with decreasing trend of eZ > eU > aU > aZ folds in both single component and mixture. Our results are in quantitative agreement with the experimental observations. The sZ folds show the lowest conformational probability in monolayer assembly (0.75% in a single component and 1.1% in a mixture). However, eU and aU folds are most probable for AMA and MMA. One striking observation is the abundance of MA conformers beyond the known WUZ convention because of the wide range distribution of intramolecular distances and change in dihedral angles. From a thermodynamic perspective, all mycolic acid monolayers in this study within the microsecond-long simulation, MA molecules self-assembled, and the self-assembled monolayer was found to be stable. The conformation of MAs corresponding to lower free energy minima in the monolayer gives rise to tighter packing and a highly dense self-assembly. Such a highly packed assembly shows higher resistance for drug permeability. Therefore, we concluded that the monolayer formed by AMA will be more densely packed and may cause more resistance for the drug molecules.

Morphology-Transport Coupling and Dissipative Structures in PEO-PS+LiTFSI Electrolytes In-Operando Conditions.

A Single-Chain-in-Mean-Field (SCMF) algorithm was introduced to study block copolymer electrolytes in nonequilibrium conditions. This method self-consistently combines a particle-based description of the polymer with a generalized diffusion equation for the ionic fluxes, thus exploiting the time scale separation between fast ion motion and the slow polymer relaxation and self-assembly. We apply this computational method to study ion fluxes in electrochemical cells containing poly(ethylene oxide)-polystyrene (PEO-PS) block copolymers with added lithium salt. Blocking of the anion fluxes by the electrodes in-operando conditions polarizes the cells and results in an inhomogeneous salt-concentration profile. This gradient of salt concentration triggers lamellae-to-disorder and disorder-to-lamellae transitions near the electrodes, in good agreement with previous experimental observations. The effects of the selectivity of the electrode surface, the salt concentration and the voltage applied to the cell are systematically studied. For PEO-selective surfaces, the lamellae parallel to the electrode that forms at low applied potentials transition to a bicontinuous morphology at high applied potentials in order to allow ion transport through the insulating PS layers. The formation of this dissipative structure, which is unexpected considering the equilibrium behavior of the material, is in line with the principle of maximum entropy production. In summary, the transport and morphology in PEO-PS electrolytes are strongly coupled: ionic currents influence self-assembly, which in turn modulates the ionic fluxes in the cell.