Structural Relaxation Research Articles

Optical pulse-induced ultrafast antiferrodistortive transition in SrTiO3

The ultrafast dynamics of the antiferrodistortive phase transition in perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using femtosecond optical pulses, we initiate a thermally driven tetragonal-to-cubic structural transformation and detect the crystal phase through changes in the frequency of Brillouin oscillations (BO) induced by propagating acoustic phonons. Coupling the measured BO frequency with a spatiotemporal heat diffusion model, we demonstrate that, for a sample kept in the tetragonal phase, deposition of sufficient thermal energy induces a rapid transformation of the heat-affected region to the cubic phase. The initial phase change is followed by a slower reverse cubic-to-tetragonal phase transformation occurring on a timescale of hundreds of picoseconds. We attribute this ultrafast phase transformation in the perovskite to a structural resemblance between atomic displacements of the R-point soft optic mode of the cubic phase and the tetragonal phase, both characterized by anti-phase rotation of oxygen octahedra. The structural relaxation time exhibits a strong temperature dependence consistent with the prediction of the equation of motion describing collective oxygen octahedra rotation based on the energy landscape of the phenomenological Landau theory of phase transitions. Evidence of such a fast structural transition in perovskites can open up new avenues in information processing and energy storage sectors.

Machine learned interatomic potentials for ternary carbides trained on the AFLOW database

Large-density functional theory (DFT) databases are a treasure trove of energies, forces, and stresses that can be used to train machine-learned interatomic potentials for atomistic modeling. Herein, we employ structural relaxations from the AFLOW database to train moment tensor potentials (MTPs) for four carbide systems: CHfTa, CHfZr, CMoW, and CTaTi. The resulting MTPs are used to relax ~6300 random symmetric structures, and are subsequently improved via active learning to generate robust potentials (RP) that can relax a wide variety of structures, and accurate potentials (AP) designed for the relaxation of low-energy systems. This protocol is shown to yield convex hulls that are indistinguishable from those predicted by AFLOW for the CHfTa, CHfZr, and CTaTi systems, and in the case of the CMoW system to predict thermodynamically stable structures that are not found within AFLOW, highlighting the potential of the employed protocol within crystal structure prediction. Relaxation of over three hundred (Mo1−xWx)C stoichiometry crystals first with the RP then with the AP yields formation enthalpies that are in excellent agreement with those obtained via DFT.

Bacterial chromatin proteins, transcription, and DNA topology: Inseparable partners in the control of gene expression.

DNA in bacterial chromosomes is organized into higher-order structures by DNA-binding proteins called nucleoid-associated proteins (NAPs) or bacterial chromatin proteins (BCPs). BCPs often bind to or near DNA loci transcribed by RNA polymerase (RNAP) and can either increase or decrease gene expression. To understand the mechanisms by which BCPs alter transcription, one must consider both steric effects and the topological forces that arise when DNA deviates from its fully relaxed double-helical structure. Transcribing RNAP creates DNA negative (-) supercoils upstream and positive (+) supercoils downstream whenever RNAP and DNA are unable to rotate freely. This (-) and (+) supercoiling generates topological forces that resist forward translocation of DNA through RNAP unless the supercoiling is constrained by BCPs or relieved by topoisomerases. BCPs also may enhance topological stress and overall can either inhibit or aid transcription. Here, we review current understanding of how RNAP, BCPs, and DNA topology interplay to control gene expression.

Dual-phase coexistence enables to alleviate resistance drift in phase-change films

The amorphous phase-change materials with spontaneous structural relaxation leads to the resistance drift with the time for phase-change neuron synaptic devices. Here, we modify the phase change properties of the conventional Ge2Sb2Te5 (GST) material by introducing an SnS phase. It is found that the resistance drift coefficient of SnS-doped GST was decreased from 0.06 to 0.01. It can be proposed that the origin originates from the precipitation of GST nanocrystals accompanied by the precipitation of SnS crystals compared to single-phase GST compound systems. We also found that the decrease in resistance drift can be attributed to the narrowed bandgap from 0.65 to 0.43 eV after SnS-doping. Thus, this study reveals the quantitative relationship between the resistance drift and the band gap and proposes a new idea for alleviating the resistance drift by composition optimization, which is of great significance for finding a promising phase change material.

Unraveling the complexity of Fe-based bulk metallic glasses: Insights into dynamic mechanical relaxation in atomic-scale

This research conducts a thorough investigation into the structural and mechanical evolution of Fe74B20Nb2Hf2Si2 bulk metallic glass (BMG) subjected to thermal aging. Utilizing X-ray diffraction (XRD) and selected area electron diffraction (SAED), the study verifies the amorphous structure of the as-cast BMG, highlighting evidence of short-range ordering (SRO). Thermal aging conducted between 342 and 570 K induces substantial changes, notably the formation of Fe23B6 and α-Fe phases and reduced defect concentrations. During dynamic mechanical analysis, a significant shift was observed, which is characterized by a decrease in the intensity of β-relaxation and an increase in β-activation energy from 333.3±2.1 kJ/mol (for the sample aged at 342 K) to 384.6±5.6 kJ/mol (for the sample aged at 570 K). Nanoindentation evaluations reveal an increase in hardness from 11.85±0.22 GPa (for the as-cast BMG) to 14.65±0.41 GPa (for the sample aged at 570 K), accompanied by an increase in Young’s modulus from 211.61±3.00–244.24±2.00 GPa. These changes imply tighter atomic packing and diminished free volume due to aging, suggesting an advanced stage of structural relaxation. Additionally, the material demonstrates a decrease in plastic strain from 0.0052 in its as-cast state to 0.0026 at 342 K, progressing to brittle behavior with further aging. This study shows the pivotal role of precise aging temperature control in optimizing BMGs for diverse applications, illustrating the intricate interplay between thermodynamics and kinetics in material science.

Impact of cyclic thermal shocks on the electrochemical and tribological properties of Fe-based amorphous coating

Fe-based amorphous coatings hold immense potential for marine industries due to their remarkable properties, including high hardness, exceptional corrosion resistance, and outstanding wear resistance. However, their performance under thermal shock conditions, particularly in high-temperature applications, remains a topic requiring further investigation. In this work, a Fe-based amorphous coating with a composition of Fe48Mo14Cr15Y2C15B6 was successfully developed using High-velocity oxygen fuel thermal spraying. To assess the thermal shock resistance of the amorphous coating, we subjected them to thermal cycles at 300 °C for 150 times, followed by cooling in two different mediums: saltwater quenching and air cooling. The results revealed that the coating maintained excellent contact with the substrate and preserved mainly amorphous structure both in the as-sprayed condition and after thermal shocks. Interestingly, the differential scanning calorimetry (DSC) results indicated that the air-cooled samples exhibited greater structural relaxation and crystallization compared to the brine-quenched samples. This microstructure changes in the air-cooled samples resulted in inferior mechanical properties, such as wear resistance and hardness, compared to the brine-quenched and as-sprayed samples.

Role of fictive temperature distribution involved in CO2 laser polishing of fused silica and its optimization for achieving even heat-affected zones

CO2 laser polishing process is a highly effective way for enhancing the surface quality and laser-induced damage threshold of fused silica optics. Nevertheless, due to the thermal history and spatial gradient of the thermodynamic temperature, the heat-affected zones can be formed unevenly on the surface with an increased fictive temperature, which causes the material densification and residual stress. In this work, a 3D model of structural relaxation coupled with temperature and flow fields was innovatively established to explore the role of fictive temperature distribution involved in CO2 laser polishing of fused silica. The Raman spectrum was applied to verify this model. Subsequently, the impacts of laser beam power, scanning speed, track overlapping, substrate temperature on the fictive temperature were discussed. The results indicate that decreasing the scanning speed and increasing the substrate temperature can decrease the fictive temperature. Additionally, a higher track overlapping can reduce the height of the fictive temperature “tool mark”. Moreover, an optimization strategy based on laser beam scanning speed was proposed based on the uniform distribution of fictive temperature. The depth difference of the heat-affected zones was reduced from 164.26 μm to 31.74 μm. This work could be used to quantitatively predict the 3D fictive temperature distribution and proposed an optimization strategy based on scanning speed to suppress the non-uniformity of fictive temperature distribution, which can provide significant theoretical guidance for CO2 laser polishing technology of fused silica optics.

Hydrogen bond symmetrisation in D2O ice observed by neutron diffraction

Hydrogen bond symmetrisation is the phenomenon where a hydrogen atom is located at the centre of a hydrogen bond. Theoretical studies predict that hydrogen bonds in ice VII eventually undergo symmetrisation upon increasing pressure, involving nuclear quantum effect with significant isotope effect and drastic changes in the elastic properties through several intermediate states with varying hydrogen distribution. Despite numerous experimental studies conducted, the location of hydrogen and hence the transition pressures reported up to date remain inconsistent. Here we report the atomic distribution of deuterium in D2O ice using neutron diffraction above 100 GPa and observe the transition from a bimodal to a unimodal distribution of deuterium at around 80 GPa. At the transition pressure, a significant narrowing of the peak widths of 110 is also observed, attributed to the structural relaxation by the change of elastic properties.

Mode coupling behavior and fragile to strong transition of trehalose in a binary mixture with water upon supercooling.

We perform molecular dynamics simulations of a binary mixture of water and trehalose with the TIP4P/Ice water model. We analyze the slow dynamics of trehalose molecules in the mildly supercooled region for concentrations of 3.66 and 18.57 wt. %. We previously studied the dynamics of water in the same mixtures. Supercooled TIP4P/Ice water solvating trehalose molecules was found to follow the Mode Coupling Theory (MCT) and to undergo a transition from a fragile to a strong behavior for both concentrations. Here, we show that also the dynamics of trehalose molecules follows the MCT and displays a fragile to strong crossover (FSC). The results show that trehalose in binary mixtures with water shares with it the dynamical behavior typical of glass forming liquids. Moreover, the FSC for trehalose structural relaxation times is found to occur at temperatures close to those previously obtained for water in the same solutions, showing that the dynamics of the solute is strongly coupled to that of the solvent. We also perform a MCT test showing that the trehalose dynamics obeys the MCT time-temperature superposition principle and that the exponents derived from the theory and the ones obtained from fitting procedure of the relaxation times are comparable, confirming that trehalose molecules in supercooled water solutions follow the MCT of glassy dynamics. Moreover, as predicted by the theory, trehalose particles have MCT parameters comparable to those of water in the same mixtures. This is an important result, given that MCT was originally formulated for monoatomic particles.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

The change in the transport properties (i.e., water diffusivity, shear viscosity, etc.) when adding salts to water has been used to classify ions as either being chaotropic or kosmotropic, a terminology based on the presumption that this phenomenon arises from respective breakdown or enhancement of the hydrogen-bonding network structure. Recent quasi-elastic neutron scattering measurements of the collective structural relaxation time, τC, in aqueous salt solutions were interpreted as confirming this proposed origin of ion effects on the dynamics of water. However, we find similar changes in τC in the same salt solutions based on molecular dynamics (MD) simulations using a coarse-grained water model in which no hydrogen bonding exists, challenging this conventional interpretation of mobility change resulting from the addition of salts to water. A thorough understanding of specific ion effects should be useful in diverse material manufacturing and biomedical applications, where these effects are prevalent, but poorly understood.

Transcriptomic and histologic analyses preliminarily reveal the immune-metabolic response mechanism to saline-alkaline in large yellow croaker (Larimichthys crocea)

There are large areas of saline-alkaline waters worldwide, the utilization of which would greatly enhance the development of aquaculture productivity. To elucidate the regulatory mechanisms underlying the adaptation of large yellow croaker (Larimichthys crocea) to saline-alkaline water, this study analyzed the growth performance, tissue histology, and gills transcriptome profiles of L. crocea in both seawater (CK) and saline-alkaline water (EX) groups. Growth indices statistics revealed that L. crocea can adapt to saline-alkaline water, with growth performance comparable to that of the CK group. Histological examination revealed partial cellular detachment and structural relaxation in the gills tissue of the EX group, while liver and kidney tissues appeared normal. Transcriptome analysis revealed 3821 differentially expressed genes (DEGs), with 1541 DEGs up-regulated and 2280 DEGs down-regulated. GO enrichment analysis indicated that up-regulated DEGs were enriched in terms related to metabolite production during biological activities, while down-regulated DEGs were associated with terms related to maintaining cellular activities. KEGG enrichment analysis revealed that up-regulated DEGs were enriched in pathways related to the synthesis and metabolism of amino acids and lipids, such as the PPAR signaling pathway and glutathione metabolism. The down-regulated DEGs were predominantly enriched in immune-related signaling pathways, including the Toll-like receptor signaling pathway and NOD-like receptor signaling pathway. Further analysis revealed that genes such as lipoprotein lipase A (lpla), branched-chain amino acid aminotransferase 2 (bcat2), interleukin 8 (il8), interleukin 10 (il10), and interferon regulatory factor 7 (irf7) were involved in the adaptation of L. crocea to saline-alkaline water culture conditions. This study provides a basis for understanding the adaptability of large yellow croaker to saline-alkaline water and lays the foundation for the rational utilization of fishery water resources.

Structure of Polaronic Centers in Proton-Intercalated AWO4 Scheelite-Type Tungstates.

The studies of polaronic centers in a homologous series of scheelite-type compounds AWO4 (A = Ca, Sr, Ba) were performed using the W L3-edge and Sr K-edge X-ray absorption spectroscopy combined with the reverse Monte Carlo simulations, X-ray photoelectron spectroscopy (XPS), and first-principles calculations. Protonated scheelites HxAWO4 were produced using acid electrolytes in a one-step route at ambient conditions. The underlying mechanism behind this phenomenon can be ascribed to the intercalation of H+ into the crystal structure of tungstate, effectively resulting in the reduction of W6+ to W5+, i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W5+ was confirmed by XPS experiments. The relaxation of the local atomic structure around the W5+ polaronic center was determined from the analysis of the extended X-ray absorption fine structures using the reverse Monte Carlo method. The results obtained suggest the displacement of the W5+ ions from the center of [W5+O4] tetrahedra in the structure of AWO4 scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.

Direct Numerical Analysis of Dynamic Facilitation in Glass-Forming Liquids.

We propose a computational strategy to quantify the temperature evolution of the timescales and length scales over which dynamic facilitation affects the relaxation dynamics of glass-forming liquids at low temperatures, which requires no assumption about the nature of the dynamics. In two glass models, we find that dynamic facilitation depends strongly on temperature, leading to a subdiffusive spreading of relaxation events which we characterize using a temperature-dependent dynamic exponent. We also establish that this temperature evolution represents a major contribution to the increase of the structural relaxation time.

Feedback boundary control of multi-dimensional hyperbolic systems with relaxation

This paper is concerned with boundary stabilization of multi-dimensional hyperbolic systems of partial differential equations. By adapting the Lyapunov function previously proposed by the second author for linearized hyperbolic systems with relaxation structure, we derive certain control laws so that the corresponding solutions decay exponentially in time. The result is illustrated with an application to water flows in open channels. The effectiveness of the derived control laws is confirmed by numerical simulations.

Ab initio property characterisation of thousands of previously unexplored 2D materials

We perform extensive density functional theory calculations to determine the stability and elementary properties of 4249 previously unexplored monolayer crystals. The monolayers comprise the most stable subset (energy within 0.1 eV/atom of the convex hull) of a larger portfolio of two-dimensional (2D) materials recently discovered using a deep generative model and systematic lattice decoration schemes. The relaxed 2D structures are run through the basic property workflow of the Computational 2D Materials Database (C2DB) to evaluate the dynamical stability and obtain the stiffness tensor, piezoelectric tensor, deformation potentials, Born and Bader charges, electronic band structure, effective masses, plasma frequency, Fermi surface, projected density of states, magnetic moments, magnetic exchange couplings, magnetic anisotropy, topological indices, optical- and infrared polarisability. We provide statistical overviews of the property data and highlight a few specific examples of interesting materials. Our work exposes previously unknown parts of the 2D chemical space and provides a basis for the discovery of 2D materials with specific properties. All data is available in the C2DB.

Adsorption of dihalogen molecules X2 (X = F, Cl, Br and I) on the Fe/W(110) substrate

We report on spin-polarized density-functional theory study of adsorption of dihalogen molecules X2 (X = F, Cl, Br and I) on the Fe/W(110) substrate, i.e., X2/Fe/W(110) systems. We considered different molecular orientations and adsorption sites of the halogens and obtained their corresponding ground-state structures. We obtained initial molecular orientation (IMO) and initial adsorption site (IAS), i.e., IMO-IAS combinations that give the minimum energy configurations for each of the X2/Fe/W(110) systems. Our results shows that all the molecules studied in this work are chemisorbed on the Fe surface. Also, the halogen atoms may be adsorbed dissociatively on the hollow sites in such a way that an X2 separates into two X atoms with each of the atoms located at two nearby hollow sites. Similarly, we found IMO-IAS combinations which resulted in a non-dissociative adsorption. In the latter, the pre-relaxed IMO-IAS is maintained even after the structural relaxation. The most stable configuration for the X2 dihalogen molecule in this case is either the top or bridge site while the halogen is in perpendicular orientation to the Fe surface. We conclude therefore that, the final relaxed configurations of the X2 halogen depends on the IMO through which is deposited on the Fe/W(110) substrate. The trend in the adsorption energy EA for the most stable configurations for the dissociative adsorption is EA (F) > EA (Cl) > EA (Br) > EA (I). The trend of EA for non-dissociative adsorption is similar to that of dissociative adsorption, however, the latter is the more energetically favorable. Electronic structure calculations show hybridization between the p and d orbitals of X and Fe atoms respectively. Furthermore, we have found antiferromagnetic coupling between the interfacial W atoms and the Fe overlayer atoms while ferromagnetic coupling is found between the halogens and the Fe atoms. Our work represents a detailed study of adsorption properties of highly reactive halogens in contact with the Fe/W(100) surface.

Achieving high fatigue endurance of nanocrystalline Ni/Ni-W layered composites through thermally and mechanically-induced relaxations

Stabilizing non-equilibrium nanograin boundaries has been shown to enhance the strength of nanocrystalline metals. Nevertheless, a thorough comprehension of how these stable grain boundaries impact the fatigue behavior in nanocrystalline metallic layered composites with heterogeneous interfaces is currently lacking. Here, aiming to develop high-performance small components in microelectromechanical systems, we prepared nanocrystalline Ni/Ni-W layered composites that underwent various degrees of annealing treatment. Our results reveal that the Ni/Ni-W layered composites annealed at 200 °C exhibit significantly higher fatigue strength, surpassing that of the as-prepared composites by 40 % and the Pt-10 at.% Au alloy by 17 % which is one of the best candidates for the microelectromechanical systems switches at present. The enhanced fatigue strength is primarily attributed to the annealing-induced grain boundary relaxation and mechanically-induced structural relaxation. Grain boundary relaxation enhances the strength by improving GB stability, while the mechanically-induced structural relaxation results in the coarsening and expansion of triangular columnar grains towards the Ni-W layer during fatigue loading. As a result, the dispersive strain localized regions triggered by the triangular columnar grains undertook cyclic strain accumulation and weakened localized damage accumulation in Ni layers, and thus further improved the fatigue resistance. The finding of the underlying mechanisms may offer a promising approach for designing high-performance materials for microelectromechanical systems switches operating at elevated temperatures.

A model on the coupling between cyclic fatigue and microstructure evolution in a metallic glass

Establishing the intrinsic correlation between microstructural heterogeneity and mechanical properties is a challenging issue of metallic glasses. The ratchet behavior was examined in a Zr-based metallic glass under cyclic tensile loading well below the yield point, particularly near the glass transition temperature. It is found that strain evolution during cyclic loading shows heightened sensitivity to temperature and stress rate. Also, creep behavior mirrors the ratchet strain induced by cyclic loading. The proposed constitutive model, integrating the Burgers model with defect concentration based on free volume theory, effectively describes strain evolution during cyclic loading near glass transition temperature. Both macroscopic and microscopic perspectives are included in this model. The results verify that metallic glasses exhibit significant viscous characteristics, displaying noticeable creep deformation under low stress rates and amplitudes, which contributes to ratchet behavior. The fitted parameters show that plastic viscosity decreases with temperature and increases with stress rate, corroborating the decrease of tensile yield stress with temperature increasing; also, the fitted relaxation time increases with loading frequency, reflecting evolution of defect concentration. Structural relaxation competes favorably against stress-driven rejuvenation throughout the cyclic process, suggesting potential in tuning metallic glasses properties through innovation thermos-mechanical processing techniques.

First Order Methods for Geometric Optimization of Crystals: Experimental Analysis

AbstractThe geometric optimization of crystal structures is a procedure widely used in computational chemistry that changes the geometrical placement of the particles inside a structure. It is called structural relaxation and constitutes a local minimization problem with a non‐convex objective function whose domain complexity increases according to the number of particles involved. This work studies the performance of the two most popular gradient methods in structural relaxation, Steepest Descent and Conjugate Gradient. Although frequently employed, there is a lack of their study in this context from an algorithmic point of view. The algorithms are initially benchmarked on the basis of a constant step size. Three concepts for designing dynamic step size rules are then examined in detail and analyzed. Results show that there is a trade‐off between convergence rate and the possibility of an experiment to succeed. In order to address this, a function is proposed as a formal means for assigning utility to each method based on preference. The function is built according to a recently introduced model of preference indication concerning algorithms with deadline and their run time. It introduces the quantification of the optimization algorithms' performance according to convergence speed and success rate, thus enabling the appointment of a specific algorithmic recipe as the best choice for balanced preferences.

Epitaxial growth of GaInAsBi thin films on Si (001) substrate using pulsed laser deposition

The results of a comprehensive study of growing a Ga1-yInyAs1-xBix quaternary alloy on a Si (001) substrate, determining the composition, studying structural properties, relaxation mechanisms and surface morphology are presented. Using pulsed laser deposition, epitaxial growth of the Ga1-yInyAs1-xBix thin film was carried out on the Si (001) substrate. Using X-ray diffraction, it was established that the Ga1-yInyAs1-xBix film was grown with a large lattice mismatch (Δa/a = 7.838 %). It is shown that the film has a well-pronounced texture in the [001] growth direction, a lattice parameter of 5.856 Å. Using Medium Energy Ion Scattering, a decrease in the In concentration after a layer thickness ≈15 nm from 0.5 to 0.4 mol frac. was detected. An important result is the homogeneous distribution of Bi throughout the layer thickness. Structural studies allow for the conclusion that stress relaxation occurred because of shears by means of a nucleation of dislocations and a slip close-packed {111} planes, as well as twinning, which led to a change in composition after a thickness of ≈15 nm from the substrate and an increase in the surface roughness of the layer.