Interaction Strength Research Articles

Interlayer and interfacial Dzyaloshinskii-Moriya interaction in magnetic trilayers: First-principles calculations

We determine the Dzyaloshinskii-Moriya interaction within and between two magnetic cobalt layers separated by a nonmagnetic spacer through calculations. We investigate different materials for the nonmagnetic layer, focusing on the experimentally realized Co/Ag/Co system. We laterally shift the atoms in the nonmagnetic layer to achieve the symmetry breaking required for the interlayer Dzyaloshinskii-Moriya interaction. We compare the resulting interactions with the Levy-Fert model and observe a good overall agreement between the model and the calculations for the dependence on the atomic positions. Additionally, we derive a formula for the strength of the interlayer isotropic exchange interaction depending on the position of the atoms in the nonmagnetic layer and compare it to the first-principles results. We investigate the limitations of the Levy-Fert model by turning off the spin-orbit coupling separately on the nonmagnetic and magnetic atoms and by studying the effect of band filling. Our work advances the understanding of the microscopic mechanisms of the interlayer Dzyaloshinskii-Moriya interaction and gives insight into possible new material combinations with strong interlayer Dzyaloshinskii-Moriya interaction. Published by the American Physical Society 2024

Unraveling the Structural Basis of Biased Agonism in the β2-Adrenergic Receptor Through Molecular Dynamics Simulations.

Biased agonism in G protein-coupled receptors is a phenomenon resulting in the selective activation of distinct intracellular signaling pathways by different agonists, which may exhibit bias toward either Gs, Gi, or arrestin-mediated pathways. This study investigates the structural basis of ligand-induced biased agonism within the context of the β2-adrenergic receptor (β2-AR). Atomistic molecular dynamics simulations were conducted for β2-AR complexes with two stereoisomers of methoxynaphtyl fenoterol (MNFen), that is, compounds eliciting qualitatively different cellular responses. The simulations reveal distinct interaction patterns within the binding cavity, dependent on the stereoisomer. These changes propagate to the intracellular parts of the receptor, triggering various structural responses: the dynamic structure of the intracellular regions of the (R,R)-MNFen complex more closely resembles the "Gs-compatible" and "β-arrestin-compatible" conformation of β2-AR, while both stereoisomers maintain structural responses equidistant from the inactive conformation. These findings are confirmed by independent coarse-grained simulations. In the context of deciphered molecular mechanisms, Trp313 plays a pivotal role, altering its orientation upon interactions with (R,R)-MNFen, along with the Lys305-Asp192 ionic bridge. This effect, accompanied by ligand interactions with residues on TM2, increases the strength of interactions within the extracellular region and the binding cavity, resulting in a slightly more open conformation and a minor (by ca. 0.2 nm) increase in the distance between the TM5-TM7, TM1-TM6, TM6-TM7, and TM1-TM5 pairs. On the other hand, an even slighter decrease in the distance between the TM1-TM4 and TM2-TM4 pairs is observed.

Optical characterization of molecular interaction strength in protein condensates.

Biomolecular condensates have been identified as a ubiquitous means of intracellular organization, exhibiting very diverse material properties. However, techniques to characterize these material properties and their underlying molecular interactions are scarce. Here, we introduce two optical techniques - Brillouin microscopy and quantitative phase imaging (QPI) - to address this scarcity. We establish Brillouin shift and linewidth as measures for average molecular interaction and dissipation strength, respectively, and we used QPI to obtain the protein concentration within the condensates. We monitored the response of condensates formed by FUS and by the low-complexity domain of hnRNPA1 (A1-LCD) to altering temperature and ion concentration. Conditions favoring phase separation increased Brillouin shift, linewidth, and protein concentration. In comparison to solidification by chemical crosslinking, the ion-dependent aging of FUS condensates had a small effect on the molecular interaction strength inside. Finally, we investigated how sequence variations of A1-LCD, that change the driving force for phase separation, alter the physical properties of the respective condensates. Our results provide a new experimental perspective on the material properties of protein condensates. Robust and quantitative experimental approaches such as the presented ones will be crucial for understanding how the physical properties of biological condensates determine their function and dysfunction.

Quantum wires with local particle loss: Transport manifestations of fluctuation-induced effects

We investigate the transport properties of a quantum wire of weakly interacting fermions in the presence of local particle loss. We calculate current and conductance in this system due to applied external chemical potential bias that can be measured in experimental realizations of ultracold fermions in quasi one-dimensional traps. Using a Keldysh field theory approach based on the Lindblad equation, we establish a perturbative scheme to study the effect of imbalanced reservoirs. Logarithmically divergent terms are resummed using a renormalization group method, and a novel powerlaw behavior for the conductance as a function of the potential bias across the wire is found. In contrast to the equilibrium case of a potential barrier in a Luttinger liquid, the conductance exhibits a scaling behavior, which depends on the interaction strength and on the loss probability. Repulsive interactions reduce the conductance of the wire while attractive interactions enhance it. However, perfect reflectivity and transparency are only reached in the absence of particle loss. Published by the American Physical Society 2024

Pairing phase favored by magnetic frustration.

The interplay between magnetic frustration and pairing is investigated by adopting a BCS-like pairing mechanism on the frustrated $J_1-J_2$ Ising model on the square lattice. The ground-state and thermal phase transitions of the model are analyzed using a fermionic formulation within a cluster mean-field method. In this approach, the lattice system is divided into identical clusters, where the intracluster dynamic is exactly solved, and the intercluster interactions are replaced by self-consistent mean fields. We introduce a framework with two pairing couplings: an intracluster local coupling, $g$, which controls the electron pairing and its mobility within the clusters, and an intercluster coupling, $g'$, which adjusts the pairing mechanism between clusters. Tuning $g'/g$ allows evaluating how the pairing phase evolves from a weak pairing coupling between clusters (clustered system) to a strong one ($g' \rightarrow g$, homogeneous system). In the range $0 \le g'/g \le 1$, we find that a gradual increase in $g'/g$ favors the pairing phase and induces a change in criticality. In particular, our results reveal the presence of tricriticality for a certain range of $g'/g$. In addition, an increase in competing magnetic interactions weakens the magnetic orders, causing the pairing phase to occur at lower strengths of pairing interactions, especially when $g' = g$. Therefore, our findings support that magnetic frustration favors the pairing phase, contributing to the onset of a superconducting state.

Effects of Diffusing Squalene on the Plastic Deformation of Ultrahigh-Molecular-Weight Polyethylene─Insights from Molecular Dynamics Simulations.

Ultrahigh-molecular-weight polyethylene (UHMWPE) stands out as a popular artificial joint material. However, wear limits its service life, which is mainly caused by accumulation of plastic deformation. The plastic deformation on the frictional interface reflects the early wear of UHMWPE. To investigate the effect of squalene, a typical component in the body fluid, on the tribological properties of UHMWPE at microscopic scale, the diffusion behavior of squalene into polyethylene and its influence on the plastic deformation of polyethylene are discussed using the molecular dynamics (MD) simulation. The lubrication model shows that polyethylene reconstructed from the interface to lower substrate, with refactor gaps between polyethylene chains. This promotes squalene molecules to gradually diffuse into polyethylene from these gaps and causes the polyethylene structure to become loose. On the other hand, in the diffused model, squalene in polyethylene substrates increases the plastic deformation of polyethylene. The separation of squalene reduces the interaction strength between adjacent polyethylene chains and accelerates the disentanglement of polyethylene. The flexibility of "C═C" bonds in squalene allows the continuous adjustment of its spatial structures to adapt the space between polyethylene chains. The squalene fragments will not hinder the plastic flow of polyethylene.

Abstract 4122024: Fibroblasts Are the Primary Contributors to a Disrupted Micro-Environment in End-Stage Pediatric Hypertrophic Cardiomyopathy

Background: Hypertrophic cardiomyopathy (HCM) is a relatively rare but debilitating diagnosis in pediatric patients. This represents a vulnerable population who often require heart transplantation. Here, we examine the transcriptome in ventricular tissue to identify underlying cellular processes unique to pediatric HCM (pHCM). Methods: We performed single-nucleus RNA sequencing on explanted hearts at transplant in female patients with end-stage pHCM (n=3) and compared findings to pediatric control (n=2) and adult HCM (n=5; Figure 1A-1B). Results: We generated 91,682 single-cell transcriptomes from 8 tissue samples. Principal component analysis exhibited the largest source of transcriptional variation separated pHCM and controls (Figure 1C). We identified distinct underlying cellular processes in cardiomyocytes, fibroblasts, endothelial cells, and myeloid cells in pHCM. pHCM was enriched in cardiomyocytes with stressed signatures and pathways associated with hypertrophy (Figure 1D); we noted depletion of tissue-resident macrophages (Figure 1D) and increased vascular remodeling in endothelial cells in pHCM. Fibroblasts exhibited activation signatures and compared to adult HCM, showed heightened fibrotic processes and a unique pro-fibrotic cluster with increased metabolic stress, mitochondrial turnover, and anti-apoptotic properties (Figure 2). In cell-to-cell communication, the differential number and weight of interactions were higher in pHCM compared to controls and fibroblasts had the highest strength of outgoing interactions. Conclusions: Our analysis provides the first single-nuclei analysis focused on pHCM. Fibroblast-mediated cellular processes contributed most to the disrupted micro-tissue environment; they had more downstream processes associated with fibrosis and may respond to metabolic stress differently compared to adult HCM. This advances our understanding of the early presentation of pHCM, providing a crucial steppingstone toward improved patient outcomes.

Role of harmonic and anharmonic interactions in controlling the cooperativity of spin-crossover molecular crystals

We present a theoretical model of spin transitions in homogeneous spin-crossover (SCO) materials, based on the description of the interactions between SCO molecules by the Lennard-Jones potential in which the equilibrium distances depend on the spin states of the linked molecules. By expanding the potential to order 3 considering a homogeneous lattice spacing, we demonstrate that the present model is isomorphic with an Ising-like model combining infinite long-range “ferromagnetic-like” interactions, competing with short-range “antiferromagnetic-like” between nearest neighbors in the case of harmonic interactions. When an anharmonic contribution is included, the short-range interaction becomes dependent on the average high-spin fraction, and changes its sign along the spin transition. The thermodynamic properties of the model were first investigated analytically in mean field approximation for both harmonic and anharmonic cases, for which we clarify the conditions for obtaining gradual and first-order transitions. Moreover, we demonstrated that within this approximation, the strength of the antiferromagnetic-like interactions are insufficient to produce multistep transitions. In contrast, the resolution of the previous emerging Ising-like model by Monte Carlo simulations demonstrated the possibility of obtaining gradual, first-order and two-step transitions as a result of the antagonism between long- and short-range interactions. Overall, the obtained results demonstrate the suitability of the present theoretical model to describe cooperative SCO materials. The key message of our paper is that our model can be used to predict complex behavior of spin-crossover crystals. Published by the American Physical Society 2024

An explicitly magnetic modified embedded atom method formalism for coupled spin dynamics and molecular dynamics

Abstract In this paper, we augment the modified embedded atom method formalism to include magnetic spin-spin interactions for elements with a persistent magnetic moment. While previous spin coupling methods have been based on pair potentials, our Magnetic MEAM formalism, which we term MagMEAM, incorporates the many-body and angular effects of MEAM allowing for the strength of the magnetic interaction to vary with atomic environment. In particular, this allows potentials using this formalism to differentiate the magnetic interaction of different stable phases of magnetic elements such as the ferritic and austenitic phases of iron. This, in turn, allows for a more robust and realistic description of magnetism in polymorphic materials than was previously possible. The motivation for MagMEAM, including the insufficiency of magnetic pair potentials, is presented and the structure of the formalism is developed. A sample iron potential is developed using this formalism and shown to exceed the capabilities of existing magnetic pair potentials by simultaneously reproducing the magnetic energy of both martensite and austenite as well as the dynamic mechanical and magnetic properties of martensite. This newly designed formalism will allow for deeper explorations in the the complex interaction between different phases of polymorphic magnetic materials at the molecular dynamics scale.

Theoretical Study on Intramolecular Hydrogen Bonds of Flavonoid Cocrystals.

This study investigates the role of intramolecular hydrogen bonds in the formation of cocrystals involving flavonoid molecules, focusing on three active pharmaceutical ingredients (APIs): chrysin (CHR), isoliquiritigenin (ISO), and kaempferol (KAE). These APIs form cocrystals with different cocrystal formers (CCFs) through intramolecular hydrogen bonding. We found that disruption of these intramolecular hydrogen bonds leads to decreased stability compared to molecules with intact bonds. The extrema of molecular electrostatic potential surfaces (MEPS) show that flavonoid molecules with disrupted intramolecular hydrogen bonds have stronger hydrogen bond donors and acceptors than those with intact bonds. Using the artificial bee colony algorithm, dimeric structures of these flavonoid molecules were explored, representing early-stage structures in cocrystal formation, including API-API, API-CCF, and CCF-CCF dimers. It was observed that the number and strength of dimeric interactions significantly increased, and the types of interactions changed when intramolecular hydrogen bonds were disrupted. These findings suggest that disrupting intramolecular hydrogen bonds generally hinders the formation of cocrystals. This theoretical study provides deeper insight into the role of intramolecular hydrogen bonds in the cocrystal formation of flavonoids.

PH-controlled-ion-mobility of Laponite-phosphate dispersion for physical hydrogel with improved mechanical properties and sensitivity to CO2

Self-assembled physical nanocomposite hydrogels based on bio-based poly(itaconic acid) and Laponite® nanoclays were prepared and tested as possible CO2 sensors in smart food packaging. The structure and mechanical properties of the fabricated hydrogels were controlled by changing the initial pH of the aqueous Laponite® dispersions stabilised by tetrasodium pyrophosphate. The dispersions were studied by SAXS and analysed using circular disc and fractal model to determine clay agglomeration tendency. Sodium and pyrophosphate ions mobility was investigated by 23Na and 31P 1D as well as spin-spin relaxation times T2 NMR measurements. The rheological behaviour of the produced hydrogels was studied by oscillatory shear measurements. It was found that decreasing the pH of the Laponite® nanodispersions up to 8 increases the strength of the physical interactions between the nanoclays, dispersant, and polyelectrolyte chains, which enables production of dimensionally stable and mechanically robust hydrogels. Their potential application as the CO2-sensitive matrix for construction of environmentally friendly colorimetric sensors was demonstrated showing the CO2 sorption capacity of 0.07 mmol CO2/g. A prototype device of hydrogel sensor was built and tested in a food packaging application to monitor the plum fruit respiration processes.

The strength of interspecies interaction in a microbial community determines its susceptibility to invasion.

Previous work shows that a host's resident microbial community can provide resistance against an invading pathogen. However, this community is continuously changing over time due to adaptive mutations, and how these changes affect the invasion resistance of these communities remains poorly understood. To address this knowledge gap, we used an experimental evolution approach in synthetic communities of Escherichia coli and Salmonella Typhimurium to investigate how the invasion resistance of this community against a bacterium expressing a virulent phenotype, i.e., colicin secretion, changes over time. We show that evolved communities accumulate mutations in genes involved in carbon metabolism and motility, while simultaneously becoming less resistant to invasion. By investigating two-species competitions and generating a three-species competition model, we show that this outcome is dependent on the strength of interspecies interactions. Our study demonstrates how adaptive changes in microbial communities can make them more prone to the detrimental effects of an invading species.

Dynamics of spin oscillation in double barrier synthetic antiferromagnet based magnetic tunnel junction in presence of spin-transfer torque

In this work, we have studied the spin dynamics of a synthetic antiferromagnet (AFM)/heavy metal/ferromagnet double barrier magnetic tunnel junction in the presence of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, interfacial Dzyaloshinskii–Moriya (iDM) interaction, Néel field, and Spin–Orbit Coupling (SOC) with different Spin-Transfer Torque (STT). We employ the Landau–Lifshitz–Gilbert–Slonczewski equation to investigate the AFM dynamics of the proposed system. We found that the system exhibits a transition from regular to damped oscillations with the increase in strength of STT for systems with a weaker strength of iDM interaction than RKKY interaction while displaying sustained oscillations for systems having the same order of RKKY and iDM interactions. On the other hand, the systems with sufficiently strong iDM interaction strength exhibit self-similar but aperiodic patterns in the absence of the Néel field. In the presence of the Néel field, the RKKY interaction dominating systems exhibit chaotic oscillations for low STT but display sustained oscillations under moderate STT. Our results suggest that the decay time of oscillations can be controlled via SOC. The system can work as an oscillator for low SOC but displays non-linear characteristics with the rise in SOC for systems having weaker iDM interaction than RKKY interactions. In contrast, opposite characteristics are noticed for iDM interaction dominating systems. We found periodic oscillations under low external magnetic fields in RKKY interaction dominating systems. However, moderate fields are necessary for sustained oscillation in iDM interaction dominating systems. Moreover, the system exhibits saddle-node bifurcations and chaos under moderate Néel field and SOC with suitable RKKY and iDM interactions. In addition, our results indicate that the magnon lifetime can be enhanced by increasing the strength of iDM interaction for both optical and acoustic modes.

Peer Interaction Dynamics and Second Language Learning Trajectories During Study Abroad: A Longitudinal Investigation Using Dynamic Computational Social Network Analysis

AbstractUsing computational Social Network Analysis (SNA), this longitudinal study investigates the development of the interaction network and its influence on the second language (L2) gains of a complete cohort of 41 U.S. sojourners enrolled in a 3‐month intensive study‐abroad Arabic program in Jordan. Unlike extant research, our study focuses on students’ interactions with alma mater classmates, reconstructing their complete network, tracing the impact of individual students’ positions in the social graph using centrality metrics, and incorporating a developmental perspective with three measurement points. Objective proficiency gains were influenced by predeparture proficiency (negatively), multilingualism, perceived integration of the peer learner group (negatively), and the number of fellow learners speaking to the student. Analyses reveal relatively stable same‐gender cliques, but with changes in the patterns and strength of interaction. We also discuss interesting divergent trajectories of centrality metrics, L2 use, and progress; predictors of self‐perceived progress across skills; and the interplay of context and gender.

Plant phenology predicts the stability of primary production in three ecosystems of the northern Chihuahuan Desert

Abstract The temporal stability of plant productivity affects species' access to resources, exposure to stressors and strength of interactions with other species in the community, including support to the food web. The magnitude of temporal stability depends on how a species allocates resources among tissues and across phenological stages, such as vegetative growth versus reproduction. Understanding how plant phenological traits correlate with the long‐term stability of plant biomass is particularly important in highly variable ecosystems, such as drylands. We evaluated whether phenological traits predict the temporal stability of plant species productivity by correlating 18 years of monthly phenology observations with biannual estimates of above‐ground plant biomass for 98 plant species from semi‐arid drylands. We then paired these phenological traits with potential climate drivers to identify abiotic contexts that favour specific phenological strategies among plant species. Phenological traits predicted the stability of plant species above‐ground biomass. Plant species with longer vegetative phenophases not only had more stable biomass production over time but also failed to fruit in a greater proportion of years, indicating a growth–reproduction trade‐off. Earlier leaf‐out dates, longer fruiting duration and longer time lags between leaf and fruit production also predicted greater temporal stability. Species with stability‐promoting traits began greening in drier conditions than their unstable counterparts and experienced unexpectedly greater exposure to climate stress, indicated by the wider range of temperatures and precipitation experienced during biologically active periods. Our results suggest that bet‐hedging strategies that spread resource acquisition and reproduction over long time periods help to stabilize plant species productivity in variable environments, such as drylands. Read the free Plain Language Summary for this article on the Journal blog.

Enhancing the Encapsulation Performances of Liposomes for Amphiphilic Copolymers by Computer Simulations.

Liposomes, which encapsulate drugs into an inner aqueous core and demonstrate high drug-loading capacity, have attracted considerable interest in the field of drug delivery. Herein, the encapsulation processes for amphiphilic copolymers within liposomes have been investigated systematically to enhance the encapsulation capacity and optimize the structures using dissipative particle dynamics simulations. The results indicate that the physicochemical properties of lipids, receptors, and amphiphilic copolymers collectively determine the encapsulation behaviors of liposomes. Adjusting the hydrophobic interaction between hydrophobic tails of lipids (receptors) and hydrophobic blocks of copolymers, along with modulating the specific interaction between ligands and the functional head groups of receptors, can lead to various encapsulation capacities. Significantly, a medium hydrophobic interaction strength or a strong specific interaction is conducive to achieving a higher degree of encapsulation for amphiphilic copolymers. Furthermore, varying the key parameters, such as the hydrophobic interaction, the specific interaction, as well as the concentrations of lipids and receptors, can induce seven typical aggregate structures: heterogeneous, fully encapsulated, partially encapsulated, saturated-encapsulated, unsaturated-encapsulated, multilamellar, and column-like structures. The final phase diagrams are also constructed to provide a guideline for designing various structures of liposomes encapsulated with amphiphilic copolymers. These results significantly contribute to the illumination of strategies for the rational construction of the self-assembly system that facilitates the efficient encapsulation of amphiphilic copolymers within the inner aqueous core of liposomes, thereby providing valuable insights into the optimal design of liposome carriers for future biomedical applications.

Detection of electron paramagnetic resonance of two electron spins using a single NV center in diamond

An interacting spin system is an excellent testbed for fundamental quantum physics and applications in quantum sensing and quantum simulation. For these investigations, detailed information on the interactions, e.g., the number of spins and their interaction strengths, is often required. In this study, we present the identification and characterization of a single nitrogen vacancy (NV) center coupled to two electron spins. In the experiment, we first identify a well-isolated single NV center and characterize its spin decoherence time. Then, we perform NV-detected electron paramagnetic resonance (EPR) spectroscopy to detect surrounding electron spins. From the analysis of the NV-EPR signal, we precisely determine the number of detected spins and their interaction strengths. Moreover, the spectral analysis indicates that the candidates of the detected spins are diamond surface spins. This study demonstrates a promising approach for the identification and characterization of an interacting spin system for realizing entangled sensing using electron spin as quantum reporters.

Quantum many-body scars in few-body dipole-dipole interactions

We simulate the dynamics of Rydberg atoms resonantly exchanging energy via two-, three-, and four-body dipole-dipole interactions in a one-dimensional array. Using simplified models of a realistic experimental system, we study the initial-state survival probability, mean level spacing, spread of entanglement, and properties of the energy eigenstates. By exploring a range of disorders and interaction strengths, we find regions in parameter space where the three- and four-body dynamics either fail to thermalize or do so slowly. The interplay between the stronger hopping and weaker field-tuned interactions gives rise to quantum many-body scar states, which play a critical role in slowing the dynamics of the three- and four-body interactions. Published by the American Physical Society 2024

The Ordered Structures Formed by Janus-like Particles on a Triangular Lattice.

The formation of ordered structures by Janus-like particles, composed of two parts (A and B), with orientation-dependent interactions on a triangular lattice was studied using Monte Carlo methods. The assumed lattice model allows each particle to take on one of the six orientations. The interaction between the A parts of neighboring particles was assumed to be attractive, while the AB and BB interactions were assumed to be repulsive. Moreover, it was assumed that the interaction between a pair of neighboring particles depended on the degrees to which their AA, AB, and BB parts face each other. It was shown that several ordered phases of different densities and structures may appear, depending on the magnitudes of AB and BB interactions. In particular, we found several structures composed of small clusters consisting of three (OT), four (OR), and seven (S) particles, surrounded by empty sites, the lamellar phases (OL, OL1, and OL3), the structures with hexagonal symmetry (R3×3 and K), as well as the structures with more complex symmetry (R5×5 and LAD). Several phase diagrams were evaluated, which demonstrated that the stability regions of different ordered phases are primarily determined by the strengths of repulsive AB and BB interactions.

Eigenstate plateau transition and equilibration in one-dimensional quantum lattice models

We report a remarkable spectral phenomenon in a generic quantum lattice gas model. As the interaction strength increases, eigenstates spontaneously reorganize, leading to plateaus in the interaction energy. Gaps between plateaus can emerge similarly to continuous phase transitions. Our perturbation analysis indicates that, aside from these gaps, the plateau structure of the spectrum remains stable in the thermodynamic limit. The structured spectrum naturally manifests in far-from-equilibrium dynamics, exhibiting multiple stages. Our findings reveal a connection between emergent properties in high-energy states and equilibration dynamics in closed quantum systems, offering insights into the impact of interactions across the entire energy spectrum. These results directly relate to experiments probing equilibration in quantum spin systems and lattice gases. Published by the American Physical Society 2024