Intrinsic Coupling Research Articles

Multiphysics analysis of RANS-based turbulent transport of solid fission products in the Molten Salt Fast Reactor

The analysis of innovative reactor concepts such as the Molten Salt Fast Reactor (MSFR) requires the development of new modeling and simulation tools. In the case of the MSFR, the strong intrinsic coupling between thermal–hydraulics, neutronics and fuel chemistry has lead to the adoption of the multiphysics approach as a state-of-the-art paradigm. One of the peculiar aspects of liquid-fuel reactors such as the MSFR is the mobility of fission products (FPs) in the reactor circuit. Some FP species appear in form of solid precipitates carried by the fuel flow and can deposit on reactor boundaries (e.g., heat exchangers), potentially representing design issues related to the degradation of heat exchange performance or radioactive hotspots. Other precipitates might be present in the primary system as well, e.g. due to oxidation of fissile species which might lead to local criticality issues. The integration of transport models for solid particles in multiphysics codes is therefore relevant for the prediction of deposited fractions. To this aim, a previously developed Eulerian single-phase transport model is employed to analyze the distribution of solid FPs in two simplified MSFR-related geometries. The effect of physical parameters and of distributed particle sources on the numerical requirements needed to resolve particle concentration gradients at reactor boundaries in the considered geometries is investigated with the aid of analytical results. Analytical estimates of concentration gradients, even though not in full agreement with simulations, prove useful to drive the choice of adequate mesh refinements. Furthermore, the influence of different RANS turbulence modeling approaches on the prediction of particle distributions and deposition is tested. Results show a limited influence of the choice of turbulence models and parameters on the deposited fraction and on concentration gradients. As a result, it is found that deposition rates are scarcely affected by the choice of turbulent Schmidt number, with lower diffusivities being compensated by larger gradients. Large deposited fractions are indeed predicted, suggesting the efficiency of FPs transport mechanisms in the reactor and the need for the integration of adequate FPs transport models within state-of-the-art MSFR codes.

AlScN films prepared by alloy targets and SAW device characteristics

In this paper, we reported a surface acoustic wave (SAW) device prepared and optimized by piezoelectric films containing AlN, AlScN (Sc-20 at%) and AlScN (Sc-30 at%) by reactive magnetron sputtering using Al and AlSc alloy targets. We calculated the material intrinsic electromechanical coupling coefficient kt2 of AlScN (Sc-20 at%) and AlScN (Sc-30 at%) which are much better than AlN. It can be explained by the lattice softening. Furtherly, the results were confirmed by transmission electron microscopy (TEM) observation of the microstructure. Then the SAW devices based on three thin films were tested by vector network analysis obtaining the device equivalent electromechanical coupling coefficient keff2. The value of AlScN (Sc-20 at%) keff2, which equals to 1.94%, is higher than that of AlN and AlScN (Sc-30 at%) while the value of AlScN (Sc-30 at%) kt2 is higher than that of others. It is shown in our study that the crystallinity and orientation of the material still have a greater impact on keff2 but it does not have influence on kt2 in the actual device preparation process.

Optical conductivity and far-field radiation of silicene

Silicene is a silicon analogue of graphene. From symmetry considerations, we develop the matrix of Hamiltonian, including spin-orbit coupling in silicene. This work analyzes the heat and angular momentum radiation from silicene through the theory of far-field radiation and optical conductivity. We discuss the relation between the radiation and spin-orbit coupling parameters. Our results show the effects of different parameters, such as temperature and sublattice potential, on optical conductivity and radiation of silicene. In particular, the threshold frequency of optical conductivity causes the appearance of the second peak in energy radiation spectrum. We also find that the ratio of the perpendicular electric field to the intrinsic spin-orbit coupling is related to the strength of angular momentum radiation, which may promote the application of silicene.

Spontaneous spin polarization at the edge of the chiral superconducting states in a spin Hall metal

We discuss the chiral superconducting state in a square lattice system with an intrinsic spin-orbit coupling causing the spin Hall effect. We estimate the spontaneous spin polarization (local moment) 〈mi 〉 at the i-th site and its sum M = ∑ i 〈mi 〉 in ribbons. In the zigzag ribbon, we see 〈mi 〉 ≠ 0 near the edge. The energy spectra of the chiral edge states show the spin splitting and then we have non-zero M. Moreover, we find that M is enhanced significantly at a topological phase transition point, where the energy gap is closing. We also have non-zero 〈mi 〉 near the edge of the straight ribbon, but in contrast, there is no spin splitting in the spectra and M vanishes.

Evidence for a single-layer van der Waals multiferroic.

Multiferroic materials have attracted wide interest because of their exceptional static1-3 and dynamical4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurementswith theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Coupling effect of boundary tribofilm and hydrodynamic film

Reducing the friction and wear of dynamic interfaces has been a constant endeavor in the field of sustainability. Lubricants with two-dimensional atomic materials are operative frameworks proposed as promising candidates: they can together generate both a boundary tribofilm and a liquid hydrodynamic film in the interfacial domain. However, a quantitative characterization of the film formation and their coupling behaviors in the finite domain is lacking. Here, we report the coupling effect of tribofilms and hydrodynamic films regulated by typical representatives of two-dimensional material family (graphene, hBN, MoS 2 , and WS 2 ). The anti-wear properties of the interfaces are attributed to the behavior of the tribofilm, whereas the anti-friction properties of the interfaces are attributed to the viscosity behavior of the hydrodynamic film. This study provides an understanding of the intrinsic coupling mechanism of a tribofilm and a hydrodynamic film and a possible strategy for the design of interfacial lubricating systems. • Understanding of coupling effect of boundary tribofilm and hydrodynamic film • Anti-wear property of the interfaces is dominated by the tribofilm • Anti-friction property of interfaces is dominated by the hydrodynamic film • Strategy for the design of interfacial lubricating systems Jin et al. report the coupling effect of tribofilms and hydrodynamic films regulated by 2D materials (e.g., graphene, hBN, MoS 2 , and WS 2 ). They study the intrinsic coupling mechanism between a tribofilm and a hydrodynamic film and propose a possible strategy for the design of interfacial lubricating systems.

Variability of Electron and Hole Spin Qubits Due to Interface Roughness and Charge Traps

Semiconductor spin qubits may show significant device-to-device variability in the presence of spin-orbit coupling mechanisms. Interface roughness, charge traps, layout or process inhomogeneities indeed shape the real space wave functions, hence the spin properties. It is, therefore, important to understand how reproducible the qubits can be, in order to assess strategies to cope with variability, and to set constraints on the quality of materials and fabrication. Here we model the variability of single qubit properties (Larmor and Rabi frequencies) due to disorder at the Si/SiO$_2$ interface (roughness, charge traps) in metal-oxide-semiconductor devices. We consider both electron qubits (with synthetic spin-orbit coupling fields created by micro-magnets) and hole qubits (with intrinsic spin-orbit coupling). We show that charge traps are much more limiting than interface roughness, and can scatter Rabi frequencies over one order of magnitude. We discuss the implications for the design of spin qubits and for the choice of materials.

Significant impact of spinel ferrites in evolution of magneto-electric coupling in novel tri-phase composites

Multiferroic materials have played an important role in recent technological breakthroughs because of their intrinsic magneto-electric coupling characteristics. In this framework, a cost effective and reliable sol–gel auto-combustion method was used to synthesize the 0.8[(1-x) BiFeO3 + xFe3O4] + 0.2Cr2O3 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1) nanocomposites. X-ray diffraction was used to identify three phases and their unique crystal parameters. Elemental composition of the samples was verified using an energy dispersive X-ray spectroscopy. Field emission scanning electron microscopy revealed distinct, well-shaped, and spherical grains when the spinel-ferrite contents were introduced to the series. An irregular behavior in the bulk porosity was observed, courtesy to the ImageJ software. The vibrating sample magnetometer, operated at ±7 kOe, showed an increase in saturation magnetism. A rigorous analysis of recoverable energy density, energy loss density, and other ferroelectric parameters had been presented, demonstrating the potential of the proposed recipe for energy storage devices. The synthesized specimens demonstrated remarkable magneto-electric coupling at optimal content levels.

Radiation modulated spin coupling in a double-stranded DNA model

The spin activity in macromolecules such as DNA and oligopeptides, in the context of the chiral induced spin selectivity has been proposed to be due to the atomic spin–orbit coupling (SOC) and the associated chiral symmetry of the structures. This coupling, associated with carbon, nitrogen and oxygen atoms in biological molecules, albeit small (meV), can be enhanced by the geometry, and strong local polarization effects such as hydrogen bonding. A novel way to manipulate the spin degree of freedom is by modifying the spectrum using a coupling to the appropriate electromagnetic radiation field. Here we use the Floquet formalism in order to show how the half filled band Hamiltonian for DNA, can be modulated by the radiation to produce up to a tenfold increase of the effective SOC once the intrinsic coupling is present. On the other hand, the chiral model, once incorporating the orbital angular momentum of electron motion on the helix, opens a gap for different helicity states (helicity splitting) that chooses spin polarization according to transport direction and chirality, without breaking time reversal symmetry. The observed effects are feasible in physically reasonable parameter ranges for the radiation field amplitude and frequency.

Full‐Polarization Frequency Controlled Multimode Spoof Surface Plasmon Polaritons Excitation via Anisotropic Metastructure

AbstractSurface plasmon polaritons (SPPs) have great applications in imaging, sensing, waveguide, and communication. Especially in the trending of electromagnetic (EM) metastructure field, researchers used the phase gradient design to achieve the efficient surface wave propagating performance ranging from GHz to THz. However, due to the intrinsic coupling interference from the current structure design, the phase and amplitude of different polarization are hard to freely manipulate simultaneously, making the design of multifunctional channels SPPs still a challenging project. Here, an anisotropic structure is proposed to realize independent control of both polarization and amplitude along different component of the wavefront. By analyzing the polarization dependent mechanism, the hexagon and half‐hexagon meta‐atom are introduced to the coupler structure to tailor the desired phase gradient along different directions under different polarized EM waves. Then the dispersion of coupler is further coupled with the propagating patch metasurface by optimization algorithm. As a result, the multifunctional SPP is achieved, and the multimode excitation can be generated under both linear and circular polarized incidence. Both simulation and experiment validate the effectiveness and wideband of spoof SPPs excitation performance, verifying that this method reduces the excitation condition of surface wave and improves the multiplexing effect of propagation channel.

Ferromagnetic and nonmagnetic 1T′ charge density wave states in transition metal dichalcogenides: Physical mechanisms and charge doping induced reversible transition

The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., $\mathrm{Cr}{X}_{2}$) or the $M\text{\ensuremath{-}}M$ ($M$ is the metal atom) dimerization (e.g., $\mathrm{Mn}{X}_{2}$). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, $\mathrm{Cr}{X}_{2}$ has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in ${\mathrm{CrS}}_{2}$, leading to significant actuation strain output (i.e., 12.17% and 5.93% along $x$ and $y$ directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.

Relativistic domain-wall dynamics in van der Waals antiferromagnet MnPS3

The discovery of two-dimensional (2D) magnetic van der Waals (vdW) materials has flourished an endeavor for fundamental problems as well as potential applications in computing, sensing and storage technologies. Of particular interest are antiferromagnets, which due to their intrinsic exchange coupling show several advantages in relation to ferromagnets such as robustness against external magnetic perturbations. Here we show that, despite of this cornerstone, the magnetic domains of recently discovered 2D vdW MnPS3 antiferromagnet can be controlled via magnetic fields and electric currents. We achieve ultrafast domain-wall dynamics with velocities up to ~3000 m s−1 within a relativistic kinematic. Lorentz contraction and emission of spin-waves in the terahertz gap are observed with dependence on the edge termination of the layers. Our results indicate that the implementation of 2D antiferromagnets in real applications can be further controlled through edge engineering which sets functional characteristics for ultrathin device platforms with relativistic features.

Complete characterization of a lithium battery electrolyte using a combination of electrophoretic NMR and electrochemical methods.

Improving transport properties of electrolytes is important for developing lithium-ion batteries for future energy storage applications. In Newman's concentrated solution theory, electrolytes are characterized by three transport parameters, conductivity, diffusion coefficient, and transference number, in addition to the thermodynamic factor. In this work, these parameters are all determined for an exemplar liquid electrolyte, lithium bis(trifluoromethanesulfonyl)imide mixed in tetraethylene glycol dimethyl ether, using electrochemical methods. The intrinsic coupling between parameters obtained by electrochemical methods results in large error bars in the transference number that obscure the transport behavior of the electrolyte. Here, we use electrophoretic NMR (eNMR) to measure the electric-field-induced ion and solvent velocities to obtain the transference number directly, which enables determination of the thermodynamic factor with greater certainty. Our work indicates that the combination of eNMR and electrochemical methods provides a robust approach for complete characterization of battery electrolytes.

Revealing intrinsic spin coupling in transition metal-doped graphene.

Graphene materials offer attractive possibilities in spintronics due to their unique atomic and electronic structures, which is in contrast to their limited applications in the design of sophisticated spintronic devices. This should be attributed to the lack of knowledge about the intrinsic characteristics of graphene materials, especially the diverse correlations between sites within the materials and their roles in spin-signal generation and propagation. This work comprehensively studies the spin couplings between transition metal atoms doped on graphene and reveals their potential application in spintronic device design through the realization of various logic gates. In addition, the effects of the distance between doped metal atoms and the number of carbon layers on the logic gate implementation further verify that the spin-coupling effect can exhibit a certain distance dependence and space propagation. The achievements in this work uncover the potential value of graphene materials and are expected to open up new avenues for exploring their application in the design of sophisticated spintronic devices.

Stochastic Robust H∞ Decentralized Network Formation Tracking Control of Large-Scale Team Satellites via Event-Triggered Mechanism

The team formation of large-scale satellites for communication services of future smart cities in the 5G and 6G era is an important research topic currently. In this study, a robust H∞ with an event-triggered mechanism is proposed for the formation of large-scale satellites for communication tasks. At first, a satellite in the team formation is described by a nonlinear stochastic system with intrinsic random fluctuation, external disturbance, and coupling from other satellites. The proposed team formation tracking control for each satellite needs to consider the effect of network-induced delay and packet-dropout as well as save communication resources via an event-triggered mechanism. Further, the desired team formation of large-scale satellites can be prescribed by a set of reference models with the desired formation shape embedded in their reference inputs. Therefore, the large-scale team formation design problem can be simplified as an independentH∞ network model reference tracking control design problem for each satellite to efficiently attenuate the worst-effect of external disturbance, coupling of other satellites, intrinsic random fluctuation, network-induced delay, and packet-dropout on the model reference tracking performance. In order to avoid solving a nonlinear partial differential Hamillion-Jacobin inequality(HJI) for the H∞ decentralized network formation control for each satellite in the team the formation, the T-S fuzzy model is employed to interpolate several local linearized systems to approximate nonlinear satellite systems. Then the HJI in the H∞ decentralized network team formation design problem can be transformed into a set of linear matrix inequalities (LMIs) which can be easily solved by LMI TOOLBOX in MATLAB. Finally, a simulation example of team formation composed of ten satellites is given to illustrate the design procedure and to validate the proposed method in comparison with other methods.

Explainable Contrastive Multiview Graph Representation of Brain, Mind, and Behavior.

Understanding the intrinsic patterns of human brain is important to make inferences about the mind and brain-behavior association. Electrophysiological methods (i.e. MEG/EEG) provide direct measures of neural activity without the effect of vascular confounds. The blood oxygenated level-dependent (BOLD) signal of functional MRI (fMRI) reveals the spatial and temporal brain activity across different brain regions. However, it is unclear how to associate the high temporal resolution Electrophysiological measures with high spatial resolution fMRI signals. Here, we present a novel interpretable model for coupling the structure and function activity of brain based on heterogeneous contrastive graph representation. The proposed method is able to link manifest variables of the brain (i.e. MEG, MRI, fMRI and behavior performance) and quantify the intrinsic coupling strength of different modal signals. The proposed method learns the heterogeneous node and graph representations by contrasting the structural and temporal views through the mind to multimodal brain data. The first experiment with 1200 subjects from Human connectome Project (HCP) shows that the proposed method outperforms the existing approaches in predicting individual gender and enabling the location of the importance of brain regions with sex difference. The second experiment associates the structure and temporal views between the low-level sensory regions and high-level cognitive ones. The experimental results demonstrate that the dependence of structural and temporal views varied spatially through different modal variants. The proposed method enables the heterogeneous biomarkers explanation for different brain measurements.

Emergence of magnetic order and enhanced magnetoelectric coupling in Lu-doped Sm2BaCuO5

The green-phase compound Sm2BaCuO5 has recently drawn significant attention due to the prominent linear magnetoelectric (ME) coupling effect. Herein, we report a successful chemical doping control of magnetic order and ME effect in Lu3+-doped multiferroic Sm2BaCuO5. Upon a small amount of Lu3+ doping, it is suggested that the majority of Cu ions interact through Sm3+ and order at TN1 ∼ 23 K, whereas some Cu2+ interacting through Lu3+ can lead to another transition at T* ∼21 K–16 K upon doping Lu3+ ions from 5% to 20%. More interestingly, this emergent magnetic order is concomitant with a broad valley in the pyroelectric current and dielectric anomaly, providing clear evidence of intrinsic coupling between magnetic and ferroelectric orders. In contrast to the parent phase, significant enhancement of the linear ME coefficient is obtained. Our results unveil the importance of interplay of 4f-3d magnetic interactions in inducing ferroelectricity and offer an alternative route to achieve high performance in ME coupling.

Charge-spin interconversion in graphene-based systems from density functional theory

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the $k$-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.

Data-driven low-fidelity models for multi-fidelity Monte Carlo sampling in plasma micro-turbulence analysis

The linear micro-instabilities driving turbulent transport in magnetized fusion plasmas (as well as the respective nonlinear saturation mechanisms) are known to be sensitive with respect to various physical parameters characterizing the background plasma and the magnetic equilibrium. Therefore, uncertainty quantification is essential for achieving predictive numerical simulations of plasma turbulence. However, the high computational costs of the required gyrokinetic simulations and the large number of parameters render standard Monte Carlo techniques intractable. To address this problem, we propose a multi-fidelity Monte Carlo approach in which we employ data-driven low-fidelity models that exploit the structure of the underlying problem such as low intrinsic dimension and anisotropic coupling of the stochastic inputs. The low-fidelity models are efficiently constructed via sensitivity-driven dimension-adaptive sparse grid interpolation using both the full set of uncertain inputs and subsets comprising only selected, important parameters. We illustrate the power of this method by applying it to two plasma turbulence problems with up to 14 stochastic parameters, demonstrating that it is up to four orders of magnitude more efficient than standard Monte Carlo methods measured in single-core performance, which translates into a runtime reduction from around eight days to one hour on 240 cores on parallel machines.

A numerical study of the coupling between Rayleigh streaming and heat transfer at high acoustic level.

Complex coupling between thermal effects and Rayleigh streaming in a standing wave guide at high acoustic levels is analyzed numerically. The approach is guided by the recent analytical study, showing that reverse streaming cells can form if the nonlinear Reynolds number exceeds a value depending on the wave frequency and thermophysical properties of the fluid and solid wall. A numerical configuration is introduced to investigate the evolution of the streaming flow structure and the average temperature field at high acoustic levels. Special attention is given to inhibit the development of shock waves. The heat conduction is accounted for in the wall. As the acoustic level is increased, the average temperature field becomes stratified transversely. The simulations show the relevance of the criterion for characterizing the appearance of additional contrarotating streaming cells near the acoustic velocity antinodes. For higher acoustic levels, these additional cells evolve into increasingly large stagnant zones, where the streaming flow is of very small amplitude and the contours of temperature are stratified longitudinally. The overall outer streaming flow decreases. These results are consistent with previous experimental observations, showing that the intrinsic coupling between the thermal effects and acoustic streaming at high levels is very well described.