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Evidence and origin of anomalous diffusion of Ag+ ion in amorphous silica: A molecular dynamics study with neural network interatomic potentials

The release of Ag+ ions into the environment through silica layers is a promising strategy for the development of anti-microbial surface coating devices. The aim of the present study is to provide some insight into the elementary mechanisms of diffusion of Ag+ ions through silica with the objective of proposing control strategies. Thanks to the development of interaction potentials based on neural networks, the diffusion processes were studied via molecular dynamics simulations. Silver diffusion was found to be anomalous and sub-diffusive, the origin of which could be attributed to deceleration and temporal anti-correlations. This sub-diffusion has been attributed primarily to the disordered nature of the silica matrix. Furthermore, it is magnified by the presence of coordination defects within the silica matrix. These defects, in particular the under-coordinated oxygen atoms, act as traps for Ag+ by forming O–Ag bonds, thereby limiting the jump length and retaining the ion for long duration. By comparison with existing diffusion models, the diffusion mechanism in the absence of defects appears to be of the fractional Brownian motion type, substantially modified by the presence of defects. Two possible approaches have emerged to tune the release of Ag+ ions through the silica layer: the monitoring of the number of defects and the opening/closing of diffusion paths via, e.g., a modification of the silica density.

Encapsulation of volatile fission products by the intermetallic electride Sr3CrN3:e–

Electrides are characterized by their unique structural architectures, where excess electrons are trapped in specific sites such as cages, channels, or layers within the lattice. The trapped electrons have significant potential for trapping volatile fission products, especially anionic species such as Br, I, and Te, released during spent nuclear fuel reprocessing. Here, density functional theory simulations are used to investigate the encapsulation efficacy of various volatile fission products, including Kr, Xe, Br, I, Te, Rb, and Cs, in a recently identified one-dimensional Sr3CrN3:e− electride and compare to values in Ca3CrN3:e− and Ba3CrN3:e−. It is shown that the encapsulation energies for Kr, Xe, Rb, and Cs are endothermic, indicating that these species are unstable when encapsulated within this electride. In contrast, the encapsulation of Br, I, and Te is highly exothermic, suggesting that the process is energetically favorable for these anions. Additionally, when homonuclear dimers (Br2, I2, and Te2) are encapsulated, the simulations predict dissociation within the electride lattice, forming pairs of adjacent anions instead of intact molecules.

Function domains and the universal matrix functional of multi-state density functional theory

On the basis of recent advancements in the Hamiltonian matrix density functional for multiple electronic eigenstates, this study delves into the mathematical foundation of the multistate density functional theory (MSDFT). We extend a number of physical concepts at the core of Kohn–Sham DFT, such as density representability, to the matrix density functional. In this work, we establish the existence of the universal matrix functional for many states as a proper generalization of the Lieb universal functional for the ground state. Consequently, the variation principle of MSDFT can be rigorously defined within an appropriate domain of matrix densities, thereby providing a solid framework for DFT of both the ground state and excited states. We further show that the analytical structure of the Hamiltonian matrix functional is considerably constrained by the subspace symmetry and invariance properties, requiring and ensuring that all elements of the Hamiltonian matrix functional are variationally optimized in a coherent manner until the Hamiltonian matrix within the subspace spanned by the lowest eigenstates is obtained. This work solidifies the theoretical foundation to treat multiple electronic states using density functional theory.

Flat-plate underwater carbon nanotube low-frequency transducer

This paper presents a novel, lightweight, low-frequency transducer based on the thermoacoustic effect, specifically tailored for underwater applications. The design leverages the simplicity of the thermoacoustic effect, which is particularly effective in the low-frequency range, to develop a flat-plate carbon nanotube transducer. A three-dimensional model incorporating electric–thermal–acoustic coupling is proposed to investigate the primary factors that influence the transducer's acoustic performance, using the finite element method as the analytical framework. To validate the model, a prototype was constructed, and its acoustic performance was evaluated. The results revealed a maximum output of 118.7 dB at a distance of 15 cm at 620 Hz and a minimum of 95.6 dB within the 490–770 Hz frequency range, achieved under current intensities ranging from 1 to 2.5 A. This study aims to establish and validate a three-dimensional simulation model, grounded in the thermoacoustic effect and finite element methodology, to optimize the performance of carbon nanotube transducers in underwater environments. The finite element simulation focused on assessing the transducer's behavior under constrained low-frequency conditions. The optimization process guided the design of the transducer, which was then subjected to rigorous testing. The results demonstrated several key advantages, including a low operating frequency, a compact and lightweight design, and straightforward installation. These characteristics make the transducer particularly well-suited for deployment on small underwater platforms, enhancing its versatility and practical application in underwater systems.

A century of quantum physics

Revealing correlation mechanisms through nonorthogonal multiconfiguration self-consistent field calculations

The presence of spin and spatial symmetry breaking upon variational optimization of mean-field wavefunctions is known to be an indicator of nondynamical electron correlation. However, a single mean-field wavefunction may not have sufficient flexibility to flag the correlated orbital space where there are multiple correlation mechanisms present. In such situations, there are multiple nearly degenerate self-consistent field solutions that describe different correlation mechanisms, but it is often not possible to know a priori when such situations will occur or if sufficient solutions have been obtained. In this work, we examine the role of spin and spatial symmetries of nonorthogonal multiconfigurational self-consistent field (NOMCSCF) calculations in revealing correlation mechanisms. We provide details of the theory for optimization of NOMCSCF wavefunctions with desired symmetries, establish which types of symmetries recover the most correlation energy when the symmetry constraints are relaxed, and discuss how the different-orbitals for different-configuration wavefunctions reveal the different correlation mechanisms present.

Strained Fe-doped ferromagnetic semiconductor (In,Fe)As thin films grown on InP (001) substrates: Ferromagnetism and electronic structure

The n-type Fe-doped ferromagnetic semiconductor (FMS) (In,Fe)As is a promising material for spintronics devices and a topological superconducting platform, thanks to its unique features such as carrier-induced ferromagnetism, spontaneous spin splitting, and spin-triplet superconductivity. In this study, we have successfully grown (In,Fe)As (9.6% Fe) thin films on InP (001) substrates, where variable in-plane compressive strain is applied to the (In,Fe)As films by growing on (In1−y,Aly)As graded buffer layers. When increasing the in-plane compressive strain (by increasing y), we observed enhancement of the Curie temperature of the (In,Fe)As films, comparing with those grown on AlSb buffer layers reported in our previous studies. We found that the (In,Fe)As thin films with strong in-plane compressive strain are highly resistive or insulating at low temperature while exhibiting ferromagnetism: This insulating and ferromagnetic feature is quite different from the conventional electron-induced ferromagnetism in (In,Fe)As. By combining our experiments and first-principles calculation, we suggest that interstitial Fe atoms increase with increasing the compressive strain and resulting band-structure change can explain these transport and magnetic properties. This work opens an avenue to harmonically tune various properties of (In,Fe)As such as the lattice constant, electrical resistance, and Curie temperature, thus giving more freedom in material design for device applications.

Inherent loss of parahydrogen-induced polarization for systems with magnetically equivalent nuclei in magnetic field cycling experiments

In the present work, we elucidate the inherent loss of net magnetization (⟨Iz⟩) in parahydrogen-induced polarization (PHIP) experiments with magnetic field cycling (MFC) for spin systems containing magnetically equivalent protons. The effects are shown for propane and diethyl ether as representative examples of potential hyperpolarized MRI contrast agents, but the findings of this work are equally applicable to other multispin systems in the liquid or gas phase. These results are relevant to both adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiments (where 1H nuclei are polarized) and MFC protocols used to transfer parahydrogen spin order to a heteronucleus such as 13C. The investigated effects should be incorporated for a correct evaluation of both the maximum possible NMR signal enhancement and the pairwise selectivity, which are useful in the context of mechanistic studies in the field of catalytic hydrogenation. Among signal enhancement damping factors in ALTADENA, such as T1 relaxation and insufficient adiabaticity of a field sweep, the inherent loss of net magnetization in spin systems containing magnetically equivalent protons (especially in PHIP systems commonly used for mechanistic studies such as propene or propane) has not been thoroughly considered and needs to be clarified. The maximum possible net magnetization in ALTADENA for diethyl ether and propane was shown to be ∑|⟨Iiz⟩| ≈ 0.56 for diethyl ether and ∑|⟨Iiz⟩| ≈ 0.45 for propane, respectively. The inherent loss of net heteronuclear magnetization of the same order of magnitude with an increase in the number of magnetically equivalent protons was also demonstrated for AmMnX-type spin systems.

Patterned electroconvection under AC and DC voltages with strong unipolar charge injection

The phenomenon of electroconvection has attracted attention because it has the potential to improve ion transport on polarized surfaces, reducing the plateau region of limiting current. Previous observations for DC voltages indicate that patterned surfaces or unipolar charge injection significantly enhance ion transport through electroconvection. However, creating and maintaining the convection cells crucial for electroconvection is challenging under AC voltages due to the alternating direction of the electric field, which can cause instabilities. In this article, we explore how electroconvection can be induced using a patterned membrane and strong unipolar charge injection with both DC and AC voltages. We use a flow simulation with a specialized adaptive time-stepping algorithm to simulate electroconvection and find the best pattern ratio (R) for achieving the highest time-averaged current density. The system’s performance is assessed at different injection levels and mobility parameters across various frequencies. We compare patterned surfaces with homogeneous membranes. Our demonstration shows that an asymmetric patterned membrane with alternating cation-selective and ion-collector membranes can effectively alleviate the limitations posed by AC voltages to a significant extent. These findings contribute to optimizing ion transport under AC conditions, offering valuable insights for applications in biomedicine, micro/nanofluidics, and electrochemical systems, including DNA diagnostics, lab-on-a-chip devices, supercapacitors, and batteries where precise control of ion transport is essential.

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Revisiting the manganese dimer on the base of first-principles theory

Manganese is one of the most intriguing elements showing multiple magnetic phases. In order to shed some light on the complex behavior, the manganese dimer has been the focus of extensive interest in theoretical research. Various quantum techniques have been utilized to comprehend the characteristics of the Mn dimer. Several approaches and functionals have been employed that suggest that the ferromagnetic (FM) state is its lowest energy configuration. Nevertheless, these findings are inconsistent with the experimental results showing that Mn2 has an antiferromagnetic (AFM) Σg+1 configuration at an interatomic Mn–Mn distance of dMn–Mn = 3.40 Å. This work presents a comparative assessment of outcomes obtained through several levels of the exchange–correlation functional: generalized gradient approximation (GGA), meta-GGA, GGA+U, and the hybrid Heyd–Scuseria–Ernzerhof (HSE06), the Perdew–Burke–Ernzerhof 0, and the Becke, 3-parameter, Lee–Yang–Parr. The results of our investigation are discussed based on previous theoretical and experimental reports. We found that the best description is obtained with the hybrid HSE06 functional. The Mn2 has a FM coupling at short distances and the characteristic AFM Σg+1 state at dMn–Mn = 3.27 Å. Furthermore, we obtained a magnetic moment (μ) per Mn atom of μ = 4.527 μB, a stretching frequency of ω = 80 cm−1, and a binding energy of Eb = −195 meV, which is in good agreement with the experimental results.

Characterization of time-dependent dielectric degradation and breakdown in bulk hexagonal BN/Si structures

Hexagonal boron nitride (h-BN) is a promising material for designing future electronic devices because of its superior dielectric properties. In this study, we fabricated bulk h-BN (sp2-bonded BN nano-network structure) on Si substrates using magnetically confined arc discharge plasma under various conditions. The effects of process gas conditions (Ar/N2 and N2) and impurity [tungsten (W)] incorporation were discussed. Regardless of the gas conditions, the presence of W atoms was found to significantly modulate the optical energy gap, which is supported by first-principles calculations. We investigated time-dependent dielectric breakdown (TDDB) mechanisms under constant voltage stress (CVS) and constant current stress (CCS). The time evolutions of the leakage current and the applied gate voltage during the TDDB measurements were analyzed to clarify the carrier-trapping and defect-generation mechanisms toward the final catastrophic dielectric breakdown. The field acceleration factors in the CVS-TDDB lifetime prediction fell within the general trend of SiO2-based films and were found to be a weak function of the gas condition and W concentration (CW), whereas the carrier-trapping and defect-generation dynamics during electrical stress depend on the gas conditions and CW. Based on the obtained results, we propose a prediction model for bulk h-BN degradation dynamics during CVS. We found that carrier trapping into preexisting sites and the probability of defect generation were enhanced by the bombardment of ions with higher energy during the bulk h-BN formation and a larger number of incorporated W atoms. These findings provide fundamental guidelines for the reliability assessment of bulk h-BN films for various applications.

A relativistic third-order algebraic diagrammatic construction theory for electron detachment, attachment, and excitation problems

We present the theory and implementation of a relativistic third-order algebraic diagrammatic construction [ADC(3)] method based on a four-component (4c) Dirac–Coulomb Hamiltonian for the calculation of ionization potentials (IPs), electron affinities (EAs), and excitation energies (EEs). Benchmarking calculations for IP, EA, and EE were performed on both atomic and molecular systems to assess the accuracy of the newly developed four-component relativistic ADC(3) method. The results show good agreement with the available experimental data. The Hermitian nature of the 4c-ADC(3) Hamiltonian, combined with the perturbative truncation of the wave function, offers significant computational advantages over the standard equation-of-motion coupled-cluster approach, particularly for property calculations. The method’s suitability for property calculations is further demonstrated by computing oscillator strengths and excited-state dipole moments for heavy elements.

Effect of magnetic field on interfacial instabilities of rapid solidification in additive manufacturing

The performance of additive manufacturing products is significantly influenced by the microstructural morphology developed during rapid solidification, where instability at the solid–liquid interface can result in various microstructural modes. The application of a magnetic field to modify the hydrodynamic mechanism within a molten pool presents a promising approach to controlling the microstructure evolution in additive manufacturing. This study establishes a mathematical and physical model of rapid directional solidification and investigates the impact of the magnetic field on the stability of the solidification interface through linear stability analysis. The results show that an external magnetic field decreases both the maximum and cutoff wave numbers in the steady mode, delaying the onset of instability. Furthermore, the extent of the unstable region in the steady mode diminishes as the Hartmann number increases. In the oscillatory mode, the application of the magnetic field increases the maximum and cutoff wave numbers, thereby promoting interfacial instabilities. However, the magnetic effect on the unstable region is not remarkable. Utilizing dimensionless drag velocity as a control parameter for rapid solidification reveals that the magnetic field can reduce the cell/dendrite domain and the solute banding region, thereby enhancing interfacial stability. The enhancement can be understood as a result of the external magnetic field promoting melt flow, with the associated stirring effect leading to increased supercooling and solidification rates, ultimately improving structural uniformity. The conclusions provide some theoretical guidance for utilizing the magnetic field to reduce defects and enhance high-quality products in additive manufacturing, potentially paving the way for further experimental investigations.

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Langevin integration for isothermal–isobaric condition with a large time step

We propose an accurate method for evaluating temperature and pressure in Langevin integration, based on the approach by Leimkuhler and Matthews (J. Chem. Phys. 138, 174102). This method improves the quality of configuration space than other Langevin dynamics methods. However, it encounters issues in pressure evaluation due to inaccuracies in momentum space. In particular, the conventional approach for calculating kinetic temperature using the full-time step momentum introduces errors proportional to the square of the time step (Δt2), leading to unreliable results when employing a large time step under isothermal–isobaric conditions. By calculating kinetic energy using the half-time step momentum in pressure evaluation, we can reduce the numerical errors. We performed molecular dynamics (MD) simulations using our refined pressure evaluation and improved accuracy and stability in the isothermal–isobaric MD simulations even with a long time step (Δt = 5 fs).

Selective excitation of heavy water solution under infrared pulse irradiation: A molecular dynamics simulation study

The heating effect of infrared pulses with varying frequencies on heavy water solutions of different concentrations was investigated using non-equilibrium molecular dynamics simulations. Numerical calculations indicate that when the pulse frequencies are 39 and 72 THz, the infrared pulses excite the heavy water molecules, while the temperature of the light water molecules remains unchanged. At pulse frequencies of 53 and 99 THz, the infrared pulses excite the light water molecules. Additionally, in the far infrared region (1–20 THz), the infrared pulses excite both heavy and light water molecules simultaneously, albeit with a significantly reduced energy absorption efficiency. We also conducted a comparative analysis of the energy absorption efficiency of water molecules under three distinct heating modes: Vibrational excitation, rotational excitation, and microwave heating. The results reveal that the energy absorption efficiency of vibrational excitation is seven times that of rotational excitation, and the efficiency of rotational excitation is seven times that of microwave heating. This conclusion holds great potential for practical applications.

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Attaining high accuracy for charge-transfer excitations in non-covalent complexes at second-order perturbation cost: The importance of state-specific self-consistency

Intermolecular charge-transfer (xCT) excited states important for various practical applications are challenging for many standard computational methods. It is highly desirable to have an affordable method that can treat xCT states accurately. In the present work, we extend our self-consistent perturbation methods, named one-body second-order Møller–Plesset and its spin-opposite scaling variant (O2BMP2), for excited states without additional costs to the ground state. We then assessed their performance for the prediction of xCT excitation energies. Thanks to self-consistency, our methods yield small errors relative to high-level coupled cluster methods and outperform other same scaling (N5) methods, such as CC2 and ADC(2). In particular, O2BMP2, whose scaling can be reduced to N4, can even reach the accuracy of CC3 (N7) with errors less than 0.1 eV. This method is thus highly promising for treating xCT states in large compounds vital for applications.

A general spectral collocation method for computing the dispersion relations of guided acoustic waves in multilayer dissipative structures

A spectral collocation method is proposed to compute the complex wavenumber–real frequency dispersion relations of guided acoustic waves in multilayer structures involving dissipative materials. The nature of these dissipative materials is initially considered to be arbitrary, i.e., poroelastic, viscoelastic, or viscoacoustic. For a given frequency, the complex wavenumbers as well as the physical fields, which are further used to evaluate the Poynting vectors and analyze the energy flux, are obtained by solving a generalized eigenvalue problem. The latter arises from a set of discretized equations of motion and appropriate boundary (coupling) conditions. These equations of motion and boundary (coupling) conditions are imposed by the nature of the material composing each layer of the structure. A focus is made on poroelastic layers. The dispersion relation of a two-layer elastic–poroelastic structure is analyzed, as well as the energy flows in the structure. The results as calculated with the present spectral collocation method are validated against those obtained with a classical complex root-finding (Müller) method and experiments.