First-principles Simulations Research Articles

Out-of-equilibrium chiral magnetic effect from simulations on Euclidean lattices

The status of the chiral magnetic effect (CME) response in full quantum chromodynamics (QCD) has been controversial so far, with previous lattice QCD studies indicating either its strong suppression or vanishing in thermal equilibrium state. We introduce the Euclidean-time correlator of axial charge and electric current as an observable that can be used to study the finite out-of-equilibrium CME response in first-principle lattice QCD simulations with background magnetic field. This observable directly reflects the fact that in the background magnetic field, a state with nonzero axial charge features nonzero electric current. For free fermions, the axial-vector correlator only receives contributions from the lowest Landau level, and exhibits a linear dependence on both magnetic field and temperature with a universal coefficient. With an appropriate regularization, nonvanishing axial-vector correlator is compatible with the vanishing of the CME current in thermal equilibrium state with nonzero chiral chemical potential μ5. We demonstrate that the real-time counterpart of the Euclidean-time axial-vector correlator is intimately related to the real-time form of the axial anomaly equation, which strongly limits possible corrections in full QCD. We present numerical results for the Euclidean-time axial-vector correlator in SU(2) lattice gauge theory with Nf=2 light quark flavors, demonstrating reasonable agreement with free fermion result on both sides of the chiral crossover. The proposed methodology should help to answer the question whether the QCD corrections might be responsible for nonobservation of CME in heavy-ion collision experiments such as the RHIC isobar run. Published by the American Physical Society 2024

Hot carrier transfer from plasmon decay in Ag20at H-Si(111) surface: real-time TDDFT simulation in Wannier gauge.

Plasmon decay is believed to play an essential role in inducing hot carrier transfer at the interfaces between plasmonic nanoparticles and semiconductor surfaces. In this work, we employ real-time time-dependent density functional theory (RT-TDDFT) simulation in the Wannier gauge to gain quantum-mechanical insights into the nonlinear dynamics of the plasmon decay in the Ag20nanoparticle at a semiconductor surface. The first-principles simulations show that the plasmon decay is more than two times faster when the Ag20nanoparticle is adsorbed on a hydrogen-terminated Si(111) surface, taking place within 100 femtoseconds of the plasmon excitation. Hot carrier transfer across the interface is observed as the plasmon decay takes place, and nearly 30% of holes are generated deep in the valence band of the semiconductor surface. The use of Wannier gauge in RT-TDDFT simulation is particularly convenient for gaining quantum-mechanical insights into non-equilibrium electron dynamics in complex heterogeneous systems.

Thermal conductivity of WC: Microstructural design driven by first-principles simulations

The relationships between the microstructure and the thermal conductivity of binderless WC have been quantified, considering crystal orientation, isotopic abundance, porosity, and grain size. A significantly higher conductivity is predicted in the out-of-plane (c-axis) direction vs. the in-plane (a-axis) direction, using first principles simulations. Isotopic enrichment of the tungsten sublattice is predicted to increase conductivity, e.g., by a factor of 4–5 in the absence of boundary scattering. The results suggest that for an isotopically pure single crystal a thermal conductivity exceeding 1000 W m−1 K−1 may be achievable normal to the basal plane. The conductivity of samples with various porosities could be well fit by a minimum surface area (exponential) model, with a porosity exponent of b = 4.4. Experiment and simulation show a strong grain size dependence to conductivity below 1 µm, with a saturation beyond ∼10 µm. The experimental plateau values for κ were ∼45 % lower than those of the simulations due to deviations from perfect stoichiometry. We also find a higher scattering coefficient in the experiments, likely due to effects of grain size distribution and elongation. Our study clarifies the physical origin of disagreeing literature reports as being predominantly due to grain boundary scattering and enables microstructural design for thermally demanding environments.

First-principles study on the lithiation process of amorphous SiO anode for Li-ion batteries with Bayesian optimization.

Amorphous silicon monoxide (a-SiO), which contains Si atoms with various valence states, has attracted much attention as a high-performance anode material for lithium (Li) ion batteries (LIBs). Although current experiments have provided some information during charge/discharge cycles, further investigation of structural changes at the atomic scale is needed. To investigate the lithiation process of a-SiO using first-principles simulations and machine learning techniques, we developed a computational code employing Bayesian optimization to efficiently identify stable sites for Li insertion in the large search-space of amorphous models to reproduce the actual lithiation process and compared this approach to the conventional random scheme by applying it to an a-SiO model previously generated with neural network potentials. The lithiation process based on Bayesian optimization resulted in lower formation energies compared to the conventional random scheme, indicating a more stable structure. During lithiation, Li atoms tended to enter the silicon (Si) phase after the SiO2 phase, in agreement with experimental results. We analyzed the structural changes and observed significant differences in the structural evolution between the conventional and new schemes. Our study highlights the significant influence of the lithiation process on the structural transformation of a-SiO materials, which in turn affects the reversible capacity of the material. These findings will provide a framework for improving the performance and lifetime of a-SiO materials.

Large Adsorption Capacity for Space Molecular Pollutants Realized by a Metal-Organic Framework.

The rapid development of capable spacecraft coatings for removing volatile organic compounds (VOCs) demands materials with a large adsorption capacity to ensure a long-duration exploration mission in a low Earth orbit (LEO). MIL-53(Al) with abundant sites exhibits resistance to the space environment in the LEO and shows great potential for VOC removal. This study combined an adsorption experiment and first-principles simulations to investigate the adsorption performance of MIL-53(Al). MIL-53(Al) demonstrated an excellent adsorption capacity for various VOCs, reaching 606.04 mg/g for m-xylene and 22.69 cm3/g for methane, which is 6 times higher than traditional adsorption methods. First-principles simulations revealed that the adsorption capacity is significantly influenced by micropores through the pore effect and breath effect, which restrict diffusion and enhance adsorption, respectively. Additionally, we predicted the adsorption properties of VOCs that are difficult to characterize through ground-based simulated experiments, further validating the potential application of MIL-53(Al) for VOC removal in the LEO. This work expands the application range of MIL-53(Al), optimizes VOC removal strategies, and offers a novel approach to protecting spacecraft in the LEO. Our findings contribute to the development of advanced materials for space exploration and provide valuable insights into the design of efficient VOC removal systems for spacecraft in the LEO.

Engineering Lattice Oxygen Regeneration of NiFe Layered Double Hydroxide Enhances Oxygen Evolution Catalysis Durability.

The lattice oxygen mechanism (LOM) endows NiFe layered double hydroxide (NiFe-LDH) with superior oxygen evolution reaction (OER) activity, yet the frequent evolution and sluggish regeneration of lattice oxygen intensify the dissolution of active species. Herein, we overcome this challenge by constructing the NiFe hydroxide/Ni4Mo alloy (NiFe-LDH/Ni4Mo) heterojunction electrocatalyst, featuring the Ni4Mo alloy as the oxygen pump to provide oxygenous intermediates and electrons for NiFe-LDH. The released lattice oxygen can be timely offset by the oxygenous species during the LOM process, balancing the regeneration of lattice oxygen and assuring the enhancement of the durability. In consequence, the durability of NiFe-LDH is significantly enhanced after the modification of Ni4Mo with an impressively durability for over 60 h, much longer than that of NiFe-LDH counterpart with only 10 h. In-situ spectra and first-principle simulations reveal that the adsorption of OH- is significantly strengthened owing to the introduction of Ni4Mo, ensuring the rapid regeneration of lattice oxygen. Moreover, NiFe-LDH/Ni4Mo-based anion exchange membrane water electrolyzer (AEMWE) presents an impressive durability for over 150 h at 100 mA cm-2. The oxygen pump strategy opens opportunities to balance the evolution and regeneration of lattice oxygen, enhancing the durability of efficient OER catalysts.

N-doping strategy for enhancing the adsorption performance of Ti2CT2 MXene for Sr ion

Radionuclides, such as strontium (Sr), are hazardous radioactive isotopes commonly found in nuclear waste, posing serious environmental and health risks due to their long half-lives and ability to bioaccumulate. Inspired by the electronic modification effect, this study theoretically predicts that doping can significantly enhance the adsorptive performance of MXenes for radionuclides. Specifically, we employed comprehensive first-principles simulations to investigate the impact of nitrogen (N) doping on the adsorption behavior of Ti2CT2 (T = O, F, OH) MXenes for Sr ions, focusing on surface N doping and C-site N doping as effective strategies to improve adsorption. The results confirmed that both types of N doping are beneficial for the adsorption performance of Ti2CT2, and the adsorption strengths of Ti2CT2 with surface N doping are significantly enhanced. This was analyzed and attributed to the ability of N doping to enhance the charge transfer between the Ti2CT2 surface and Sr ions, thus enhancing the adsorption properties. By elucidating the N doping mechanisms, this study is expected to provide theoretical guidance for the design of high-performance MXene materials in radionuclides remediation applications.

Surrogate model of turbulent transport in fusion plasmas using machine learning

Abstract The advent of machine learning (ML) has revolutionized the research of plasma confinement, offering new avenues for exploration. It enables the construction of models that effectively streamline the simulation process. While previous first-principles simulations have provided physics-based transport information, they have been inadequate fast for real-time applications or plasma control. In order to address this challenge, we introduce SExFC, a surrogate model based on the Gyro-Landau Extended Fluid Code (ExFC). An approach of physics-based database construction is detailed, as well the validity is illustrated. Through harnessing the power of ML, SExFC offers the capability to deliver rapid and precise predictions, facilitating real-time applications and enhancing plasma control. The proposed model integrates the recurrent neural network (RNN) algorithm, specifically leveraging the Gated Recurrent Unit (GRU) for iterative prediction of flux evolutions based on radial profiles. Therefore, the SExFC model has the potential to enable rapid and physics-based predictions that can be seamlessly integrated into future real-time plasma control systems.

Self-Adaptive Layering Structure of Nanoconfined Fe-Ni Liquids: Origin of the Liquid-Liquid Phase Transition.

Metallic liquids under confinement exhibit different properties compared to those of their corresponding bulk phases, such as miscibility, diffusion, and phase transitions. Unfortunately, the challenges in experimentally characterizing Fe-Ni liquids at the nanoscale and the high cost of first-principles simulations hindered the atom-level understanding that is necessary for controlling Fe-Ni liquids. Here, we report a comprehensive molecular dynamics study of the liquid Fe-Ni alloy confined within nanoslits. Driven by the slit size, the confined Fe-Ni liquid experiences the liquid-liquid phase transition (LLPT) that is characterized by layering transitions. Interestingly, during the LLPT, a transition liquid phase appears, separating two layering phases, which is accompanied by abnormal variations in density, potential energy, and pressure perpendicular to the wall. The Voronoi cluster analysis reveals a self-adaptive local structural evolution in confined Fe-Ni liquids during the LLPT. The effect of temperature, pressure, and composition on the LLPT is investigated. Based on the pressure-confinement phase diagram, the LLPT is mainly induced by confinement under high pressure, while under low pressure, the LLPT is mainly pressure-driven. Our research will stimulate more interest in the phase transition under confinement in a metallic system.

An investigation on the surface properties of B4C for advancing its nuclear applications

B4C is an important material in diverse nuclear applications. However, a systematic examination of its surface properties is still missing. In this work, we employ first-principles simulations to investigate the energetic stability of 16 distinct slab models representing (001), (100), (101), (110), and (111) surfaces, which are constructed by minimizing dangling bonds. Our results show that C-terminated (001) surface exhibits significantly greater stability than other surfaces under both the carbon and boron-rich conditions. Besides, we also study the defect formation energies on the C-terminated (001) surface and compare them with the cases in bulk. The high formation energies of the defects suggest a low likelihood of their occurrence on this surface, despite their formation energies being lower compared to bulk cases. Furthermore, mid-gap surface states are revealed for the top atomic layers of the C-terminated (001) surface, which are deduced at the deeper layers, and the band structures of the middle layers of this slab recover to the bulk band gap. These surface mid-gap states allow electron excitation from the valence band to these states, resulting in a reduced optical band gap compared to the bulk band gap of B4C. This provides a plausible explanation for the significantly smaller band gap observed in experiments compared to the larger gap predicted by theoretical models. Our study not only sheds light on the surface properties of B4C but also lays the groundwork for advancing this material for more advanced nuclear applications.

Universal Polar Instability in Highly Orthorhombic Perovskites.

The design of novel multiferroic ABO3 perovskites is complicated by the presence of necessary magnetic cations and ubiquitous antiferrodistortive modes, both of which suppress polar distortions. Using first-principles simulations, we observe that the existence of quadlinear and trilinear invariants in the free energy, coupling tilts, and antipolar motions of A and B sites to the polar mode drives an avalanche-like transition to a non-centrosymmetric Pna21 symmetry in a wide range of magnetic perovskites with small tolerance factors, overcoming the above restrictions. We find that the Pna21 phase is especially favored with tensile epitaxial strain, leading to an unexpected but technologically useful out-of-plane polarization. We use this mechanism to predict various novel multiferroics, displaying interesting magnetoelectric properties with small polarization switching barriers.

Ultralow lattice thermal conductivity in K2AuBi and its thermoelectric properties

Thermoelectric materials are best known for their prowess to transform the environment’s waste heat into electricity. In an endeavor to explore new thermoelectric prospects, in the present study, we investigate K2AuBi using density functional theory-based first-principles simulations. From our simulations, we find an intrinsically low lattice thermal conductivity of 0.43 W m−1 K−1 at 300 K in K2AuBi. Based on our detailed analysis, we find the reasons for such a low value of lattice thermal conductivity as, low phonon group velocities, short phonon lifetimes, anharmonicity in the lattice vibrations, and significant mean square displacements of K and Au atoms. The large mean square displacements hint at weak bonding and anharmonicity in the lattice vibrations, favoring more phonons scattering. We also find that the vibrations of K-atoms can be related to rattlers, conducive to low lattice thermal conductivity. Our simulations predict a high value, ∼784 μV K−1, of Seebeck coefficient at 700 K on account of the large density of states in the vicinity of Fermi level. Combining our computed lattice thermal conductivity with electrical transport properties, we obtain a high figure of merit, ZT∼ 1.04, at 700 K in K2AuBi.

Piezoelectric GaGeX2 (X = N, P, and As) semiconductors with Raman activity and high carrier mobility for multifunctional applications: a first-principles simulation.

In the present work, we propose GaGeX2 (X = N, P, As) monolayers and explore their structural, vibrational, piezoelectric, electronic, and transport characteristics for multifunctional applications based on first-principles simulations. Our analyses of cohesive energy, phonon dispersion spectra, and ab initio molecular dynamics simulations indicate that the three proposed structures have good energetic, dynamic, and thermodynamic stabilities. The GaGeX2 are found as piezoelectric materials with high piezoelectric coefficient d 11 of -1.23 pm V-1 for the GaGeAs2 monolayer. Furthermore, the results from electronic band structures show that the GaGeX2 have semiconductor behaviours with moderate bandgap energies. At the Heyd-Scuseria-Ernzerhof level, the GaGeP2 and GaGeAs2 exhibit optimal bandgaps for photovoltaic applications of 1.75 and 1.15 eV, respectively. Moreover, to examine the transport features of the GaGeX2 monolayers, we calculate their carrier mobility. All three investigated GaGeX2 systems have anisotropic carrier mobility in the two in-plane directions for both electrons and holes. Among them, the GaGeAs2 monolayer shows the highest electron mobilities of 2270.17 and 1788.59 cm2 V-1 s-1 in the x and y directions, respectively. With high electron mobility, large piezoelectric coefficient, and moderate bandgap energy, the GaGeAs2 material holds potential applicability for electronic, optoelectronic, piezoelectric, and photovoltaic applications. Thus, our findings not only predict stable GaGeX2 structures but also provide promising materials to apply for multifunctional devices.

First-principles calculations to investigate thermodynamic, mechanical and electronic properties of Penta-C72 carbon under pressure effect

The current work explores the mechanical characteristics, band structure modifications, elastic constant changes, and anisotropy of the carbon allotrope Penta-C72 at high pressure. First-principles simulations were used to analyse the structural features, stability, electronic, mechanical, and thermodynamic properties of Penta-C72, a metastable sp3 -bonded structure made up of carbon pentagons joined by bridge-like connections. The structural and dynamical stability of Penta-C72 under high pressure is confirmed using formation energy, phonon band maps, and thermodynamic properties. Besides, Born's mechanical stability criterion is fulfilled based on the elastic constants of Penta-C72, indicating its mechanical stability. Also, the anisotropic mechanical behaviour is shown by the material. Using the Reuss, Voigt, and Hill approximations, the elastic moduli under high pressure were computed, including bulk modulus (K), shear modulus (G), and Young's modulus (E). Besides, Penta-C72 exhibits a rise in anisotropy and moduli with increasing pressure. This investigation explores the changes in the band gap upon variation in the pressure. It is observed that there is a transition from semiconducting to metallic property above 10 GPa. Beyond 10 GPa, the material exhibits metallic behaviour. The study emphasizes that Penta-C72’s exceptional mechanical stability, directional anisotropy, and customisable electronic characteristics make it a strong contender for various engineering applications.

First-principles simulation of electronic properties of MoB/Si3N4 superlattices via machine learning

MBene materials and Si3N4 materials have excellent properties, but there are still many technical bottlenecks in the preparation of high-performance and flexible structure electronic device materials. In this paper, MBene is combined with Si3N4 materials to form a superlattice structure, which is composed of six superlattice heterostru ctures. Firstly, the first-principles method was used to study the electronic properties of MBene/Si3N4 superlattice materials. The results show that with the increase of stacking times, the material structure becomes more stable, and the interlayer electrons are transferred from the MoB layer to the Si3N4 layer. Second, in the machine learning part, the Extreme Gradient Boosting (XGBoost) algorithm and the Convolutional Neural Network (CNN) algorithm were used to predict the density of states of six superlattice heterojunctions through ensemble learning, and it was found that the best prediction results were achieved when the input data with a combination of global descriptors and local descriptors were used. The lowest MAE is only 0.03, which not only indicates that the predicted density of states model is excellent, but also proves that the more refined the characterization of the surrounding environment of the atom, the more accurate the prediction of the density of states of the material and the higher the generalization performance of the prediction model.

Hydrogen and vacancy concentrations in [formula omitted]-iron under high hydrogen gas pressure and external stress: A first-principles neural network simulation study

The metal-hydrogen-vacancy interaction in α-iron is crucial for understanding hydrogen embrittlement behavior and developing reliable materials for green gaseous hydrogen applications; however, it remains underexplored, particularly in contexts involving external stress and high hydrogen gas pressure. In this study, we performed quantitative analyses of metal-vacancy–hydrogen interactions in α-iron, measured by hydrogen solubility and the thermodynamics of vacancy–hydrogen complexes, under such challenging conditions using a reliable first-principles neural network potential. High hydrogen gas pressures reaching up to 2 GPa were investigated. As the atomic concentration surpasses approximately 0.01, hydrogen solubility is dominated by hydrogen-induced lattice distortion (primarily volumetric expansion) and the interactions between hydrogen atoms within the metal matrix, resulting in significant deviations from Sieverts’ law. Our results reveal a significant effect of shear stress on hydrogen solubility, deviating from previously used equations. Moreover, the influence of external stress on the thermodynamics of the vacancy–hydrogen complex, particularly vacancy formation free energy, is uniquely characterized by hydrogen solubility, regardless of the external stress magnitude. It thereby offers a rapid method to estimate the vacancy properties under external stress based on readily accessible external-stress-free data.

Study on thermoelectric properties and atomic-scale mechanism of B-site molybdenum-modified La0.1Sr0.9TiO3

Extensive studies on strontium titanate thermoelectric materials have shown that their poor conductivity results in a low ZT value. However, elemental doping can effectively improve the thermoelectric properties of these materials. In this study, a two-step sintering process was applied, using physical mixing to introduce 1 % molybdenum powder as a modification to the lanthanum-doped strontium titanate solid powder (La0.1Sr0.9TiO3). The obtained sample exhibited a maximum conductivity of 7830 S/m, with a maximum ZT of 0.12. Furthermore, this study combined experimental data with first-principles simulation calculations to elucidate the mechanism at the atomic scale. The incorporation of Mo6+ in place of Ti4+ at the B-site by molybdenum (Mo) adds two free electrons, leading to an elevated carrier concentration. This change concurrently reduces the Fermi level of the material and improves mobility, collectively resulting in a substantial increase in electrical conductivity. Phonon simulation indicated that Mo doping increased structural defects, thereby reducing lattice thermal conductivity. This significant enhancement in electrical conductivity and decrease in lattice thermal conductivity collectively increased the ZT value of the material.

Analysis and simulation of opto-electronics characterization of two-dimensional Janus monolayers for energy applications

First-principles simulations are conducted to investigate the absorption and optoelectronic efficacy of molybdenum–sulfur–selenium, referred to here as MoSSe, and molybdenum–sulfur–oxygen, referred to here as MoSO, Janus monolayers. The materials MoSSe and MoSO demonstrate characteristics of semiconductors, as they possess bandgaps of 2.00 eV (direct) and 1.61 eV (indirect), respectively. This property renders them highly suitable for efficient light absorption. The efficiency of absorption of the device was calculated for the MoSSe and MoSO families, leading to the observation that these material families demonstrate a broad absorption range spanning from the infrared to the ultraviolet regions of the electromagnetic spectrum. This finding represents a novel discovery. Furthermore, the design as a topmost cell is particularly attractive due to its exceptional device absorption efficiency and broader bandgap. This particular family ensures that its band edges remain in alignment with the water-redox potentials. Molybdenum sulfide and molybdenum selenide exhibit promising potential as photocatalysts and in optoelectronic device applications. This is attributed to their appealing photocatalytic properties and notable efficiency in absorbing light for the purpose of water splitting.

Effects of metals (X = Pd, Ag, Cd ) on structural, electronic, mechanical, thermoelectric and hydrogen storage properties of LiXH3 perovskites

Using the WIEN2K code, the hydrogen storage capabilities of lithium compositions like LiXH3 (X = Pd, Ag, Cd) hydrides are examined. Structural, electronic, mechanical, thermoelectric, and hydrogen storage properties of these hydrides are analyzed using first-principles simulations. Structural analysis of these compositions reveals that the hydrides are stable and belong to the cubic space group number (221 Pm-3 m). The thermodynamic stability of these hydrides are given in terms of formation energies and stability is checked through phonon dispersion curves. The purpose of the study is to calculate formation energy and breakdown temperature to determine stability of these hydrides. The metallic nature of all compositions are confirmed by band plots and density of states. The elastic properties are calculated to check the applicability of these compositions for applications involving hydrogen storage. The present paper represents the initial theoretical approach towards the future exploration of these materials for hydrogen storage applications.

Ionic conduction and cathodic properties of CaMO3 (M=Fe and Mn) electrode materials via molecular dynamics and first-principles simulations

Calcium-ion(Ca-ion) batteries are gaining ever-increasing attention for next-generation energy storage systems due to affordability, highly abundant, high energy density, high theoretical capacity, and low redox potential close to Li-ion. In this work, we deployed the first-principles and classical molecular dynamics simulations to investigate the electronic and diffusive properties of isostructural ternary perovskite CaMO3 (M= Fe and Mn). The transport properties at various temperatures from ion dynamics and electronic properties of CaMO3 perovskites are examined using classical molecular dynamics and quantum mechanical simulations, respectively. We present the microscopic origin of the diffusion of multivalent ions like Ca2+ within the crystal structure of perovskite material and the effects of two transition metals, manganese and iron. Dynamic studies of Ca-ions were performed using molecular dynamic simulation, which depicts the diffusivity and conductivity of Ca-ion in CaMO3 material. We find that the diffusivity in both the crystals increases with temperature; as a result, conductivity increases. Among both the crystals, CaFeO3 requires less activation energy for diffusion and ionic conduction than CaMnO3. Using density functional theory, we calculated specific capacity, electronic density of states, phase stability and equilibrium cell voltage, and charge transfer process during intercalation-deintercalation from first-principles calculations. The electronic behavior of these materials shows that CaFeO3 has better electronic and transport properties than CaMnO3.