Local Density Research Articles

Chemically Accurate Singlet-Triplet Gaps of Arylcarbenes from Local Hybrid Density Functionals.

Singlet-triplet (ST) gaps are key descriptors of carbenes, because their properties and reactivity are strongly spin-dependent. However, the theoretical prediction of ST gaps is challenging and generally thought to require elaborate correlated wave function methods or double-hybrid density functionals. By evaluating two recent test sets of arylcarbenes (AC12 and AC18), we show that local hybrid functionals based on the "common t" local mixing function (LMF) model achieve mean absolute errors below 1 kcal/mol at a computational cost only slightly higher than that of global hybrid functionals. An analysis of correlation contributions to the ST gaps suggests that the accuracy of the common t-LMF model is mainly due to an improved description of nondynamical correlation which, unlike exchange, is not additive in each spin-channel. Although spin-nonadditivity can be achieved using the local spin polarization alone, using the "common", i.e., spin-unresolved, iso-orbital indicator t for constructing the LMF is found to be critical for consistent accuracy in ST gaps of arylcarbenes. The results support the view of LHs as vehicles to improve the description of nondynamical correlation rather than sophisticated exchange mixing approaches.

The importance of long range Fe–Fe interactions in magnetically ordered 2D Fe0.2Ni0.8/Ni(001) and Fe0.3Ni0.7/Ni(001) alloy systems

We develop a theoretical approach to study the magnonic properties of the 2D ordered surface Fe–Ni alloys with 20% and 30% Fe on a face-centered cubic Ni(001) surface substrate. The surface alloy nanostructures in stable equilibrium are determined by computing the lowest energy configurations of constitutive supercells of the ordered alloys. The magnetic exchange constants of the ground state of the stable systems are calculated to define their Heisenberg Hamiltonians using the DFT Spin-Polarized Relativistic Korringa–Kohn–Rostoker (SPRKKR) method and considering Fe–Fe interactions up to third nearest neighbors. To determine the propagating exchange spin-wave modes, evanescent modes, and the consequent magnonic properties, the spin dynamics of the magnetic surface alloy nanostructures are solved using the phase field matching theory (PFMT). Our results indicate that the Fe–Fe exchange interactions up to third nearest neighbors are vital for accurately describing their magnonics. In particular, the exchange interactions influence the form of dispersion branches for the high-energy spin-wave modes of the Fe–Ni surface alloys considered. Appropriate Green’s functions are also derived from the PFMT method to compute the local densities of states (LDOS) for the atomic sites at the surface boundary. For the case of 20% Fe surface alloy, the Fe–Fe exchange interactions shift the Fe LDOS peaks to lower frequency intervals. We also distinguish the emergence of remarkable LDOS peaks for the he case of the 30% Fe surface alloy at specific Ni sites. These peaks change positions under second and third nearest neighbor Fe–Fe interactions, heralding new accessible spin-wave channels. Our results contribute to establishing a systematic understanding of the magnonics of the magnetic surface alloys of transition metal (TM) materials. This new knowledge will enable the study of the quantum transport of spin waves at interfacial alloy nanojunctions composed of such materials. The present DFT-PFMT theoretical approach is general and can be applied to other surface TM alloy magnetic nanostructures.

Real-time confinement regime detection in fusion plasmas with convolutional neural networks and high-bandwidth edge fluctuation measurements

A real-time detection of the plasma confinement regime can enable new advanced plasma control capabilities for both the access to and sustainment of enhanced confinement regimes in fusion devices. For example, a real-time indication of the confinement regime can facilitate transition to the high-performing wide-pedestal (WP) quiescent H-mode, or avoid unwanted transitions to lower confinement regimes that may induce plasma termination. To demonstrate real-time confinement regime detection, we use the 2D beam emission spectroscopy (BES) diagnostic system to capture localized density fluctuations of long wavelength turbulent modes in the edge region at a 1 MHz sampling rate. BES data from 330 discharges in either L-mode, H-mode, quiescent H (QH)-mode, or WP QH-mode were collected from the DIII-D tokamak and curated to develop a high-quality database to train a deep-learning classification model for real-time confinement detection. We utilize the 6×8 spatial configuration with a time window of 1024 µs and recast the input to obtain spectral-like features via fast Fourier transform preprocessing. We employ a shallow 3D convolutional neural network for the multivariate time-series classification task and utilize a softmax in the final dense layer to retrieve a probability distribution over the different confinement regimes. Our model classifies the global confinement state on 44 unseen test discharges with an average F 1 score of 0.94, using only ∼1 ms snippets of BES data at a time. This activity demonstrates the feasibility for real-time data analysis of fluctuation diagnostics in future devices such as ITER, where the need for reliable and advanced plasma control is urgent.

Unveiling the Role of Spin Currents on the Giant Rashba Splitting in Single-Layer WSe2.

The Rashba spin splitting in uniaxial, inversion-asymmetric materials has attracted considerable interest for spintronic applications. The most widely used theoretical framework to model such states is Kohn-Sham density functional theory (DFT) in combination with standard (semi)local exchange-correlation density functional approximations (DFAs). However, in the presence of spin-orbit coupling, DFT misses contributions due to modification of the many-body interaction by spin currents J⃗. Inclusion of the latter effects requires a spin current DFT (SCDFT) formulation, which is seldom considered. We investigate the giant Rashba splitting in single-layer WSe2, and we quantify the effect of including spin currents in DFAs of the SCDFT. Crucially, we show that SCDFT allows fully capturing the giant Rashba band splitting in single-layer WSe2, otherwise previously systematically underestimated by standard (semi)local DFAs within the DFT framework. We find the inclusion of J⃗ on the DFA increases the Rashba splitting by about 20%.

Transformation of a cellular skyrmion to polyomino-like structures

Topological spin structures with transformable shapes may have potential implications for data storage and computation. Here, we demonstrate that a square cellular skyrmion on an artificial grid pinning pattern can be manipulated by programmed current pulses. We find that parallel short pulses could result in elongation of the skyrmion mainly in the current direction, while parallel long pulses are able to induce elongation in the direction perpendicular to the current due to the intrinsic skyrmion Hall effect. Consequently, a programmed sequence of parallel pulses could lead to transformation of the skyrmion to I-, L-, and Z-shaped polyomino-like structures without affecting the topological charge. In addition, we find that orthogonal pulses could lead to the transformation to more complex polyomino-like structures, including T-shaped and irregular ones. Particularly, when a small T-shaped structure is formed, the topological charge of the system is found to be non-integer due to incomplete compensation of local topological charge densities; however, the T-shaped structure is stable on the attractive pinning pattern. Our results offer an effective way to create polyomino-like spin structures toward functional applications.

Influence of the Hall-bar geometry on texture-induced topological spin transport in two-dimensional Rashba spin–orbit ferromagnets

While the recent prediction and observation of magnetic skyrmions bears inspiring promise for next-generation spintronic devices, how to detect and track their position becomes an important issue. In this work, we investigate the spin transport in a two-dimensional magnetic nanoribbon with the Hall-bar geometry in the presence of Rashba spin–orbit coupling and magnetic skyrmions. We employ the Kwant tight-binding code to compute the Hall conductance and local spin-polarized current density. We consider two versions of the model: One with single skyrmion and one with two separate skyrmions. It is found that the size and position of the skyrmions strongly modulate the Hall conductance near the Hall-bar position. The geometry of the Hall bar also has a strong influence on the Hall conductance of the system. With the decreasing of the width of Hall leads, the peak of Hall conductance becomes sharper. We also show the spatial distribution of the spin-polarized current density around a skyrmion located at different positions. We extend this study toward two separate skyrmions, where the Hall conductance also reveals a sizable dependence on the position of the skyrmions and their distance. Our numerical analysis offers the possibility of electrically detecting the skyrmion position, which could have potential applications in ultrahigh-density storage design.

Anomalous Superfluid Density in a Disordered Charge-Density-Wave Material: Pd-Intercalated ErTe_{3}.

We image local superfluid density in single crystals of Pd-intercalated ErTe_{3} below the superconducting critical temperature T_{c}, well below the onset temperature T_{CDW} of (disordered) charge-density-wave order. We find no detectable inhomogeneities on micron scales. We observe a rapid increase of the superfluid density below T_{c}, deviating from the behavior expected in a conventional Bardeen-Cooper-Schrieffer superconductor, and show that the temperature dependence is qualitatively consistent with a combination of quantum and thermal phase fluctuations.

Local existence and uniqueness of solutions to the time-dependent Kohn–Sham equations coupled with classical nuclear dynamics

We prove the short-time existence and uniqueness of solutions to the initial-value problem associated with a class of time-dependent Kohn–Sham equations coupled with Newtonian nuclear dynamics, combining Yajima's theory for time-dependent Hamiltonians with Duhamel's principle, based on suitable Lipschitz estimates. We consider a pure power exchange term within a generalisation of the so-called local-density approximation, identifying a range of exponents for the existence and uniqueness of H2 solutions to the Kohn–Sham equations.

Investigation of current density and temperature distribution characteristics under transient conditions based on a 108 cm2 segmented proton exchange membrane fuel cell

Visualizing the electrochemical and temperature distribution characteristics inside proton exchange membrane fuel cell (PEMFC) should greatly contribute to the product development and performance optimization. In the study, a customized 27-segment 108 cm2 fuel cell was fabricated based on the printed circuit board methodology to achieve the local current density and local voltage measurement. In addition, 27 thermocouples were embedded in the segmented cathode graphite gas flow plate for local temperature measurement. Detailed information of fuel cell core components, multi-channel data acquisition system, external temperature control system, and uncertainty analysis was presented. According to the test results of 4000 s current rising conditions and 2400 s current decreasing conditions, the measurement deviation of total current between data acquisition system and TOYO test station is 2.4%–3.2%. Meanwhile, the deviation of output voltage ranges from 0.1% to 2.6%. To quantify the degree of distribution non-uniformity, the range and standard deviation of local parameter values as well as contour map are demonstrated. The non-uniformity of local current density and local temperature becomes more significant under higher operating current conditions. Furthermore, the degree of parameter non-uniformity is more noticeable under current rising conditions compared with current decreasing conditions. The transient increase of operating current leads to the sudden overshoot phenomena of local current range, and the change of local current density among all segments is instantaneously finished. However, the range of current demonstrates relatively smooth transition trends when reactant flow rate rises. Higher temperature area appears in the middle of membrane electrode assembly, especially in the position with higher local current density. The parameter distribution and transition characteristics could provide helpful guidance for fuel cell optimization and diagnosis.

Tailoring local electron density and molecular oxygen activation behavior via potassium/halogen co-tuned graphitic carbon nitride for enhanced photocatalytic activity

Graphite carbon nitride (g-C3N4) is a promising photocatalyst,but its inadequate reactive sites, weak visible light responsiveness, and sluggish separation of photogenerated carriers hamperthe improvement of photodegradation efficiency. In this work, potassium (K) and halogen atoms co-modified g-C3N4 photocatalysts (CN-KX, X = F, Cl, Br, I) were constructed to adjust the electrical and band structure for enhanced generation of reactive oxygen species. Through an integration of theoretical calculation and experimental exploration, the doping sites of halogen atoms as well as the evolution of crystal, band, and electronic structures were investigated. The results show that a covalent bond is formed between the F atom and the C atom, substitution of the N atom occurs with a Cl atom, and doping of Br, I, or K atoms takes place at the interstitial site. CN-KX photocatalysts exhibits lower band gap, faster photogenerated electron migration, and enhanced photocatalytic activity. Specifically, the CN-KI photocatalyst exhibits the highest photodegradation efficiency because of its smaller interplanar spacing, formation of the midgap state, and adjustable local electron density. Equally, the doping of I atom not only provides a stable adsorption site for oxygen (O2) but also facilitates electron transfer, promoting the production of superoxide radicals (O2−) and contributing to the process of photodegradation.

Electronic structure, bonding, and mechanical strength at the [formula omitted]-Al2O3 (0001)/L12-Al3Zr (111) interface by first-principles calculations

The interface adhesion energy, interfacial energy, charge density difference, local density of states, and first-principles computational tensile test of α-Al2O3 (0001)/L12-Al3Zr (111) interface were investigated by first-principles calculations. The dominant factors affecting the interfacial bond strength are the type of chemical bonds and the number of electronic orbital hybrids at the interface. The first-principles computational tensile test demonstrates that Al-fcc and Al2-hcp configurations fracture at the interface, while the O-fcc configuration fractures at the Al3Zr slab rather than at the interface and has the best tensile strength. The proposed interface bonding criteria were based on the analysis of the Al2O3/Al3Zr interface bonding data. These criteria will guide the establishment of the interface model. These results will provide theoretical guidance for understanding the failure mechanism of the Al2O3/Al3Zr interface and designing new interfaces.

Home range overlaps of the brushtail possum (Trichosurus vulpecula): implications for disease transmission

Understanding how bovine tuberculosis (TB) is maintained in wildlife reservoirs is critical for the management of this disease impacting cattle in many countries. For the primary wildlife reservoir of the disease in New Zealand, the brushtail possum (Trichosurus vulpecula), transmission of this contagious bacterial disease among possums is often assumed to be linked to home range overlap. Identifying drivers of possum home range, and home range overlap between individuals, is thus important for informing wildlife reservoir TB management in New Zealand. We monitored four sub-populations of free-living possums in New Zealand native forests during 10 consecutive months using live trapping, to give the first direct insight into how the frequency and area of overlaps alters with density, sex and age. A total of 832 individuals were captured (average 9.3, range from 1 to 40 captures per animal with a median value of 7) and 35,820 home range overlaps were recorded. Both the number and area of overlaps were significantly associated with age class, with 66.6% of overlaps occurring between adults, 30% between adults and juveniles, and only 3.4% between juveniles. Overall, adult males showed significantly higher numbers of overlaps than expected, while adult and juvenile females showed significantly lower numbers of overlaps than expected and no differences were observed in juvenile males. In addition, males exhibited more and larger overlaps than females. The number and size of overlaps per individual decreased with increasing local population density. Understanding shared areas of activity among individuals can provide insights into the interactions occurring and potential pathways for diseases transmitted by contact such as TB. These results can inform to develop effective strategies for the control of diseases carried and dispersed by possums.

Potassium Phthalimide Doped with Delta‐Site Structures to Construct Ultra‐Long Cycle Life Aqueous Zn‐Polymer Batteries

AbstractAqueous Zn‐polymer batteries hold great promise for environmentally friendly energy storage systems. However, the poor cycling performance caused by irreversible structural deformation and polymer chain entanglement during the embedding and de‐embedding processes of Zn2+ and H+ ions is one of the key challenges facing their development. Here a new polyaniline cathode doped is developed with potassium phthalimide using a secondary doping approach. This polymeric electrode integrates the conjugated backbone for charge transport with an anionic phthalimide dopant for constructing a triangular‐site structure, as illustrated by the density functional theory calculations. This unique molecular structure is favorable for improving conductivity and structural stability by effectively modulating the localized electron cloud density. The Zn‐polymer battery holds an outstanding capacity retention of 76.1% after 25,000 cycles at a current density of 1 A g−1 with the coulombic efficiency maintained above 99.4%. This secondary doped polymer strategy offers a new host alternative for the ultra‐long cycle life of Zn‐polymer batteries. The molecular design strategy and the insight into the relationship between the structure and performance may provide some guidelines for developing polymer cathodes with high stability.

Constructing Sn-Cu2O Lithiophilicity Nanowires as Stable and High-Performance Lithium Metal Anodes.

Lithium (Li) metal batteries (LMBs) have garnered significant research attention due to their high energy density. However, uncontrolled Li dendrite growth and the continuous accumulation of "dead Li" directly lead to poor electrochemical performance in LMBs, along with serious safety hazards. These issues have severely hindered their commercialization. In this study, a lithiophilic layer of Sn-Cu2O is constructed on the surface of copper foam (CF) grown with Cu nanowire arrays (SCCF) through a combination of electrodeposition and plasma reduction. Sn-Cu2O, with excellent lithiophilicity, reduces the Li nucleation barrier and promotes uniform Li deposition. Simultaneously, the high surface area of the nanowires reduces the local current density, further suppressing the Li dendrite growth. Therefore, at 1 mA cm-2, the half cells and symmetric cells achieve high Coulombic efficiency (CE) and stable operation for over 410 cycles and run smoothly for more than 1350 h. The full cells using an LFP cathode demonstrate a capacity retention rate of 90.6% after 1000 cycles at 5 C, with a CE as high as 99.79%, suggesting excellent prospects for rapid charging and discharging and long-term cyclability. This study provides a strategy for modifying three-dimensional current collectors for Li metal anodes, offering insights into the construction of stable, safe, and fast-charging LMBs.

Manipulating chiral Majorana mode with additional potential in superconductor-Chern insulator heterostructures

We employed the self-consistent Bogoliubov–de Gennes equations to explore the states of chiral Majorana mode in quantum anomalous Hall insulators in proximity to a superconductor, leading to the development of an extensive topological phase diagram. Our investigation focused on how an additional potential affects the separation of chiral Majorana modes across different phase conditions. We substantiated our findings by examining the zero-energy Local Density of States spectrum and the probability distribution of the chiral Majorana modes. We established the universality of chiral Majorana mode separation by applying an additional potential. This finding serves as a vital resource for future endeavors aimed at controlling and detecting these particles, thereby contributing to the advancement of quantum computing and condensed matter physics.

Interfacial engineering of Ag/C catalysts for practical electrochemical CO2 reduction to CO.

Electrochemical CO2 reduction to value-added chemicals by renewable energy sources is a promising way to implement the artificial carbon cycle. During the reaction, especially at high current densities for practical applications, the complex interaction between the key intermediates and the active sites would affect the selectivity, while the reconfiguration of electrocatalysts could restrict the stability. This paper describes the fabrication of Ag/C catalysts with a well-engineered interfacial structure, in which Ag nanoparticles are partially encapsulated by C supports. The obtained electrocatalyst exhibits CO Faradaic efficiencies (FEs) of over 90% at current densities even as high as 1.1 A/cm2. The strong interfacial interaction between Ag and C leads to highly localized electron density that promotes the rate-determining electron transfer step by enhancing the adsorption and the stabilization of the key *COO‒ intermediate. In addition, the partially encapsulated structure prevents the reconfiguration of Ag during the reaction. Stable performance for over 600 h at 500 mA/cm2 is achieved with CO FE maintaining over 95%, which is among the best stability with such a high selectivity and current density. This work provides a novel catalyst design showing the potential for the practical application of electrochemical reduction of CO2.

Approximate Functionals for Multistate Density Functional Theory.

Recently Lu and Gao [J. Phys. Chem. Lett. 2022, 13 (33), 7762-7769] published a new, rigorous density functional theory for excited states and proved that the projection of the kinetic and electron-repulsion operators into the subspace of the lowest electronic states are a universal functional of the matrix density D(r). This is the first attempt to find an approximation to the multistate universal functional . It is shown that (i) does not explicitly depend on the number of electronic states and (ii) is an analytic matrix functional. The Thomas-Fermi-Dirac-von Weizsäcker model and the correlation energy of the homogeneous electron gas are turned into matrix functionals guided by two principles: that each matrix functional should transform properly under basis set transformations and that the ground state functional should be recovered for a single electronic state. Lieb-Oxford-like bounds on the average kinetic and electron-repulsion energies in the subspace are given. When evaluated on the numerically exact matrix density of LiF, this simple approximation reproduces the matrix elements of the electron-repulsion operator in the basis of the exact eigenstates accurately for all bond lengths. In particular the off-diagonal elements of the effective Hamiltonian that come from the interactions of different electronic states can be calculated with the same or better accuracy than the diagonal elements. Unsurprisingly, the largest error comes from the kinetic energy functional. More exact conditions that constrain the functional form of are needed to go beyond the local density approximation.

Study of structural and electronic properties of AgCl and AgBr binaries

This modest work was devoted to studying the structural and electronic properties of the AgCl and AgBr binaries in the cubic NaCl phase. In this study, we used the FP-LAPW method based on functional density theory (DFT). For the treatment of exchange potential and correlation, we used the local density approximation (LDA). The objective of this work is to predict and compare the structural and electronic properties of our materials with other theoretical and experimental results. Most of the results obtained were approximate.

Benchmarking Functionals for Strong-Field Light-Matter Interactions in Adiabatic Time-Dependent Density Functional Theory.

In recent years, time-dependent density functional theory (TDDFT) has been extensively employed for highly nonlinear optics in molecules and solids, including high harmonic generation (HHG), photoemission, and more. TDDFT exhibits a relatively low numerical cost while still describing both light-matter and electron-electron interactions ab initio, making it highly appealing. However, the majority of implementations of the theory utilize the simplest possible approximations for the exchange-correlation (XC) functional-either the local density or generalized gradient approximations, which are traditionally considered to have rather poor chemical accuracy. We present the first systematic study of the XC functional effect on molecular HHG, testing various levels of theory. Our numerical results suggest justification for using simpler approximations for the XC functional, showing that hybrid and meta functionals (as well as Hartree-Fock) can, at times, lead to poor and unphysical results. The specific source of the failure in more elaborate functionals should be topic of future work, but we hypothesize that its origin might be connected to the adiabatic approximation of TDDFT.

Bioconvective reversible (an esterification process) Casson nanofluid flow over a radiative spinning disc with microorganism

This study describes the Casson (a non-Newtonian fluid) nanofluid bioconvection flow across a spinning disc in the presence of gyrotactic microorganisms, many slips and thermal radiation. Also, the flow is considered as a reversible flow. The esterification process is taken into account. Using the proper variables, a system of extremely nonlinear PDEs is converted into a system of ODEs. To arrive to the solution of such equations, a numerical approach is used. Using the bvp4c approach, nonlinear flow equations can be numerically solved. Investigated are the effects of different numbers on the thermal field, volumetric concentration of nanoparticles and microbiological field. The key characteristics of the parameters in relation to the profiles of the velocity, temperature, concentration and microorganisms are graphically assessed with appropriate physical effects. A graphical explanation is provided for the wall shear stress, local Nusselt number, local Sherwood number and local motile density number. Rate of motile density number shows a prominent difference between reversible and irreversible flows for Brownian motion and Peclet number. The results of the theoretical simulations have dynamic applications in the fields of biotechnology and thermal engineering.