Charge Neutrality Condition Research Articles

Revealing the Potential-Dependent Rate-Determining Step of Oxygen Reduction Reaction on Single-Atom Catalysts.

Single-atom catalysts (SACs) have attracted widespread attention due to their potential to replace platinum-based catalysts in achieving efficient oxygen reduction reaction (ORR), yet the rational optimization of SACs remains challenging due to their elusive reaction mechanisms. Herein, by employing ab initio molecular dynamics simulations and a thermodynamic integration method, we have constructed the potential-dependent free energetics of ORR on a single iron atom catalyst dispersed on nitrogen-doped graphene (Fe-N4/C) and further integrated these parameters into a microkinetic model. We demonstrate that the rate-determining step (RDS) of the ORR on SACs is potential-dependent rather than invariant within the operative potential range. Specifically, under the charge-neutral condition, the RDS is calculated to be water desorption with the highest barrier, while as the potential increases, it gradually transitions to the protonation of *OH species, O2* species, and O* species, regardless of the protonation of *OH species as the potential-determining step. Moreover, we reveal the critical role of the dynamic adsorption of axially adsorbed water in facilitating the release of the single-atom site, thus enhancing the ORR rate. Our work has resolved the long-standing controversies over the RDS of ORR on SACs and implies that the step with the lowest exothermicity is not always synonymous with the RDS, highlighting the importance of examining the kinetic barriers under realistic potential conditions for understanding the electrocatalytic performance.

The dense QCD equation of state within a modified NJL model

In this paper, we study the 2-flavor equation of state of the quantum chromodynamics at zero temperature and finite chemical potentials with a modified Nambu–Jona–Lasinio model, where the beta equilibrium and electric charge neutrality conditions of the system (including u, d quarks, electrons and muons) are considered. The related chiral phase transition is also discussed in this paper. For comparison, we show the results with four different parameter sets, and find only quantitative differences. As chemical potential increases, the crossover instead of first-order chiral phase transition happens. Finally, we calculate the binding energy per baryon for different parameter sets, and find that the 2-flavor quark system with a smaller [Formula: see text] (or a larger m) possesses the lower binding energy per baryon, indicated to be more stable than the other case.

Stellar model with non-zero strange quark mass (ms≠0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_s\ e 0$$\\end{document}) and Mak–Harko density profile admitting observational results

This article describes the configuration of strange quark stars composed of three flavour quarks in hydrostatic equilibrium considering the energy density profile of Mak and Harko and non-zero strange quark mass (ms≠0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_s\ e {0}$$\\end{document}). A suitable stellar model is proposed assuming equation of state of interior matter as modified MIT equation of state in bag model p=13(ρ-4Bg)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p=\\frac{1}{3}(\\rho -4B_g)$$\\end{document}, where Bg\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B_g$$\\end{document} is known as bag constant, to predict the viability of strange stars. The interior of such compact stars is mainly composed of quarks and electrons to establish charge neutrality condition. We have checked the various stability windows depending on the energy per baryon for different values of bag constant Bg\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B_g$$\\end{document} and mass of strange quark ms\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_s$$\\end{document}. We have studied the effect of non-zero mass of the strange quark (ms\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_s$$\\end{document}) on the physical properties of the strange star in the context of the modified MIT bag equation of state. We note that the maximum stellar mass decreases with increasing ms\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_s$$\\end{document}. The model is suitable for predicting the radius, central density and other properties of compact stars having mass ≤2.01M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\le 2.01M_{\\odot }$$\\end{document}, such as 4U1820-30\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4U~1820-30$$\\end{document}, PSRJ1614-2230\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$PSR~J1614-2230$$\\end{document} etc, which are supposed to be strange stars. The radius of few recently observed compact stars are predicted in our model and they are found to be compatible with the value as predicted from observation. The model is found to be suitable in view of all the necessary criterion along with fulfilment of stability of the stellar configuration as well as stable in view of small radial perturbations.

Lossless Spin-Orbit Torque in Antiferromagnetic Topological Insulator MnBi_{2}Te_{4}.

We formulate and quantify the spin-orbit torque (SOT) in intrinsic antiferromagnetic topological insulator MnBi_{2}Te_{4} of a few septuple-layer thick in charge-neutral condition, which exhibits pronounced layer-resolved characteristics and even-odd contrast. Contrary to traditional current-induced torques, our SOT is not accompanied by Ohm's currents, thus being devoid of Joule heating. We study the SOT-induced magnetic resonances, where in the tri-septuple-layer case we identify a peculiar exchange mode that is blind to microwaves but can be exclusively driven by the predicted SOT. As an inverse effect, the dynamical magnetic moments generate a pure adiabatic current, which occurs concomitantly with the SOT and gives rise to an overall reactance for the MnBi_{2}Te_{4}, enabling a lossless conversion of electric power into magnetic dynamics.

First-principles calculations of proton defect properties in Ca-doped YPO4.

YPO4 has a xenotime-type structure with one-dimensional percolating channels along the c-axis, and Ca-doped YPO4 exhibits proton conduction. In the present study, using first-principles calculations, we investigate the behaviors of proton solutions in 3 mol% Ca-doped YPO4 in the intermediate temperature range on the basis of point-defect formation energies, defect concentrations, and migration barriers. Although the charge-neutrality condition is mainly satisfied by and defects within the defect formation energy diagrams, the defect complex has the lowest formation energy and the highest concentration among examined defects under various temperature and partial-pressure conditions. The migration barriers of isolated protons in the [100] and [001] directions, as obtained from nudged elastic band (NEB) calculations, are 0.49 and 0.17 eV, respectively, confirming that protons in the YPO4 crystal exhibit anisotropic diffusion and are likely to migrate along the c-axis channels. The activation energy estimated by the sum of the migration energy of the isolated proton and the association energy for the defect complex is comparable to the reported experimental value. Based on concentration calculations and diffusion-property analyses, we identified the Ca doping conditions and temperature ranges that govern proton conduction in YPO4 and elucidated the diffusion pathways.

Energy optimisation predicts the capacity of ion buffering in the brain.

Neurons store energy in the ionic concentration gradients they build across their cell membrane. The amount of energy stored, and hence the work the ions can do by mixing, can be enhanced by the presence of ion buffers in extra- and intracellular space. Buffers act as sources and sinks of ions, however, and unless the buffering capacities for different ion species obey certain relationships, a complete mixing of the ions may be impeded by the physical conditions of charge neutrality and isotonicity. From these conditions, buffering capacities were calculated that enabled each ion species to mix completely. In all valid buffer distributions, the [Formula: see text] ions were buffered most, with a capacity exceeding that of [Formula: see text] and [Formula: see text] buffering by at least an order of magnitude. The similar magnitude of the (oppositely directed) [Formula: see text] and [Formula: see text] gradients made extracellular space behave as a [Formula: see text]-[Formula: see text] exchanger. Anions such as [Formula: see text] were buffered least. The great capacity of the extra- and intracellular [Formula: see text] buffers caused a large influx of [Formula: see text] ions as is typically observed during energy deprivation. These results explain many characteristics of the physiological buffer distributions but raise the question how the brain controls the capacity of its ion buffers. It is suggested that neurons and glial cells, by their great sensitivity to gradients of charge and osmolarity, respectively, sense deviations from electro-neutral and isotonic mixing, and use these signals to tune the chemical composition, and buffering capacity, of the extra- and intracellular matrices.

Impact of symmetry energy on sound speed and spinodal decomposition in dense neutron-rich matter

Using a meta model for nuclear Equation of State (EOS) with its parameters constrained by astrophysical observations and terrestrial nuclear experiments, we examine effects of nuclear EOS especially its symmetry energy \esym term on the speed of sound squared $C^2_s(\rho)$ and the critical density $\rho_t$ where $C^2_s(\rho_t)$ vanishes (indicating the onset of spinodal decomposition) in both dense neutron-rich nucleonic matter relevant for relativistic heavy-ion collisions and the cold $n+p+e+\mu$ matter in neutron stars at $\beta$-equilibrium. Unlike in nucleonic matter with fixed values of the isospin asymmetry $\delta$, in neutron stars with a density dependent isospin profile $\delta(\rho)$ determined consistently by the $\beta$ equilibrium and charge neutrality conditions, the $C^2_s(\rho)$ almost always show a peak and then vanishes at $\rho_t$. The latter strongly depends on the high-density behavior of \esym if the skewness parameter $J_0$ characterizing the stiffness of high-density symmetric nuclear matter (SNM) EOS is not too far above its currently known most probable value of about $-190$ MeV inferred from recent Bayesian analyses of neutron star observables. Moreover, in the case of having a super-soft \esym that is decreasing with increasing density above about twice the saturation density of nuclear matter, the $\rho_t$ is significantly lower than the density where the \esym vanishes (indicating the onset of isospin-separation instability and pure neutron matter formation) in neutron star cores.

Framework for phase transitions between the Maxwell and Gibbs constructions

By taking the nucleon-to-quark phase transition within a neutron star as an example, we present a thermodynamically consistent method to calculate the equation of state of ambient matter so that transitions that are intermediate to those of the familiar Maxwell and Gibbs constructions can be described. This method does not address the poorly known surface tension between the two phases microscopically (as, for example, in the calculation of the core pasta phases via the Wigner-Seitz approximation) but instead combines the local and global charge neutrality conditions characteristic of the Maxwell and Gibbs constructions, respectively. Overall charge neutrality is achieved by dividing the leptons to those that obey local charge neutrality (Maxwell) and those that maintain global charge neutrality (Gibbs). The equation of state is obtained by using equilibrium constraints derived from minimizing the total energy density. The results of this minimization are then used to calculate neutron star mass-radius curves, tidal deformabilities, equilibrium and adiabatic sound speeds, and nonradial $g$-mode oscillation frequencies for several intermediate constructions. Various quantities of interest transform smoothly from their Gibbs structures to those of Maxwell as the local-to-total electron ratio $\eta$, introduced to mimic the hadron-to-quark interface tension from $0$ (Gibbs) to $\infty$ (Maxwell), is raised from $0$ to $1$. A notable exception is the $g$-mode frequency for the specific case of $\eta=1$ for which a gap appears between the quark and hadronic branches.

Topological invariant and domain connectivity in moiré materials

Recently, a moir\'e material has been proposed in which multiple domains of different topological phases appear in the moir\'e unit cell due to a large moir\'e modulation. Topological properties of such moir\'e materials may differ from those of the original untwisted layered material. In this paper, we study how the topological properties are determined in moir\'e materials with multiple topological domains. We show a correspondence between the topological invariant of moir\'e materials at the Fermi level and the topology of the domain structure in real space. We also find a bulk-edge correspondence that is compatible with a continuous change in the truncation condition, which is specific to moir\'e materials. We demonstrate these correspondences in the twisted Bernevig-Hughes-Zhang model that describes general ${\mathbb{Z}}_{2}$-indicated topological insulator phases, by tuning its domain structure in the moir\'e unit cell. The obtained result can only be used to determine the topology at the Fermi level in the charge-neutral condition, but it can be combined with the traditional Wilson loop analysis to determine topologies of other gaps around. These results give a feasible method to evaluate a topological invariant for all occupied bands of a moir\'e material and contribute to the design of topological moir\'e materials and devices.

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Genetic significance of trace elements in hydrothermal quartz from the Xiangzhong metallogenic province, South China

Despite the Xiangzhong metallogenic province being globally the most important ore-concentrated area of antimony (Sb) mineralization, the genesis of Sb-(Au-W) deposits in this region is still under debate. In this study, trace element compositions of hydrothermal quartz from four representative Sb-(Au-W) deposits (Xikuangshan, Woxi, Banxi, and Zhazixi) in the Xiangzhong metallogenic province have been investigated to constrain the origin of ore-forming fluids and physiochemical conditions of Sb mineralization. The fsLA-ICP-MS data indicate that Li, B, Na, Al, P, K, and Sb are relatively enriched in quartz, probably mainly incorporated into the quartz crystal lattice, and dominated by the coupled substitution of (Al3+, Sb3+, B3+) + (Li+, Na+, K+, H+) ↔ Si4+ and (Al3+, Sb3+, B3+) + P5+ ↔ 2Si4+. In the Xikuangshan Sb deposit, charge-neutrality conditions of cations reveal that (Al3+, Sb3+) + Li+ ↔ Si4+ is the most important substitution mechanism in quartz from early-stage Sb mineralization (quartz-stibnite veins), whereas (Al3+, Sb3+) + (Li+ + H+) ↔ Si4+ may control the trace element contents in quartz from late-stage Sb mineralization (calcite-quartz-stibnite veins). The relatively high Al content and the low variability in trace element content suggest that quartz from the Xikuangshan Sb deposit may have precipitated from a relatively acidic and stable ore-forming fluid. Combined with the high Sr and Rb contents of quartz, a plausible explanation is that acidic hydrothermal fluids promoted water–rock reaction that dissolved large amounts of Sr and Rb, resulting in the high Al, Rb, and Sr contents in crystallized quartz from the Xikuangshan Sb deposit. In contrast, the relatively low but inhomogeneous Al content indicates that quartz from the Woxi and Banxi Sb-(Au-W) deposits may have precipitated from a less acidic ore-forming fluid and supports the idea of fluid mixing. More importantly, the relatively high contents of Rb, Ge, and Ti in quartz from the Woxi and Banxi Sb-(Au-W) deposits, combined with the low Al and Sr contents, imply that magmatic-hydrothermal systems possibly contributed to the ore-forming fluids. Based on the trace element compositions of quartz, two major types of Sb deposits were identified in the Xiangzhong metallogenic province: the giant Xikuangshan epithermal deposit in which the ore-forming fluids are relatively acidic, stable and have experienced water–rock reaction, generally similar to the Qinglong Sb deposit in the Youjiang metallogenic province. The second type is the orogenic-type deposit represented by the Woxi Sb-Au-W deposit, including the Banxi Sb deposit, where the ore-forming fluids have undergone fluid mixing and relatively weak water–rock reaction, and may have been joined by a small amount of magmatic-hydrothermal fluids. Together with the recently published Sb mineralization ages (149–144 Ma) of the Woxi Sb-Au-W deposit, it supports the idea that this type of Sb deposit is related to the Mesozoic magmatism. The Zhazixi Sb-W deposit shows transitional features between these two types of Sb deposits.

Noise spectroscopy based numerical modelling of chemisorption on SnO2 surface for CO gas sensing applications

The gas sensor response of the tin oxide (SnO2) semiconductor toward carbon monoxide (CO) gas has been simulated using the noise spectroscopy technique. The Wolkenstein gas adsorption model is used to calculate the surface coverage of environmental oxygen ions and target gas molecules. The interaction of CO gas molecules with the negatively charged oxygen ions has been incorporated in the numerical model. The equilibrium charge neutrality condition of the semiconductor material has been solved as a function of gas adsorption parameters like surface coverage, temperature, pressure and surface potential. Eventually, the power density spectrum (PDS) of the electrical conductance of the sensing layer was calculated before and after the gas adsorption conditions. It has been observed that the semiconductor band bending increases sharply at a certain concentration of environmental oxygen gas concentration (10−4 to 10−3 atm). At a temperature of 480 K, a steep decrease in surface potential was observed. This temperature range of maximum sensor response was optimized for calculating noise spectrum of electrical conductivity fluctuations at equilibrium condition of adsorption-desorption. The cutoff frequency (109 Hz) was calculated using normalized PDS of electrical conductance. The gas sensor response was calculated as a ratio of film conductance before and after the gas adsorption as a function of gas pressure and surface temperature at the calculated cutoff frequency. The increase in sensor response was observed with the increase in CO partial pressure as a function of temperature for a specific cut off frequency. The comparison of sensor response of the presented model has been demonstrated with another published numerical model. The presented model shows improved sensor response for higher (>10−8 atm) CO partial pressure.

Diffusion of oxygen in Mg-doped α−Al2O3 : The corundum conundrum explained

It has been a puzzle for over two decades that the enhancement of oxygen diffusion in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, with respect to the amount of Mg doping, is several orders of magnitude less than expected. The standard model, which envisages that transport is mediated by oxygen vacancies induced to compensate the charge of ${\mathrm{Mg}}^{2+}$ ions substituting ${\mathrm{Al}}^{3+}$ ions, has not been able to explain this anomaly. Here, we report a detailed study of populations of point defects and defect clusters in Mg-doped $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$. By taking into account calculated defect formation energies from the literature, the condition of charge neutrality, and the environmental parameters (chemical potentials) under which the anomalous trend in oxygen diffusivities were previously observed, we are able to arrive at an explanation. A nonlinear relationship between Mg concentration in the system and key native point defects, which serve as mediators of self-diffusion in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, is predicted: The concentrations of such defects increase much more slowly in the supersaturation regime than in the presaturation regime, matching the anomalous result previously observed in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$. We identify the reason for this as buffering by positively charged Mg interstitials and Mg--oxygen vacancy clusters, which compensate the negative charges of Mg substitutional defects (${\mathrm{Mg}}_{\text{Al}}^{1\ensuremath{-}}$). This study answers part of the long-standing question about self-diffusion in alumina, referred to by Heuer and Lagerl\"of in 1999 as the corundum conundrum.

Density functional calculations of atomic structure, charging effect, and static dielectric constant of two-dimensional systems based on B-splines

We have implemented a total-energy minimization scheme to allow for relaxation of atomic positions in density functional calculations for two-dimensional (2D) materials using a mixed basis set. The basis functions consist of products of 2D plane waves in the plane of the material and localized B-splines along the perpendicular direction. By using the mixed basis approach (MBA), we have studied the atomic relaxation and charge polarization of 2D systems under an applied electric field. In contrast to the conventional super-cell approach (SCA) which adopts repeated slabs sandwiched between vacuum regions, MBA makes no requirement of compensating background charge for treating electrically charged 2D systems due to carrier injection. With MBA we can avoid using a periodic sawtooth potential for systems under the applied field as commonly adopted in SCA. Furthermore, we introduced a simple method to determine the out-of-plane dielectric constants of 2D materials without the ambiguity of defining their effective thickness. This is achieved by calculating the linear response in charge polarization to the applied field. Selected 2D systems, including graphene and transition-metal dichalcogenides are tested. Our MBA results are consistent with previous SCA calculations when both approaches are equally applicable. However, for charged systems with high carrier density, we find significant deviation from SCA results obtained by imposing artificial charge neutrality condition.

Effects of local compositional and structural disorder on vacancy formation in entropy-stabilized oxides from first-principles

Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy composition. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodynamics of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equilibrium bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions associated with the local configuration of chemical species. Vacancies in ESOs exhibit deep thermodynamic transition levels that inhibit electrical conduction. By applying the charge-neutrality condition, we determine that the equilibrium concentrations of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chemistry and associated structural distortions by varying alloy composition acts an engineering principle that enables controlled defect formation in multi-component alloys.

Can the one-zone hadronuclear model explain the hard-TeV spectrum of BL Lac objects?

Context. The intrinsic TeV emission of some BL Lacs is characterized by a hard spectrum (the hard-TeV spectrum) after correcting for the extragalactic background light. The hard-TeV spectra pose a challenge to conventional one-zone models, including the leptonic model, the photohadronic model, the proton synchrotron model, and others. Aims. In this work, we aim to investigate whether or not the one-zone hadronuclear (pp) model can be used to interpret the hard-TeV spectra of BL Lacs without introducing extreme parameters. Methods. We provide analytical calculations that can be used to study whether or not there is a parameter space, and whether or not the charge neutrality condition of the jet can be satisfied when interpreting the hard-TeV spectra of BL Lacs without introducing a super-Eddington jet power. Results. We find that in a sample of hard-TeV BL Lacs previously collected, only the hard-TeV spectrum of 1ES 0229+200 can be explained by γ-rays from π0 decay produced in the pp interactions, but at the cost of setting a small radius of the radiation region than the Schwarzschild radius of the central black hole. Combining our findings with those of previous studies of other one-zone models, we suggest that the hard-TeV spectra of BL Lacs cannot be explained by a one-zone model without introducing extreme parameters, and should originate from the multiple radiation regions.

The critical role of synthesis conditions on small polaron carrier concentrations in hematite—A first-principles study

Achieving highly efficient energy conversion with transition metal oxides necessitates overcoming conductivity limitations due to the formation of small polarons. Detailed understanding of the interplay among intrinsic defects, dopants, and electron polarons can help devise strategies for achieving higher carrier concentrations, therefore improving carrier conductivity. This work employs first-principles calculations to reliably predict electron polaron concentrations in a prominent polaronic oxide, hematite (Fe2O3), by resolving interactions between charged defects and electron polarons and keeping charge neutrality condition among all charged species. This work addresses that both VO and Fei can be primary donors in undoped hematite depending on the synthesis conditions, such as synthesis temperature and oxygen partial pressure, despite the fact that VO owns an extremely high ionization energy compared to kBT. Furthermore, from calculations of a plethora of n-type dopants (group IV and V elements), we find that Ti, Ge, Sb, and Nb are able to raise electron polaron concentrations in hematite significantly without considering dopant clustering. However, the magnitude of electron polaron concentration increase would be smaller if the dopant has a high tendency of clustering, such as Ti. We reveal the critical role of synthesis conditions on tuning electron polaron concentrations of both undoped and doped hematite. Our theoretical analysis provides important insights and general design principles for engineering more conductive polaronic oxides.

Phase structures of neutral dense quark matter and applicationto strange stars * *Supported in part by the National Natural Science Foundation of China (11905107), the National Natural Science Foundation of Jiangsu Province of China (BK20190721), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (19KJB140016), Nanjing University of Posts and Telecommunications Science Foundation (NY129032), Innovation Program of Jiangsu

In the contact interaction model, the quark propagator has only one solution, namely, the chiral symmetry breaking solution, at vanishing temperature and density in the case of physical quark mass. We generalize the condensate feedback onto the coupling strength from the 2 flavor case to the 2+1 flavor case, and find the Wigner solution appears in some regions, which enables us to tackle chiral phase transition as two-phase coexistences. At finite chemical potential, we analyze the chiral phase transition in the conditions of electric charge neutrality and equilibrium. The four chemical potentials, , , and , are constrained by three conditions, so that one independent variable remains: we choose the average quark chemical potential as the free variable. All quark masses and number densities suffer discontinuities at the phase transition point. The strange quarks appear after the phase transition since the system needs more energy to produce a -quark than an -quark. Taking the EOS as an input, the TOV equations are solved numerically, and we show that the mass–radius relation is sensitive to the EOS. The maximum mass of strange quark stars is not susceptible to the parameter we introduced.

Effects of displacement current on wave dispersion relation and polarization properties in auroral plasmas

In-situ observations from the FREJA magnetospheric research satellite and the Fast Auroral SnapshoT satellite have shown that plasma waves are frequently observed in the auroral plasma, which are believed to be fundamentally important in wave energy dissipation and particle energization. However, the effects of a displacement current on these waves have not been examined. Based on the two-fluid theory, we investigate the dispersion relation and polarization properties of fast, Alfvén, and slow modes in the presence of a displacement current, and the effects of the displacement current on these waves are also considered. The results show that the wave frequency, polarization, magnetic helicity and other properties for the fast and Alfvén modes are highly sensitive to the normalized Alfvén velocity vA /c, plasma beta β, and propagation angle θ, while for the slow mode the dependence is minor. In particular, for both fast and Alfvén modes, the magnetic helicity is obviously different with and without the displacement current, especially for the Alfvén mode with the helicity reversals from right-handed to left-handed when vA /c increases from 0 to 0.3. The charge-neutral condition of both fast and Alfvén modes with frequencies larger than the proton cyclotron frequency is invalid in the presence of the displacement current. Moreover, the presence of the displacement current leads to relatively large magnetic compressibility for the Alfvén mode and relatively large electron compressibility for the fast mode. These results can be useful for a comprehensive understanding of the wave properties and the physics of particle energization phenomena in auroral plasmas.

Constant potential simulations on a mesh.

Molecular dynamics simulations in a constant potential ensemble are an increasingly important tool to investigate charging mechanisms in next-generation energy storage devices. We present a highly efficient approach to compute electrostatic interactions in simulations employing a constant potential method (CPM) by introducing a particle-particle particle-mesh solver specifically designed for treating long-range interactions in a CPM. Moreover, we present evidence that a dipole correction term-commonly used in simulations with a slab-like geometry-must be used with caution if it is also to be used within a CPM. It is demonstrated that artifacts arising from the usage of the dipole correction term can be circumvented by enforcing a charge neutrality condition in the evaluation of the electrode charges at a given external potential.