Large Negative Values Research Articles

Геометрический фактор в сезонных вариациях среднесуточных значений геомагнитного индекса Dₛₜ

The work uses data on the geomagnetic index Dst for the period 1966–2015. Under quiet conditions, the occurrence of seasonal variations of the daily average Dst index depends on geometric factors of the interaction between the solar wind and the magnetosphere; and under disturbed conditions, on the development of a partial ring current in the magnetosphere. At large negative values of the Dst index, there is no seasonal variation in it. The imperfection of the network of Dst stations is assumed to lead to the formation of annual variation in Dst. The formation of a semiannual variation is associated with the movement of the plasma sheet relative to the plane of the geomagnetic equator during the annual rotation of Earth around the Sun. Based on the data on semiannual variations in the number of days n(Dst), the critical daily average value of the geomagnetic index Dst is determined, starting from which we can speak of disturbed days: Dst≤–24 nT.

An All-Solid Dispersion-Compensating Photonic Crystal Fiber Based on Mode Coupling Mechanism in Dual-Concentric Core

An all-solid dispersion-compensating photonic crystal fiber based on mode coupling mechanism in dual-concentric core has been proposed. The mode coupling characteristics, dispersion, confinement loss of the fiber, and the influence on dispersion of some structure parameters are simulated by full-vector finite element method. By using the relationship between phase matching wavelengths and coupling strength with the change of fiber microstructure parameters, an all-solid dual-concentric-core dispersion-compensating photonic crystal fiber is presented. The structure parameters on dispersion characteristic are investigated. The results demonstrate that the proposed fiber has a large negative dispersion value 8465 ps/(nm·km) at 1550 nm. The effective mode area and the splicing loss to the standard single mode fiber are 12.8 μm2 and 1.89 dB at 1550 nm, respectively. At 1550 nm, the confinement loss is less than 1 × 10−3 dB/km and the bending loss with 2 cm bending diameter is less than 1 × 10−2 dB/km.

Improving the optical properties of [5] circulene with different electron donors and acceptor substitutions (push-pull system).

In this research, the circulene molecule was selected, and the electron donor and acceptor groups were attached to improve its optical properties. Large negative values of enthalpies, Gibbs free energies, and exothermic energies of formation for these molecules, especially for the CN-CIR-NHLi molecule, show that their formation is highly reasonable. It was illustrated that the Eg of the circulene molecule was lowered in the electron donor and acceptor groups' presence, while the CN-CIR-NHLi and CF3-CIR-NHLi molecules have the lowest values of Eg. It was observed that the optical properties of circulene molecules were improved in electron donor and acceptor groups' presence. Between these groups, the -CN and -CF3 as electron acceptors and -NHLi substituents, as electron donor groups, yield higher enhancements on the optical properties of circulene, which is in agreement with the results obtained for Eg values. Graphical abstract.

Investigations of Ternary Interdiffusion in β (BCC) Phase Field of Ti-Al-Mo Alloy System

Solid–solid diffusion couples were investigated at 1100 °C for determination of ternary interdiffusion coefficients at several compositions of the BCC phase field of Ti-Al-Mo system. Some of the diffusion couples developed zero flux planes (ZFP), which indicate the presence of strong diffusional interactions in this system. The composition of ZFP corresponds to the cross-over point of the diffusion path of the couple with the iso-activity line passing through one of the terminal alloys. Ternary interdiffusion coefficients were evaluated at several compositions at which the diffusion paths of the couples intersected each other. The strong diffusional interactions were also evident in large values of cross interdiffusion coefficients. Large negative values of $$ \tilde{D}_{\text{TiAl}}^{\text{Mo}} $$ indicate that the interdiffusion flux of titanium is enhanced up the gradient of aluminum and reduced down its gradient. An empirical relation is proposed between the ternary interdiffusivities and composition. The diffusion profiles were regenerated reasonably well using finite difference method based on empirically fitted composition dependent diffusion data. Availability of such composition–diffusivity relations in ternary systems would help in more accurate predictive design of the alloys and processes as it avoids the assumptions of negligible cross effects and need of complex extrapolations from lower order systems.

Mixed micelles of sodium perfluorooctanoate and imidazolium based ionic liquids in aqueous solution: A SANS and Tensiometric study

Micellization and microstructure of mixed micelles from mixtures of an anionic fluorocarbon surfactant sodium perfluorooctanoate (NaPFO) and cationic alkylmethyl-imidazolium based non-amphiphilic and amphiphilic ionic liquids (ILs) with different alkyl chain length (C4, C6, C8, C10 and C12) and anions viz. chloride (Cl−), tetrafluoroborate (BF4−), hexaflurophosphate (PF6−), octylsulphate (C8SO4−) and trifluoromethane-sulphonate (CF3SO3−) were examined extensively by surface tension and small-angle neutron scattering (SANS) measurements. Low critical micelle concentration (CMC) and large negative value of interaction parameter (β) showed synergism in all the mixed systems and it depends on the molecular characteristics of ILs. Thermodynamic parameters for mixed monolayers and mixed micelles were evaluated and discussed. Micellar growth, shape changes and micelle to vesicle transitions obtained from these oppositely charged mixed systems depend on the surface activity of ILs (alkyl chain and counter ions). Influence of ILs which differ in counter ions with similar chain length and similar counter ions with different chain lengths on aggregation patterns of NaPFO micelles have been examined using SANS. The data for size, shape and aggregation number for aggregates formed by these mixed systems evaluated from the fitted SANS data are reported and discussed in terms of their distinct interaction with NaPFO micelles. The present study is a significant addition to existing knowledge on ionic liquid induced morphological changes of sodium perfluorooctanoate micelles.

Single Copper Atoms Supported on ZnS as an Efficient Catalyst for Electrochemical Reduction of CO to CH3OH

AbstractThe electrochemical reduction of carbon dioxide (CO2) or carbon monoxide (CO) into fuels and value‐added products such as methane, methanol or ethanol are promising strategies to mitigate the carbon levels in the environment. Single‐atom catalysts (SAC) have attained much attention in the recent years due to their high activity, selectivity and maximum atom utilization. In this work, single copper atoms anchored on ZnS (Zinc Blende) and ZnO (Wurtzite) were investigated for CO reduction by using density functional theory calculations for the first time. Cu atom supported on ZnS exhibited high activity as well as selectivity towards the reduction of CO to methanol (CH3OH) with a limiting potential of only −0.19 V, while Cu/ZnO exhibited a very large negative value of the limiting potential for the reduction of CO to CH3OH. This work may prove helpful for the design of efficient single‐atom catalysts for the electrocatalytic reactions.

Negative area compressibility in silver oxalate

The presence of extremely large negative linear compressibilities (NLC) in crystalline silver oxalate was discovered in a recent work by using first principles solid-state calculations. Although the minimum value of the NLC was found for a negative applied hydrostatic pressure, in this work the presence of NLC in this material for positive applied pressures is verified experimentally by means of high-pressure X-ray diffraction experiments performed at room temperature in the Beijing synchrotron radiation facility. The results of this study demonstrate with certainty that the compressibility of silver oxalate along [010] crystallographic direction is negative for applied pressures in the range from 0.0 to 0.85 GPa. Since the measured compressibility decreases largely as the pressure decreases, large negative values of the compressibility are expected for negative applied pressures in agreement with the results found using first principles methods. Furthermore, the analysis of the variation of the lattice parameters of the crystal structure of silver oxalate under pressure in the principal axes reference system revealed that this material also exhibits the largest negative area compressibility phenomenon found so far at zero pressure, the values of the compressibilities along two of the principal axes being − 16.7 and − 20.0 $${\mathrm{TPa}}^{-1}$$ . The negative compressibility phenomenon in silver oxalate can be rationalized in terms of a “chains of rotating parallelograms” structural model.

Boson stars in f(T) extended theory of gravity

Spherically symmetric configurations of the non-interacting massive complex scalar field, representing non-rotating boson stars, are considered within the framework of the modified torsion based $f(T)$ gravity, with $f(T) = T + \alpha \, T^2/2$. We find that with sufficiently large negative value of $\alpha$ the mass of the boson stars can be made arbitrarily large. This is in contrast to General Relativity where an upper bound, $M_{max} \sim M_{Planck}^2/m$, to the mass of the boson stars built from the non-interacting scalar field exists and where the masses of boson stars in the astrophysical regime can be obtained only with the introduction of the scalar field self-interaction. With sufficiently large negative $\alpha$ we also find negative gravitational binding energy for all masses, which can be seen as an indication of the stability of such configurations. In its positive regime, $\alpha$ can not be made arbitrarily large as a phase transition in the stress--energy components of the $f(T)$-fluid develops. This phenomenon has already been reported to occur in polytropic stars constructed within the $f(T)$ gravity theory.

Observation of “Outlaw” Dual Aromaticity in Unexpectedly Stable Open-Shell Metal Clusters Caused by Near-Degenerate Molecular Orbital Coupling

The Huckel’s rule, Baird’s rule, and electronic shell closure model are classical and well-established concepts in chemistry, which have long been employed in rationalizing the aromaticity/antiarom...

Effect of pressure gradients on the different stages of roughness induced boundary layer transition

The physical mechanisms of roughness-induced transition (RIT) in pressure gradient boundary layers are studied using direct numerical simulations. Recent investigations have examined RIT processes in zero-pressure-gradient boundary layers (Suryanarayanan et al., 2019). The present study uses a vorticity dynamics point of view to examine how these processes are altered by a locally accelerating or decelerating flow that strains the vorticity field and creates a net vorticity flux at the wall. Flow acceleration is imposed on specific streamwise extents of the flow. This provides an understanding about how the fundamental mechanisms in different stages of RIT are affected by pressure gradients. The present results suggest that both lift-up and subsequent amplification of the unsteady perturbations are mitigated by flow acceleration. The effect on lift-up is explained by the compression (i.e. large negative value of the stretching term) of the wall-normal vorticity by negative dv/dy. Consistent with earlier experimental observations on spots and wedges, favorable pressure gradients reduce turbulent wedge spreading and nearly arrest the spreading when sufficiently strong. This result is also explained in terms of vorticity dynamics.

Zinc based chalcogenides ZnMn2X4 (X = S, Se, Te) as promising spintronic and sustainable energy materials: Ab-initio DFT investigations

To probe the applications in spintronics and sustainable energy devices of ZnMn2X4 (X=S, Se, Te) spinels, their structural, elastic, electronic and thermoelectric properties have been studied in the present work. Structural properties reveal stability of ferromagnetic (FM) phase in these spinels which is further confirmed by computing the heat of formation for these compounds. The mechanical properties of the ZnMn2X4 spinels have been investigated by computing their elastic constants, which show that all these spinels exhibit a brittle character. Moreover, electronic band structure and density of states reveal a half-metallic FM structure in all these spinels. Analysis of the band structure of these materials reveals a semiconducting nature in the spin down channel. Exchange constants (N0α and N0β) have been computed to directly probe the state splitting and our results large splitting of Mn 3d-states by N0β owing to its larger negative value as compared to N0α. The strong p-d hybridization is found to make exchange field dominant to crystal field which consequently induces ferromagnetism. In addition, the p-d hybridization reduces the magnetic moment of Mn ions by inducing traces of magnetic moments at non-magnetic sites. The observation of thermoelectric behavior within temperature range 200K–800K reveals positive Seeback coefficient indicating holes as majority charge carriers. Although the power factor is found to increase on going from ZnMn2S4 to ZnMn2Te4, our results clearly show improvement of power factor with increasing temperature.

Recent weakening of the southern stratospheric polar vortex and its impact on the surface climate over Antarctica

The variability in the southern stratospheric polar vortex (SSPV) and its downward coupling with the troposphere are known to play a crucial role in driving climate variability over Antarctica. In this study, SSPV weakening events and their impacts on the surface climate of Antarctica are examined using in-situ observation and reanalysis data. Combining criteria from several previous studies, we introduce a new detection method for SSPV weakening events. Based on the new criteria, the occurrence frequency of SSPV weakening events has exhibited a systematic increasing trend since the 2000 s. However, the weakened anomalies of individual SSPV events are not statistically different (95% confidence level) between the earlier (1979–1999) and later (2000–2017) periods examined in this study. The recent increase in the occurrence of SSPV weakening events is largely controlled by tropospheric mechanisms, i.e. the poleward heat flux carried by southern hemispheric planetary waves and associated vertical wave propagation. Among the various scales of planetary waves, the wavenumber 1 contributes most of the poleward eddy heat flux. We show that SSPV weakening events induce statistically significant cooling over the Antarctic Peninsula (AP) region and warming over the rest of Antarctica. Typically, surface air temperature anomalies with large negative values smaller than − 0.6 °C and positive values larger than + 0.8 °C are observed over the east coast of the tip of the AP and King Edward VII Land, respectively. The influence of an SSPV weakening event on the surface lasts for approximately three months with higher height anomalies off western Antarctica, providing favorable conditions for the atmosphere to transport cold air from the interior of Antarctica to the AP via the Weddell Sea. Distinct positive surface air temperature anomalies over the rest of Antarctica are associated with the northerly circulation anomaly from the eastern Weddell Sea to east Antarctica.

Seismic Velocity Changes Caused by Water Table Fluctuation in the New Madrid Seismic Zone and Mississippi Embayment

AbstractWe use cross correlation of the ambient seismic field to estimate seasonal variations of seismic velocity in the Mississippi Embayment and to determine the underlying physical mechanisms. Our main observation is that the δt/t variations correlate primarily with the water table fluctuation, with the largest positive value from May to July and the largest negative value in September/October relative to the annual mean. The δv/v residuals after water level fluctuation correction correlate with air pressure in the short term and follow the trend of temperature in the long term. The δv/v residuals after water level fluctuation and air pressure correction correlate inversely with the wind speed. The correlation coefficients between water table fluctuation and δt/t are independent of the interstation distance and frequency, but high coefficients are observed more often between 0.3 and 1 Hz than between 1 and 2 Hz because high‐frequency coherent signals attenuate faster than low‐frequency ones. The δt/t variations lag behind the water table fluctuation by about 20 days, which suggests that the velocity changes can be attributed to the pore pressure diffusion effect. The seasonal variations of δt/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the δt/t variation. At close distances, the maximum δt/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed can be explained by a poroelastic model.

Vacuum fluctuation, microcyclic universes, and the cosmological constant problem

We point out that the standard formulation of the cosmological constant problem itself is problematic since it is trying to apply the very large scale homogeneous cosmological model to very small (Planck) scale phenomenon. At small scales, both the spacetime and the vacuum stress energy are highly inhomogeneous and wildly fluctuating. This is a version of Wheeler's "spacetime foam". We show that this "foamy" structure would produce a large positive contribution to the average macroscopic spatial curvature of the Universe. In order to cancel this contribution to match the observation, the usually defined effective cosmological constant $\lambda_{\mathrm{eff}}=\lambda_B+8\pi G\langle\rho\rangle$ has to take a large negative value. The spacetime dynamics sourced by this large negative $\lambda_{\mathrm{eff}}$ would be similar to the cyclic model of the universe in the sense that at small scales every point in space is a "micro-cyclic universe" which is following an eternal series of oscillations between expansions and contractions. Moreover, if the bare cosmological constant $\lambda_B$ is dominant, the size of each "micro-universe" would increase a tiny bit at a slowly accelerating rate during each micro-cycle of the oscillation due to the weak parametric resonance effect produced by the fluctuations of the quantum vacuum stress energy tensor. In this way, the large cosmological constant generated at small scales is hidden at observable scale and no fine-tuning of $\lambda_B$ to the accuracy of $10^{-122}$ is needed. This at least resolves the old cosmological constant problem and suggests that it is the quantum vacuum fluctuations serve as the dark energy which is accelerating the expansion of our Universe.

STRENGTHENING OF THE SURFACE LAYER OF THE WELD PARTS WHEN TURNING BY CUTTERS WITH PSTM “KIBORIT”

The results of an experimental study of hardening of the surface layer of parts deposited by electric arc welding with Np-30KhGSA wire under a flux layer of AN-348A are presented, when machined with cutters equipped with replaceable non-milling plates RNMN 070300 made of polycrystalline superhard material (PCBN) based on cubic boron nitride «ciborite». The influence of mining regimes on the surface microhardness and the thickness of the hardened surface layer of parts is studied. The nature of the lines of equal microhardness indicates a less significant effect of the cutting speed compared to the depth of cut and feed and a close degree of influence of changes in the latter. Processing speeds have less effect on the thickness of the hardened layer compared to the depth of cut and the amount of feed, although the latter has a less intense effect. The greatest hardening of the deposited parts was achieved by turning with a cutter with rake angles of (40–50)°. Treatment with chisels with PCBN «ciborite» with large negative values of the front angle γ is accompanied by the creation in the processing area of such a level of thermobaric load, which leads to structural changes in the surface layer of parts, namely, a layer of weakly etchable hardening structure – unstructured martensite H50 = 5700–5920 MPa, which is 1.8–2.1 times higher than the microhardness of the main weld material. The thickness of the reinforced deposited layer during blade-hardening processing with a tool with PCBN «ciborite» is 60–80 μm, due to the low carbon content in the material, the presence of ascending diffusion of carbon and intense plastic deformation in the contact zone. The results show that blade-hardening machining with tools with PCBN is an effective method of controlling the performance of machine parts, such as their wear resistance.

Chaotic synchronization of memristive neurons: Lyapunov function versus Hamilton function

In this paper, we consider a 5-dimensional Hindmarsh–Rose neuron model. This improved version of the original model shows rich dynamical behaviors, including a chaotic super-bursting regime. This regime promises a greater information encoding capacity than the standard bursting activity. Based on the Krasovskii–Lyapunov stability theory, the sufficient conditions (on the synaptic strengths and magnetic gain parameters) for stable chaotic synchronization of the model are obtained. Based on Helmholtz’s theorem, the Hamilton function of the corresponding error dynamical system is also obtained. It is shown that the time variation of this Hamilton function along trajectories can play the role of the time variation of the Lyapunov function—in determining the stability of the synchronization manifold. Numerical computations indicate that as the synaptic strengths and the magnetic gain parameters change, the time variation of the Hamilton function is always nonzero (i.e., a relatively large positive or negative value) only when the time variation of the Lyapunov function is positive, and zero (or vanishingly small) only when the time variation of the Lyapunov function is also zero. This, therefore, paves an alternative way to determine the stability of synchronization manifolds and can be particularly useful for systems whose Lyapunov function is difficult to construct, but whose Hamilton function corresponding to the dynamic error system is easier to calculate.

Strong phonon interaction between two distant magnetic films inserted into a cavity

Phonon and magnon excitations can be modulated by magnetoelastic and magneto-optical coupling effects, respectively. The photons not only mediate the interaction between two distant magnetic films inserted into a cavity, but also affect the phonon excitations in each film by the magnetoelastic coupling. In this work, an indirect interaction mechanism is presented for the phonons in two distant magnetic films inserted into a cavity. Although the phonons in no way pass through a very long distance from one film to the other in a vacuum environment, they can establish a correlation between two films through the magneto-optical and magnetoelastic couplings in each film. This really corresponds to an indirect interaction. In this work, the relationship between the phonon interaction and the laser intensity is studied using a quantum theory. It is found that when the laser frequency is equal to magnon frequency and the laser intensity is in a certain range, this phonon interaction energy takes very large negative values so as to satisfy the application requirement. According to a generating functional integral approach, a formula for the phonon interaction energy is derived, and an experimental approach to detect this effect is suggested. This indirect interaction effect of phonons not only adds to our understanding of the essence of matter interaction, but also reveals some potential applications to the heat management of electronic devices, long-range transfer of heat energy, and ultrasonic signal transmission in a vacuum environment.

Multiple element mapping and in-situ S isotopes of Au-carrying pyrite of Shuiyindong gold deposit, southwestern China using NanoSIMS: Constraints on Au sources, ore fluids, and mineralization processes

This study employed high-sensitivity and high-resolution secondary ionizing mass spectrometry (NanoSIMS) to determine the distribution of Au, As, Cu, Se, and S isotopes of pyrite from the Shuiyindong Au deposit, southwestern China. NanoSIMS maps show that Au occurs as solid solution, and heterogeneously distributes at micron scale in the ore pyrite, which is the overgrowth of the preexisting synsedimentary or hydrothermal pyrite core, commonly rimmed again by the As- and Au-poor outermost zone. There is no detectable Au in the synsedimentary pyrite. Invisible Au was observed to fill in the submicron-scale cracks of the ore pyrite. Meanwhile, element line profile analysis shows that Au occurs in three discrete climaxes in the ore pyrite. In-situ S isotopic analyses identified two types of S sources. The synsedimentary pyrites have large negative δ34S values ranging from −38.3‰ to −18.8‰, which is a typical biogenetic S generated by bacterial reduction of sea water sulfate. The hydrothermal pyrites with zoned texture have a narrow δ34S range of −3.5‰～−7.4‰ with a median of 2.3‰, similar to the recently published sulfur isotope data of the Southwest China’s Carlin-type gold deposits except for the Lannigou Mine. Combining the distribution of Au and S isotopes in pyrite indicates that the Au-carrying zones have lower δ34S values than that of the ore-unrelated zones. These NanoSIMS observations indicate that the Au mineralization was closely associated with strong deformation of the host rock. The Au forming the deposit was exotic, instead of the reactivated Au in the preexisting pyrite formed in the synsedimentary basin. The mineralization progress was multistage. The distribution of Au and S isotopes is hard to interpret by the viewpoint of metamorphic fluid or fluid pressure fluctuation. Instead, it is more consistent with magmatic fluid, likely contaminated by sedimentary sulfur during its ascending to the surface. In addition, the multistage ore formation pattern discovered in this study poses new challenges to determine the mineralization age of the Au deposits in southwestern China.

Iterative coordinates

Barycentric coordinates provide a simple way of expressing the linear interpolant to data given at the vertices of a triangle and have numerous applications in computer graphics and other fields. The generalization of barycentric coordinates to polygons with more than three vertices is not unique and many constructions have been proposed. Among them, mean value coordinates stand out by having a simple closed form and being well-defined for arbitrary polygons, but they may take on large negative values in the case of concave polygons, leading to artefacts in applications like shape deformation. We present a modification of mean value coordinates that is based on the observation that the mean value coordinates of some point v inside a polygon can be negative if the central projection of the polygon onto the unit circle around v folds over. By iteratively smoothing the projected polygon and carrying over this smoothing procedure to the barycentric coordinates of v, these fold-overs as well as the negative coordinate values and shape deformation artefacts gradually disappear, and they are guaranteed to completely vanish after a finite number of iterations.

Determination of thermal expansion coefficient of a monofilament polyamide fiber using digital image correlation

Coiled polymer actuators are a type of artificial muscles that are a promising development in the field of smart materials. The coefficient of thermal expansion of monofilament polyamide fibers is a crucial parameter for understanding the actuation of coiled fibers. The main purpose of this work is to develop a new methodology for estimating the coefficient of thermal expansion and the transition temperature of monofilament polymer fibers. In the experimental procedure, axial deformations of monofilament polyamide fiber samples were induced by temperature variations using a controlled thermal system. These deformations were determined from images of polyamide samples using the digital image correlation method. Two different approaches based on distinct temperature conditions were conducted. An alternative model with three parameters, including the coefficient of thermal expansion, was introduced to describe the thermal-mechanical behavior of monofilament polyamide fibers. Moreover, polyamide samples were also characterized using four conventional methodologies. Results indicated that the coefficient of thermal expansion changed of a modest negative value to a large negative value and this transition occurred around the glass transition temperature of the polyamide. The thermal expansion curves demonstrate good repeatability and all estimated parameters were in accordance with literature, indicating that the proposed approach can be suitable for the proposed study. This investigation may help in understanding of the intrinsic thermal-mechanical behavior of polymeric monofilaments employed as actuators.