Signs Of Constants Research Articles

Super-exchange theory for polyvalent anion magnets

The Goodenough–Kanamori–Anderson (GKA) rules have been widely applied for explaining the magnetic properties induced by super-exchange interaction. As conclusions of the super-exchange theory, they reveal the antiferromagnetic (ferromagnetic) ordering along with bond angle of 180° (90°) in the cation–anion–cation interaction path, in which the theory sets a pre-condition that the electronic states of cations in all paths are identical. We observed that the GKA rules are in fact not universal and even invalid to materials containing anions with different valence states, for example, the layered CrOCl crystal (with two valence states of anions: O2− and Cl−). In this study, we propose an extended super-exchange theory (ESET) related to superposed electronic states of cation in a specific path. ESET is capable of predicting not only the sign and relative magnitude of magnetic exchange constants in different cation–anion–cation paths, but also the magnetic ground state. Through our proposed theory, we conclude that the magnetic ordering along with bond angle of 90° in Cr–Cl–Cr path is moderately antiferromagnetic and of 180° in Cr–O–Cr path is strongly ferromagnetic, which are opposite to the contents of GKA rules. Moreover, we clarify that monolayer CrOCl has antiferromagnetic ordering rather than ferromagnetic as reported recently. The reliability of ESET is verified via first-principles calculation and previous experimental report as well, and its universality is also demonstrated. Thus, our theory is powerful to predict the magnetic properties, which makes it possible to design new high Curie temperature two-dimensional semiconducting ferromagnets with polyvalent anion materials.

Gravitational decoupling in 2 + 1 dimensional space-times with cosmological term

In this work we implement the minimal geometric deformation method to obtain the isotropic sector and the decoupler matter content of any anisotropic solution of the Einstein field equations with cosmological constant in 2 + 1 dimensional space-times. We obtain that the solutions of both sectors can be expressed analytically in terms of the metric functions of the original anisotropic solutions instead of formal integral as in its 3 + 1 counterpart. As a particular example we study a regular black hole solution and we show that, depending on the sign of the cosmological constant, the solutions correspond to regular black holes violating the null energy condition or to a non-regular black hole without exotic hair. The exotic/non-exotic and the regular/non-regular black hole like-dualities are discussed.

The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules

ABSTRACTMonte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species.

The effects of surface elasticity on the thermal stress around a circular nano-hole in a thermoelectric material

We analyze the contribution of surface elasticity and electric current density on the thermal stress distribution around a circular nano-hole in a thermoelectric material. Using complex variable methods, we obtain closed-form solutions describing the corresponding thermoelastic fields in the vicinity of the nano-hole. Our results indicate that the effect of surface elasticity is to generate significant normal and shear stresses on the boundary of the hole, allowing for the ability to either suppress or enhance hoop stress depending on the sign of the corresponding surface material constant. In addition, we find that positive hoop stress generated on the boundary by the remote electric current density can be neutralized by the incorporation of positive surface elasticity. In the case of the remaining boundary stress components, both surface elasticity and electric current density are found to enhance normal stress, while the maximum shear stress depends largely on the contribution of surface elasticity.

Strain sensing by exploiting the Matteucci effect in amorphous wire

Previous investigations of the Matteucci effect in amorphous microwires have yielded a basic understanding of the underlying physical mechanism. However, a complete quantitative model is still elusive particularly in explaining the role of magnetoelastic interactions. This is important because the Matteucci effect is an under-utilized phenomenon with promising potential for exploitation in stress sensing applications. In this work, we have studied the influence of tensile stress on the Matteucci effect in two types of amorphous wires (Fe77.5Si7.5B15 and Co68.15Fe4.35Si12.5B15). This paper shows how the axial stress significantly affects the Matteucci voltage and that this behaviour is dependent upon the sign of the magnetostriction constant. Measuring the Matteucci voltage as a function of stress in magnetostrictive wire produced an equivalent gauge factor over two orders of magnitude greater than that obtained in a conventional resistive strain gauge. This offers the potential for the development of a new type of wire-based strain sensor.

What if Newton’s Gravitational Constant Was Negative?

In this work, we seek a cosmological mechanism that may define the sign of the effective gravitational coupling constant, G. To this end, we consider general scalar-tensor gravity theories as they provide the field theory natural framework for the variation of the gravitational coupling. We find that models with a quadratic potential naturally stabilize the value of G into the positive branch of the evolution and further, that de Sitter inflation and a relaxation to General Relativity is easily attained.

Effect of Hydrogenation on Magnetostriction and Magnetocaloric Effect in Gadolinium Single Crystal

The gadolinium single crystal obtained by the Czochralski method was hydrogenated to the composition GdH0.15, which corresponds to a metal–hydrogen solid solution (α phase). The magnetostriction and magnetocaloric effect were measured for both the initial and hydrogenated samples. It is found that the hydrogen atoms in the hexagonal lattice of gadolinium can affect the magnitude and sign of the magnetostriction constants and cause the anisotropy of the magnetocaloric effect. The main mechanisms responsible for the observed effects are discussed.

Nuclear matter and neutron star properties with the extended Nambu-Jona-Lasinio model * * Supported by Hunan Provincial Natural Science Foundation of China (2015JJ2005), the Science Research Foundation of Education Department of Hunan Province (15C0029), the National Natural Science Foundation of China (11103001), and the construct program of the key discipline in Hunan province

An extended Nambu-Jona-Lasinio (eNJL) model with nucleons as the degrees of freedom is used to investigate properties of nuclear matter and neutron stars (NSs), including the binding energy and symmetry energy of the nuclear matter, the core-crust transition density, and mass-radius relation of NSs. The fourth-order symmetry energy at saturation density is also investigated. When the bulk properties of nuclear matter at saturation density are used to determine the model parameters, the double solutions of parameters are obtained for a given nuclear incompressibility. It is shown that the isovector-vector interaction has a significant influence on the nuclear matter and NS properties, and the sign of isovector-vector coupling constant is critical in the determination of the trend of the symmetry energy and equation of state. The effects of the other model parameters and symmetry energy slope at saturation density are discussed.

Magnetic Cationic Copper(II) Chains and a Mononuclear Cobalt(II) Complex Containing [Ln(hfac)4]- Blocks as Counterions.

Four new heterospin compounds with molecular formula {[Cu2(hfac)3(TlTrzNIT)2][Ln(hfac)4]} n·C7H16 (LnIII = Gd (1), Tb (2), or Dy (3)) and [Co(hfac)(TlTrzNIT)2][Dy(hfac)4] (4), where hfac is hexafluoroacetylacetonato and TlTrzNIT is the nitronylnitroxide radical 1-( m-tolyl)-1 H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), were obtained. All structures were determined by single-crystal X-ray diffraction. In compounds 1-3, the TlTrzNIT radical is bridge-coordinated to copper(II) ions, leading to positively charged copper(II)-radical chains containing [Ln(hfac)4]- as counterions. In compound 4, the cobalt(II) ion is coordinated to two TlTrzNIT radicals and one hfac ligand in bidentate mode leading to a mononuclear cationic complex that contains [Dy(hfac)4]- as counterion. Magnetic measurements of all complexes were performed. Magnetic data were fit considering the contributions of the copper(II)-radical chain and a paramagnetic gadolinium(III) ion for 1. The sign and magnitude of the magnetic coupling constants extracted from the fit were confirmed by density functional theory calculations. The obtained spin topology shows an alternated ferro-antiferromagnetic chain. Field-induced single molecule magnet behavior was observed for the Dy derivatives 3 and 4, in agreement with CASSCF calculations performed for the latter system.

Влияние гидрирования на магнитострикцию и магнитокалорический эффект в монокристалле гадолиния

AbstractThe gadolinium single crystal obtained by the Czochralski method was hydrogenated to the composition GdH_0.15, which corresponds to a metal–hydrogen solid solution (α phase). The magnetostriction and magnetocaloric effect were measured for both the initial and hydrogenated samples. It is found that the hydrogen atoms in the hexagonal lattice of gadolinium can affect the magnitude and sign of the magnetostriction constants and cause the anisotropy of the magnetocaloric effect. The main mechanisms responsible for the observed effects are discussed.

Features of magnetoresistance and magnetic properties in Bi95.69Mn3.69Fe0.62

A study of textured, polycrystalline Bi95.69Mn3.69Fe0.62 containing two phases: the bismuth matrix and inclusions of the α-BiMn phase. It is shown that the anomalous behavior of the magnetization temperature dependences can be linked to the reorientation transition of the Mn magnetic moments in the α-BiMn phase, caused by the change in the sign of the magnetic anisotropy constant. It is established that the temperature dependences of the electrical resistivity in a magnetic field are non-monotonic, and that the magnetoresistance is positive in all temperature ranges, reaching a maximum value of 3033% for a magnetic field of 140 kOe, when the magnetic field is perpendicular to the transport current. A strong anisotropy of electrical resistivity is measured in a magnetic field. It is assumed that the anomalous behavior of the electrical resistivity as a function of temperature in Bi95.69Mn3.69Fe0.62, as compared to pure bismuth, is linked to a change in the overlap between the electron and the hole Fermi surfaces under the influence of α-BiMn phase magnetism, as well as the magnitude and direction of the external magnetic field.

Regular solutions in f(T) -Yang-Mills theory

We consider extended covariant teleparallel $(f(T))$ gravity whose action is analytic in the torsion scalar and which is sourced by an $su(2)$ valued Yang-Mills field. Specifically, we search for regular solutions to the coupled $f(T)$ Yang-Mills system. For $f(T)=T$ we, not surprisingly, recover the Bartnik-McKinnon solitons of Einstein Yang-Mills theory. However, interesting effects are discovered with the addition of terms in the action which are nonlinear in the torsion scalar, which we specifically study up to cubic order. With the addition of the nonlinear terms the number of regular solutions becomes finite. As well, beyond critical values of the coupling constants we find that there exist \emph{no} regular solutions. These behaviors are asymmetric with respect to the sign of the nonlinear coupling constants and the elimination of regular solutions turns out to be extremely sensitive to the presence of the cubic coupling. It may be possible, therefore, that with sufficiently high powers of torsion in the action, there may be no regular Yang-Mills static solutions.

Analysis of time-resolved EPR spectra observed in the photolysis of disulphonated anthraquinones included in cyclodextrins – evidence of the S-T_ mixing of radical pair mechanism

ABSTRACTCyclodextrins (CDs) are well known hosts of inclusion compounds, and they can be used for molecular identification and separation. Several studies have already been reported from the point of view of photchemistry and spin-chemistry. These reports showed that the inside environment of CDs such as their bore-size works for their specific properties. In this report, the time-resolved EPR spectra observed in the photolysis of aqueous solution of disulphonated anthraquinones included in β-CD and γ-CD are carefully analysed. The spectral simulation employing Radical Pair Mechanism (RPM) of Chemically Induced Dynamic Electron Polarisation is carried out. Finally, it is concluded that the peculiar hyperfine-dependence observed in these particular systems is exclusively caused by the mixing RPM, despite the radical pairs (RPs) having small hyperfine coupling (HFC) constants (less than 3.4 mT). Another finding is the quenching of the spin polarisation of the RP conveyed from the excited triplet state of anthraquinones before mixing. This is explained as arising from slow relative diffusional motion of the RP members when included in the interior of the CDs. General expression of the and S-T+ mixing of multiple nuclei (I = 1/2) with different conditions such as the spin multiplicity of the precursor molecule and the sign of the HFC constants, is also suggested.

Spinor Field Nonlinearity and Space-Time Geometry

Within the scope of Bianchi type VI,VI0,V, III, I, LRSBI and FRW cosmological models we have studied the role of nonlinear spinor field on the evolution of the Universe and the spinor field itself. It was found that due to the presence of non-trivial non-diagonal components of the energy-momentum tensor of the spinor field in the anisotropic space-time, there occur some severe restrictions both on the metric functions and on the components of the spinor field. In this report we have considered a polynomial nonlinearity which is a function of invariants constructed from the bilinear spinor forms. It is found that in case of a Bianchi type-VI space-time, depending of the sign of self-coupling constants, the model allows either late time acceleration or oscillatory mode of evolution. In case of a Bianchi VI0 type space-time due to the specific behavior of the spinor field we have two different scenarios. In one case the invariants constructed from bilinear spinor forms become trivial, thus giving rise to a massless and linear spinor field Lagrangian. This case is equivalent to the vacuum solution of the Bianchi VI0 type space-time. The second case allows non-vanishing massive and nonlinear terms and depending on the sign of coupling constants gives rise to accelerating mode of expansion or the one that after obtaining some maximum value contracts and ends in big crunch, consequently generating space-time singularity. In case of a Bianchi type-V model there occur two possibilities. In one case we found that the metric functions are similar to each other. In this case the Universe expands with acceleration if the self-coupling constant is taken to be a positive one, whereas a negative coupling constant gives rise to a cyclic or periodic solution. In the second case the spinor mass and the spinor field nonlinearity vanish and the Universe expands linearly in time. In case of a Bianchi type-III model the space-time remains locally rotationally symmetric all the time, though the isotropy of space-time can be attained for a large proportionality constant. As far as evolution is concerned, depending on the sign of coupling constant the model allows both accelerated and oscillatory mode of expansion. A negative coupling constant leads to an oscillatory mode of expansion, whereas a positive coupling constant generates expanding Universe with late time acceleration. Both deceleration parameter and EoS parameter in this case vary with time and are in agreement with modern concept of space-time evolution. In case of a Bianchi type-I space-time the non-diagonal components lead to three different possibilities. In case of a full BI space-time we find that the spinor field nonlinearity and the massive term vanish, hence the spinor field Lagrangian becomes massless and linear. In two other cases the space-time evolves into either LRSBI or FRW Universe. If we consider a locally rotationally symmetric BI(LRSBI) model, neither the mass term nor the spinor field nonlinearity vanishes. In this case depending on the sign of coupling constant we have either late time accelerated mode of expansion or oscillatory mode of evolution. In this case for an expanding Universe we have asymptotical isotropization. Finally, in case of a FRW model neither the mass term nor the spinor field nonlinearity vanishes. Like in LRSBI case we have either late time acceleration or cyclic mode of evolution. These findings allow us to conclude that the spinor field is very sensitive to the gravitational one.

Nonlinear Spinor Field in Non-Diagonal Bianchi Type Space-Time

Within the scope of the non-diagonal Bianchi cosmological models we have studied the role of the spinor field in the evolution of the Universe. In the non-diagonal Bianchi models the spinor field distribution along the main axis is anisotropic and does not vanish in the absence of the spinor field nonlinearity. Hence within these models perfect fluid, dark energy etc. cannot be simulated by the spinor field nonlinearity. The equation for volume scale V in the case of non-diagonal Bianchi models contains a term with first derivative of V explicitly and does not allow exact solution by quadratures. Like the diagonal models the non-diagonal Bianchi space-time becomes locally rotationally symmetric even in the presence of a spinor field. It was found that depending on the sign of the coupling constant the model allows either an open Universe that rapidly grows up or a close Universe that ends in a Big Crunch singularity.

Theoretical Investigations of the Spin Hamiltonian Parameters for the Mononuclear Square Pyramidal [CuO5] Groups in Two Paddle Wheel Copper Complexes

The spin Hamiltonian parameters (SHPs) (g factors and hyperfine structure constants) for the mononuclear square pyramidal [CuO5] groups in two paddle wheel copper complexes {Cu2(μ2–O2CCH3)4}(OCNH2CH3) and \({}_{\infty }^{3} [{\text{Cu}}_{ 2}^{\text{I}} {\text{Cu}}_{ 2}^{\text{II}} ( {\text{H}}_{ 2} {\text{O)}}_{ 2} {\text{L}}_{ 2} {\text{Cl}}_{ 2} ]\) are theoretically investigated from the perturbation calculations of these parameters for a rhombically elongated octahedral 3d 9 group. The slightly larger anisotropy Δg (≈ g // − g⊥) of complex 1 than complex 2 is attributed to the slightly bigger deviations of the polar angles related to the ideal value 90° and relative differences between the axial and basal Cu–O distances in the former. The axiality of the EPR signals for both systems can be illustrated as the fact that the perpendicular anisotropic contributions to X and Y components of the SHPs arising from the four basal ligands with slightly distinct bond lengths and bond angles may roughly cancel one another. The signs of hyperfine structure constants are also theoretically determined for both complexes.

Theoretical Studies of the Electron Paramagnetic Resonance Parameters and Local Structure for VO2+ in Oxyfluoroborate Glasses

The electron paramagnetic resonance parameters (g factors g‖, g⊥ and hyperfine structure constants A‖, A⊥) of a tetragonal V4+ center in oxyfluoroborate glasses (20Li2O-10Li2F2-70B2O3) are theoretically investigated by using the perturbation formulas for a 3d1 ion in tetragonally compressed octahedra. The calculated results are in good agreement with the experimental data. Local structure parameters of [VO6]8- clusters are obtained from the calculation (i.e., R‖ ≈ 1.74 Å and R⊥ ≈ 1.985 Å for the metal-ligand distances parallel and perpendicular to the C4 axis, respectively). It is shown that the local structure around the V4+ ion possesses a compressed tetragonal distortion along C4 axis. The signs of the hyperfine structure constants A‖ and A⊥ for V4+ centers in oxyfluoroborate glasses were also suggested in the discussion.

Termohram quality stabilisation by the way of thermal defocus compensation

This article considers the problem of the thermograph lens defocusing under the influence of ambient temperature. The gradient of the external ambient temperature leads to a change of the design parameters of the optical system and, consequently, to thermoresponsive and the emergence of thermobarics image, which cause a sharp deterioration in the frequency and energy characteristics of the lens of the thermograph. One method of solving this problem is to compensate the influence of temperature fields on the image quality of the focusing assemblies operating in the infrared spectral range for the case of uniform heat distribution in the system.Elimination based on the characteristics of the lenses from the temperature is advantageously carried out at the design stage of the optical system by using methods of passive optical termales, which is based on the application schemes termaltake infrared (IR) lens components, made of different optical materials, transparent in the infrared region of the spectrum and have different signs of thermo-optic constants.<p >Based on the proposed method of filling thermograph thermoresponsive lens and further optimization was designed ternary diagram of temperature-compensated infrared lens for use with thermographs modern matrix receivers emitters with a size of 340×240 pixels with a size of one pixel 25x25 mkm.Thanks to this method, and the calculations in the mathematical software Mathcad have been received a number of combinations termaltake triplets infrared spectral range of 8-12 microns. At the same time it was managed to achieve thermal stabilization and system achromatization. However, this approach is only oncovin because was made from absolutely fine components and without taking into account the nonlinearity of the dependence of materials optical parameters from temperature. Therefore, for the design of termaltake working system with good performance requires further optimization of the triplet. It was implemented in the environment of computer-aided design Zemax, which allows to conduct computer modeling with the following factors: nonlinear dependence of characteristics of optical materials from the wavelength of the radiation and temperature; the analysis of passing through focusing system a large number of rays involved in image formation; obtaining a graphical representation of image quality of the optical system.The proposed lens have the following characteristics: relative aperture of 1:1 angular field of view 2ω=20°, focal length f'=38,2 mm. High image quality of the lens is confirmed by the level of modulation transfer function of 50% at a spatial frequency of 20 mm-1 for the edge of the system view field and the level of the function of energy concentration of 66% in the spot size dispersion of 25 mkm for the edge of the system view field.Back focal length of this triplet at variations of temperature in the range from 0 °C to +50 °C changes only by a fraction of a micrometer, which in turn gives the opportunity to obtain thermogram with high resolution. The stability characteristics of the lens make it possible to more accurately determine the temperature gradient adjacent portions of the object surface and the diagnosis when the temperature of the external environment changes.Ref. 11, fig. 1, tabl. 2.

Magnetic Behavior of Polycrystalline Gadolinium Subjected to a Magneto-Thermal Protocol

Magneto-thermal (M-T) hysteresis is studied in polycrystalline gadolinium (Gd). The change of the sign of the first anisotropy constant leads to easy cones of magnetization below the spin-reorientation temperature ( $T_{\mathrm {SR}} = 250$ K). Cooling a polycrystalline Gd sample in a magnetic field through the spin-reorientation temperature creates magnetic structures which persist to temperatures close to the Curie temperature, if these structures are not erased by going to fields larger than the cooling field. An M-T protocol is applied to give some systematics to these complex processes.

Local distortion and EPR parameters of copper(II) in borate glasses

The EPR parameters (g and A tensors) of the paramagnetic Cu2+ sites in CaB4O7, LiCaBO3, Li2B4O7, KLiB4O7 glasses are well explained by utilizing the fourth−order perturbation formulas for 3 d9 ions in the tetragonally elongated octahedral [CuO6]10− clusters. The magnitude of the local distortion for the [CuO6]10− clusters suffering the Jahn-Teller effect is denoted by the relative elongation ratio ρ which is proportional to the ratio Δg///Δg⊥ (Δg//= g//–gs and Δg⊥ = g⊥–gs). The g isotropies giso (=(g//+2g⊥)/3) undergo an linear increase with the decline of the covalecny of the glass systems (i.e., the augment of the orbital reduction factor k). The signs of the hyperfine structure constants are determined by computing the quantitative contributions arising from the isotropic and anisotropic copper 3d-3s (4s) orbital admixtures indicated by the core polarization constant κ and the reduction factor H, respectively. The above correlations are proved to be available for analogous borate glasses doping with copper ions, e.g., MRbB4O7 (M = Li, Na and K), 90M2B4O7·9PbO·CuO (M = Li, Na and K), 10MO·30ZnO·60B2O3 (M = Mg, Ca and Sr) and xLi2O·(30-x)Na2O·69.5B2O3 (5 ≤ x ≤ 25 mol%), and all the results are discussed.