Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

Gedanken experiments to destroy a black hole

It is widely believed that the complete gravitational collapse of a body always results in a black hole (i.e., “naked singularities” can never be produced) and that all black holes eventually “settled down” to Kerr-Newman solutions. An important feature of the Kerr-Newman black holes is that they satisfy relation m² ? a² + e² where m is the mass of the black hole, e is its charge, $\text{a =}\text{J}\text{m}$ is its angular momentum per unit mass and geometrized units G = c = 1 are used. (For m² <a² + e² the Kerr-Newman solutions describe naked singularities.) In this paper, we test the validity of the above conjectures on gravitational collapse by attempting to create a spacetime with m² <a² + e² starting with a Kerr-Newman black hole with m² = a² + e². Such a spacetime would either have to be a new black hole solution or a “naked singularity,” in violation of the above conjectures. In the first gedanken experiment we attempt to make the black hole capture a test particle having large charge and orbital angular momentum compared with energy. In the second gedanken experiment we attempt to drop into the black hole a spinning test body having large spin to mass ratio. In both cases we find that bodies which would cause violation of m² ? a² + e² will not be captured by the black hole, and, thus, we cannot obtain m² <a² + e², although we can come arbitrarily close in the sense that m² = a² + e² can be maintained in these processes.     

Final state band gap effects in UV photoemission from Cu(111)

The crystal momentum dependence of the final states involved in bulk photoemission from the sp-bands through a (111) surface of copper was investigated with a photon energy hν = 11.7 eV. Plane wave hybridization was observed and is described in terms of a 2-OPW model Hamiltonian with parameters determined to be V₀=7.1±0.1 eV for the inner potential and E_g = 1.6 ± 0.2 eV for the energy gap. The model is also shown to account for a rapid intensity variation with crystal momentum of the form |a_G(k)|², where a_G(k) is a plane wave amplitude of the final state wave function.     

Casimir self-stress on a perfectly conducting cylindrical shell

We compute the Casimir stress on a perfectly conducting cylindrical shell, due to quantum field fluctuations (zero-point energy) in both the interior and exterior regions, using a Green's dyadic formulation for the field strengths. To obtain a finite answer, a frequency cutoff must be inserted, but the result is independent of that cutoff. The Casimir stress is found to be attractive, the Casimir energy per unit length for a cylinder of radius a being $\text{E}$ = $\text{?0.014}\text{a}{}^2$.     

Photoluminescence and optical properties of He ion bombarded ultra-high molecular weight polyethylene

Ion bombardment is a suitable tool to improve the physical and chemical properties of polymer surface. In this study UHMWPE samples were bombarded with 130 keV He ions to the fluences ranging from 1 × 10¹² to 1 × 10¹⁶ cm⁻². The untreated and ion beam modified samples were investigated by photoluminescence, and ultraviolet-visible (UV-vis) spectroscopy. Remarkable decrease in integrated luminescence intensity with increasing ion fluences was observed. The reduction in PL intensity with increase of ion fluence might be attributed to degradation of polymer surface and formation of defects. The effect of ion fluence on the optical properties of the bombarded surfaces was characterized. The values of the optical band gap E_g, and activation energy E_a were determined from the optical absorption. The width of the tail of the localized states in the band gap (E_a) was evaluated using the Urbach edge method. With increasing ion fluences a decrease in both the energy gap and the activation energy were observed. Increase in the numbers of carbon atoms (N) in a formed cluster with increasing the He ion fluence was observed.     

Physical meaning of δ2z of Glansdorff and Prigogine

It is shown that δ²z of Glansdorff and Prigogine is nothing but δ²s, wheres is the entropy per unit mass, by taking into account that the inverse process of the dissipation of the macroscopic motion can occur through fluctuation.     

Truesdell transport in general relativity

The Truesdell transport of a contravariant tensor fieldX ^ab is defined with respect to a time-like vector fieldu ^a analogous to the classical definition in continuum mechanics. Truesdell stress rate of the shear tensor (σ ^ab) and the metric tensor (g ^ab ) is Truesdell transported alongl _a if and only if the medium is rigid. In case of two-dimensional projection operatorsγ ^ab andγ ^*ab are Truesdell transported along timelike vector fieldu ^a if and only if they are expansion free (alongu ^a ) with \(\tau = \lambda - \bar \sigma + \mu - \rho = 0\) . A collapsing perfect fluid is Truesdell transported with respect tou ^a if and only if it is expansion free (alongu ^a ), energy density (D), pressure (P), and density of neutrino radiation (q) are conserved alongu ^a withσt=λ, \(\alpha + \bar \beta = \tau = 0\) . In all the above cases, analogous results are found alongl ^a ,n ^a , andm ^a using freedom conditions (k=ε=π=γ+γ ^?=0) as null rays are always restricted to be geodesic (Carter [1]).     

The casimir effect as a screening effect in quantized field theory

We study the vacuum energy and the vacuum force in a system of quantized scalar fields (massive and massless) in interaction with a given screening medium. Regularization of the energy is studied and the types of determinable forces are clarified. The Casimir effect—the attraction between two conducting plates in a vacuum, and its extension to different geometries —is re-examined in this framework. Instead of the puzzling repulsion for a spherical shell conductor, an attractive force is obtained in our case. As a by-product, we obtain a potential energy between two balls of large screening power and at remote distance R, ?a₁a₂/4πR³, where a_i are the ball radii.     

Energy dissipation in the deterministic and nondeterministic Nagel-Schreckenberg models

In this paper, we numerically study energy dissipation per unit time per car E_d in the Nagel-Schreckenberg (NS) model. Numerical results show that there is a critical density over which energy dissipation occurs, but below which no energy loss happens in the deterministic NS model. Energy dissipation depends on both the car density and initial configuration in the deterministic case. The energy dissipation rate, E_d, calculated starting from a completely jammed state whose value is minimum, decreases monotonously with the increase of car density above the critical density. In the nondeterministic case, however, there is no critical density and energy dissipation happens in the whole density region, and initial configuration has no effects on energy dissipation. The dissipated energy has two different contributions: one coming from the interactions and another from the braking noise in the stochastic case. The relative contributions of the two dissipation mechanisms are presented. In the free-flow state, E_d is proportional to p(1−p) where p is the stochastic braking probability. In the case of v_max=1,E_d is proportional to the mean density of “go and stop” car per time step ρ_gs, which is equal to n₀(1−n₀) where n₀ is the fraction of stopped car. Theoretical analyses give an excellent agreement with numerical results.     

New evaluation of muon (g − 2) hadronic anomaly

The hadronic part a_H of the muon g-factor anomaly $\text{a ≡}\text{(g ? 2)}\text{2}$ is evaluated from latest data on σ(e⁺e^? → hadrons). For a p-wave ππ scattering length of a₁ = 0.04±0.005 we calculate a_H = (66±10) × 10^?9, compared to a(experiment) ? a(QED) = (60±29) × 10^?9. Half of the uncertainty on a_H is associated with the energy interval $\text{0.92 <}\text{s}\text{< 2}\text{GeV}$.     

Local equilibration and diffusive phenomena in40Ar+27Al and14N+27Al heavy ion reactions

Extensive analysis of the energy dissipation and nucleon exchange has been done with heavier systems. Here we choose the two light systems⁴⁰Ar+²⁷Al (E _lab=340 MeV) and¹⁴N+²⁷Al (E _lab=100 MeV) in order to study the correlation of the energy dissipation with the variance of the charge distributions as a function of total kinetic energy loss bins. Considerable energy damping is found to occur in the approach phase which cannot be explained by a simple Fokker-Planck diffusion model. Indeed a model which interprets the collision as a local equilibration followed by diffusive phenomena is more appropriate to fit the data.     

On the role of image forces in chemisorption theory

The Extended Anderson Hamiltonian is used to study the effect of fluctuations of an adatom charge Q on the ionic part of the chemisorption energy. It is shown that dynamical effects essentially modify the classical expression E = ? ?Q² for the energy of interaction between a static charge Q and a metal (? is the interaction energy for a unit charge). The exact solution for the one-electron two-level model as well as a variational solution for the Extended Anderson Hamiltonian model are given. Validity conditions for a variety of approximate schemes are studied. The results are presented for the Extended Anderson Hamiltonian model parameterized so as to describe some aspects of the Li/W and Li/Mo chemisorption systems.     

Specific features of the optical and electrical properties of polycrystalline CdTe films grown by the thermal evaporation method

The results of studying the physical properties of thin CdTe films obtained by the thermal evaporation method have been presented. The optical constants and the band gap of the films under study have been determined (E _g = 1.46 eV). It has been established based on the investigation of optical properties and the Raman spectrum of the films that they possess high structural quality. The activation energy of the electrical conductivity of CdTe films has been determined: E _a = 0.039 eV. The measured spectral dependences of the impedance of CdTe thin films are characteristic of the inhomogeneous medium with two time constants: τ_gb = R _gb C _gb = 1/ω_gb = 1.62 × 10^?3 s and τ_g = R _g C _g = 1/ω_g = 9.1 × 10^?7 s for grain boundaries and grains, respectively.     

A string-like self-dual solution of Yang-Mills theory

We attempt to obtain Nielsen-Olesen strings in pure Yang-Mills theory without Higgs scalars. This is inspired by recent work interpreting the Prasad-Sommerfield monopole as a static self-dual Yang-Mills solution in which A₄^a plays the role of the Higgs field. In similar fashion, we make A₃^aA₄^a serve as the two required isovector fields in an ansatz independent of x₃ and x₄. The condition of self-duality results in a single Painlevé equation of the third kind (or equivalently, a radial sinh-Gordon equation in 2 + 0 dimensions), the solution of which determines A_μ^a. We make use of the extensive analysis of the former equation carried out by Wu, McCoy and collaborators in the context of the scaling limit of the two-dimensional Ising model. Their simplest solution yields a flux value of ?2π/e just as in the Nielsen-Olesen model and flux is quantized in multiples of this unit. The string tension (action per unit time per unit distance) diverges as r^?2 (In r)^?2 as r → 0 for the same Wu-McCoy solution.     

Correlation inequalities and the mass gap inP (φ)2

We consider the infinite volume Dirichlet (or half-Dirichlet)P(φ)₂ quantum field theory withP(X)=aX ⁴+bX ⁴+bX ²?μX(a>0). If μ≠0 there is a positive mass gap in the energy spectrum. If the gap vanishes as μ → 0, it goes to zero no faster than linearly yielding a bound on a critical exponent.     

Electrical properties of narrow gap semiconductor PtSb2

Results of measurements of conductivity, Hall and Seebeck coefficients of tellurium doped n-type crystals of platinum antimonide are presented. The Hall coefficient and the Seebeck coefficient undergo sign inversion twice, below and above room temperature. The detailed analysis of the experimental results revealed that below 200 K PtSb₂ can be described by a simple conduction and valence band model. The energy gap E_g = (110?0.15 × T) (meV), the electron conductivity mass m_n^c/m₀ = 0.35, acoustic phonon limited electron mobility 〈μ_a〉_n = 3 × 10⁶ T^{?$\text{3}\text{2}$} ($\text{cm}{}^2\text{V · s}$) and mobility ratio 〈μ_a〉_n/〈μ_a〉_p = 0.4 are determined. However, at higher temperatures a more complicated valence band model is needed to account for the experimental results. The arguments for the existence of subsidiary valleys in the valence band are presented.     

A one-equation turbulence model for recirculating flows

A one-equation turbulence model which relies on the turbulent kinetic energy transport equation has been developed to predict the flow properties of the recirculating flows. The turbulent eddy-viscosity coefficient is computed from a recalibrated Bradshaw’s assumption that the constant a₁ = 0.31 is recalibrated to a function based on a set of direct numerical simulation (DNS) data. The values of dissipation of turbulent kinetic energy consist of the near-wall part and isotropic part, and the isotropic part involves the von Karman length scale as the turbulent length scale. The performance of the new model is evaluated by the results from DNS for fully developed turbulence channel flow with a wide range of Reynolds numbers. However, the computed result of the recirculating flow at the separated bubble of NACA4412 demonstrates that an increase is needed on the turbulent dissipation, and this leads to an advanced tuning on the self-adjusted function. The improved model predicts better results in both the non-equilibrium and equilibrium flows, e.g. channel flows, backward-facing step flow and hump in a channel.