Single-cavity 8 MeV race-track microtron

This paper describes the operational details of the single-cavity race track microtron of this laboratory. The machine is capable of providing electrons of 8 MeV energy at 1 mA peak current. Important parameters of the machine and beam handling system are studied and the results are reported.     

Strong coupling expansion for classical statistical dynamics

We discuss the simple, randomly driven systemdx/dt = –x –x³ +f(t), wheref(t) is a Gaussian random function or stirring force with f(t)f(t) = (t – t). We show how to obtain approximately the coefficients of the expansion of the equal-time Green's functions as power series in (1/R)ⁿ, whereR is the internal Reynolds number ()^1/2/, by using a new expansion for the path integral representation of the generating functional for the correlation functions. Exploiting the fact that the action for the randomly driven system is related to that of a quantum mechanical anharmonic oscillator with Hamiltonianp ²/2 +m ² x ²/2 +vx ⁴ +x ⁶/2, we evaluate the path integral on a lattice by assuming that thex ⁶ term dominates the action. This gives an expansion of the lattice theory Green's functions as power series in 1/(a)^1/3, wherea is the lattice spacing. Using Padé approximants to extrapolate toa = 0, we obtain the desired large-Reynolds-number expansion of the two-point function.Supported financially by the National Science Foundation and the U.S. Department of Energy.     

On Solvability of Functional Equations Relating to Dynamical Systems with Two Generators

In this paper, some solvability problems for functional equations of the form

Effect of Zr concentration on the mechanical and thermodynamic properties of NbIr3 intermetallic compounds from theoretical estimations

ABSTRACT

The iridium is an important metal which has excellent resistance to corrosion at high temperature. L1₂ intermetallic compounds i.e. Ir₃Nb and Ir₃Zr, with similar lattice parameters are ideal for working at high temperature. They are fully soluble due to their low lattice misfit. A first-principle investigation into the effect of doping Zr with different concentrations on the electronic structure, mechanical and thermodynamic properties of NbIr₃ has been studied to prompt the development of novel high-temperature materials. Nine Zr_xNb_8?xIr₂₄ compounds are carefully considered. The results show that adding Zr into these compounds can strengthen their structural stability and ductility. Nevertheless, it reduces the elastic modulus and elastic stiffness. Simultaneously, with the increase of Zr content, the thermodynamic properties of these compounds decrease. It is also found that the changes of elastic modulus are mainly attributed to the variations of bonds in these compounds.     

The electronic properties and electron transport of sawtooth penta-graphene nanoribbon under uniaxial strain: ab-initio study

ABSTRACT

The electronic properties and electron transport of a sawtooth penta-graphene nanoribbon (SSPGNR) under uniaxial strains are theoretically studied by density-functional theory (DFT) in combination with the non-equilibrium Green's function formalism. We investigated the electronic structures and the current–voltage (I–V) characteristics of the SSPGNRs under a sequence of uniaxial strains in range from 10% compression to 10% stretch. In this strained range, carbon atoms still keep a pentagon network, but with the changing bond lengths. The C–C bond lengths change almost linearly with the tolerable strain. The value of the band gap of SSPGNRs can be depicted as a parabola under uniaxial strain. Our calculations show that the current is monotonous increase with compressive strain at the same applied bias voltage. In case of tensile strain, the variable rule of the current is different that it increases at first and decrease later. The fundamental physical properties (band structure, I–V characteristic) of SSPGNRs seem to be more sensitive to compressive strain than the stretch strain. The current intensity of the compressive-SSPGNR is by 2 orders of magnitude compared to that of the tensile-SSPGNR at the same strain in range from 6% to 10%. The results obtained from our calculations are beneficial to practical applications of these strained structures in SSPGNRs-based electromechanical devices.     

Effects on the evolution of microstructure and properties of low-silicon cast aluminum alloys in service

ABSTRACT

The microstructure evolution and property change of four kinds of low silicon cast aluminum alloy exposed to heat for 0–50?h at 200°C were studied by means of Brinell hardness test, tensile property test, friction and wear property test and XRD analysis. The results show that with increasing thermal exposure time, the tensile strength of each group of samples decreased and the amount of wear increased. The tensile strength of samples with more Si content decreased slowly. When the time increased to 50?h, the increase of wear loss was the largest. The hardness of samples after thermal exposure increases compared with that before thermal exposure. The residual stress of (311) diffraction crystal surface of AlSi3.5Mg0.66 under different thermal exposure time was measured. The type of residual stress changed from residual tensile stress to residual compressive stress after thermal exposure. There is an abnormal phenomenon that the hardness of the sample increased and the amount of wear increased, and it is evident that the distribution of residual stress was inhomogeneous after thermal exposure. It is found that with increasing thermal exposure time to 50?h, the average lattice distortion ε of the low-index crystal plane and the high-index crystal plane in the aluminum alloys gradually increased.     

Theory of hydrogen solubility in binary iron alloys based on ab initio calculation results

Interaction of hydrogen atoms with three substitutional impurities (X?=?Pd, Ti, Cr) in bcc iron base solid solution was modelled ab initio using the WIEN2k package. It was shown that in spite of attraction between H and X atoms, excess energy of the H atom in tetrahedral sites in the first sphere of coordination of the X atom has a significant positive value, while the lowest negative values are observed in the second (Pd, ?0.087?eV; Ti, ?0.091?eV) or the third (Cr, ?0.032?eV) sphere. A new thermodynamic theory of hydrogen solubility in dilute bcc Fe–X alloys was developed on the basis of these results. The resulting equation was used to analyze existing experimental data on H solubility in a number of Fe–X alloys, and X–H interaction energies were determined for each case. The energies determined from high-temperature solubility data for Fe–Pd, Fe–Ti and Fe–Cr are somewhat greater than those obtained in ab initio calculations. The theory gives a new basis for analyzing hydrogen behaviour in iron-base solid solutions.     

Critical and multicritical behavior in the Ising–Heisenberg universality class

Critical behavior of three-dimensional classical frustrated antiferromagnets with a collinear spin ordering and with an additional twofold degeneracy of the ground state is studied. We consider two lattice models, whose continuous limit describes a single phase transition with a symmetry class differing from the class of non-frustrated magnets as well as from the classes of magnets with non-collinear spin ordering. A symmetry breaking is described by a pair of independent order parameters, which are similar to order parameters of the Ising and O(N) models correspondingly. Using the renormalization group method, it is shown that a transition is of first order for non-Ising spins. For Ising spins, a second order phase transition from the universality class of the O(2) model may be observed. The lattice models are considered by Monte Carlo simulations based on the Wang–Landau algorithm. The models are a ferromagnet on a body-centered cubic lattice with the additional antiferromagnetic exchange interaction between next-nearest-neighbor spins and an antiferromagnet on a simple cubic lattice with the additional interaction in layers. We consider the cases N = 1, 2, 3 and in all of them find a first-order transition. For the N = 1 case we exclude possibilities of the second order or pseudo-first order of a transition. An almost second order transition for large N is also discussed.     

Evolution of Orbits and Quantum Correlation Functions by Quadratic Hamiltonians

Quantum systems with quadratic Hamiltonians are considered. Some results about the time evolution of homogeneous polynomials and of quantum correlation functions are given. The image of arbitrary orbit of Weyl–Heisenberg group under this time evolution is shown to be again an orbit of this group. For quantum free particle it is shown that its time evolution intersects arbitrary such orbit at most once. A result about existence of more orbits having the same dispersion of some quantum position is presented.PACS: 02.20.Qs, 02.30.Sa     

Charge qubits and limitations of electrostatic quantum gates

We investigate quantum dot arrays and their application to quantum computation. The arrays analyzed contain a total of a few operating electrons with constant tunneling between the dots. We construct quantum two-level systems near the ground state with a large energy separation to the remainder of the states and with the electrostatic interaction modeled within the capacitance matrix formalism. A set of representative examples is investigated numerically.