Electron-cyclotron-resonance plasma heating with broadband microwave radiation: Anomalous effects

Affects of microwave bandwidth on the high-charge-states of ion beams extracted from a conventional minimum-B-geometry ECR ion source are first demonstrated. The high-charge-state intensities, produced with broadband microwave radiation are observed to be factors ?2 than those produced with narrow bandwidth microwave radiation at the same power level.     

Critical behaviors of bond and crystal field dilution Blume–Emery–Griffiths model

The critical behavior of the spin-1 bond and crystal field dilution Blume–Emery–Griffiths model have been investigated on simple-cubic lattice within the framework of the effective field theory. In particular, both bond dilution and random crystal field are considered at the same time. The interplay between bond and crystal field dilution constructs rich and interesting phase diagrams. Significant distinctions are exhibited. When positive ratio α changes in a certain range, there exist double tricritical points in phase-transition lines in T–D plane. Moreover, this first-order phase transition is enlarged with increasing of ratio α at a fixed crystal field dilution concentration, while this first-order phase transition will shrink when bond dilution concentration is fixed. In addition, we observe that there exist two bond percolation thresholds for negative crystal field and α>0 in T–P plane.     

Analytical solutions for the energy distribution of charged particles in a weak electric field

The paper deals with the stationary distribution of charged particles moving in a material medium, having scattering and absorption properties, in which a uniform electric field is present. The purpose of the work is finding analytical solutions in simplified but physically significant situations and comparing different approximations based on a spherical-harmonics expansion of the velocity distribution. Received: 28 July 1998     

Microscopic membrane elasticity and interactions among membrane inclusions: interplay between the shape, dilation, tilt and tilt-difference modes

A phenomenological Landau elasticity for the shape, dilation, and lipid-tilt of bilayer membranes is developed. The shape mode couples with the sum of the monolayers' tilt, while the dilation mode couples with the difference of the monolayers' tilts. Interactions among membrane inclusions within regular arrays are discussed. Inclusions modifying the membrane thickness and/or inducing a tilt-difference due to their convex or concave shape yield a dilation-induced attraction and a tilt-difference-induced repulsion. The resulting interaction can stabilize 2D crystal phases, with the possible coexistence of different lattice spacings when the dilation-tilt-difference coupling is large. Inclusions favoring crystals are those with either a long-convex or a short-concave hydrophobic core. Inclusions inducing a local membrane curvature due to their conical shape repel one another. At short inclusions separations, a tilt comparable with the inclusion's cone angle develops: it relaxes the membrane curvature and reduces the repulsion. At large separations the tilt vanishes, whatever the value of the shape-tilt coupling. Received 23 October 1998 and Received in final form 12 January 1999     

Quantum spin model of a cubic ferromagnet in a magnetic field

The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also been considered and the critical index has been found. Received 9 May 2000 and Received in final form 5 July 2000     

Shape transformation transitions in a model of fixed-connectivity surfaces supported by skeletons

A compartmentalized surface model of Nambu and Goto is studied on triangulated spherical surfaces by using the canonical Monte Carlo simulation technique. One-dimensional bending energy is defined on the skeletons and at the junctions, and the mechanical strength of the surface is supplied by the one-dimensional bending energy defined on the skeletons and junctions. The compartment size is characterized by the total number L^′ of bonds between the two-neighboring junctions and is assumed to have values in the range from L^′ = 2 to L^′ = 8 in the simulations, while that of the previously reported model is characterized by L^′ = 1, where all vertices of the triangulated surface are the junctions. Therefore, the model in this paper is considered to be an extension of the previous model in the sense that the previous model is obtained from the model in this paper in the limit of L^′↦1. The model in this paper is identical to the Nambu-Goto surface model without curvature energies in the limit of L^′↦∞ and hence is expected to be ill-defined at sufficiently large L^′. One remarkable result obtained in this paper is that the model has a well-defined smooth phase even at relatively large L^′ just as the previous model of L^′↦ 1. It is also remarkable that the fluctuations of surface in the smooth phase are crucially dependent on L^′; we can see no surface fluctuation when L^′≤ 2, while relatively large fluctuations are seen when L^′≥ 3.     

Exact results of a mixed spin-1/2 and spin-1 Ising chain with both longitude and transverse single-ion anisotropies

The mixed spin-1/2 and spin-1 Ising chain with both longitude and transverse single-ion anisotropies Dz and Dx is solved exactly by means of a mapping to the spin-1/2 Ising chain with the alternating transverse fields and the Jordan-Wigner transformation. The analytical expressions of the quasi-particles' spectra Λk, the minimal energy gap Δ₀ for exciting a fermion quasi-particle, the minimal energy gap Δh for exciting a hole, and the ground state energy are obtained. The phase diagram of the ground state is also given. The results show that when Dz?0 for any finite value of Dx, there is no quantum critical point and the ground state is always in a spin ordered phase disregard of the boundary condition in the present system.     

Application of atmospheric pressure microwave plasma source for production of hydrogen via methane reforming

In this paper, results of hydrogen production via methane pyrolysis in the atmospheric pressure microwave plasma with CH₄ swirl are presented. A waveguide-based nozzleless cylinder-type microwave plasma source (MPS) was used to convert methane into hydrogen. The plasma generation was stabilized by a CH₄ swirl having a flow rate of 87.5 L min^-1. The absorbed microwave power was 1.5–5 kW. The hydrogen production rate and the corresponding energy efficiency were 866 g (H₂) h^-1 and 577 g (H₂) kWh^-1 of microwave energy absorbed by the plasma, respectively. These parameters are better than our previous results when nitrogen was used as a swirl gas and much better than those typical for other plasma methods of hydrogen production (electron beam, gliding arc, plasmatron).     

Synthesis of intrinsic fluorescent polypyrrole nanoparticles by atmospheric pressure plasma polymerization

Intrinsic fluorescent polypyrrole (ppy) nanoparticles with different shapes were fabricated by atmospheric pressure plasma polymerization. Gradient electrical field and polarization of active particles in the plasma induce change of shape of nanoparticles from spherical to rod, when the plasma power varied from 5 W to 10 W. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results suggest that the atmospheric pressure plasma polymerization process (APPP) at the power of 5 W and 10 W can help to preserve the integrity of the structure of monomer due to the predominant role of radical polymerization in APPP at these powers. However, when the plasma power increased to 20 W, the ring structure of some pyrroles was destroyed, owing to existence of higher energy species. The polypyrrole nanoparticles exhibit the peak fluorescence around 415 nm. Fluorescent results show that the fluorescent properties of polypyrrole nanoparticles are related to the particle size of the polymer. The bigger particles would have more enlarged room for exciton diffusion, resulting in lower fluorescence intensity and red shift of the fluorescent peak.     

Investigation on the magnetocaloric effect in DyNi2, DyAl2 and Tb1−nGdnAl2 (n=0, 0.4, 0.6) compounds

The magnetocaloric effect (MCE) in the DyNi₂, DyAl₂ and Tb_1−nGd_nAl₂ (n=0, 0.4, 0.6) was theoretically investigated in this work. The DyNi₂ and DyAl₂ compounds are described considering a model Hamiltonian which includes the crystalline electrical field anisotropy. The anisotropic MCE was calculated changing the magnetic field direction from 〈1 1 1〉 to 〈0 0 1〉 in DyNi₂ and from 〈1 0 0〉 to 〈0 1 1〉 in DyAl₂. The influence of the second- and first-order spin-reorientation phase transitions on the MCE that occurs in these systems is discussed. For the calculations of the MCE thermodynamic quantities in the Tb_1−nGd_nAl₂ systems we take into account a two sites magnetic model, and good agreement with the available experimental data was obtained.