Electron beam-plasma interaction and ion-acoustic solitary waves in plasmas with a superthermal electron component

The study of non-Maxwellian plasmas is crucial to the understanding of space and astrophysical plasma dynamics. In this paper, we investigate the existence of arbitrary amplitude ion-acoustic solitary waves in an unmagnetized plasma consisting of ions and excess superthermal electrons (modelled by a kappa-type distribution), which is penetrated by an electron beam. A kappa (kappa-) type distribution is assumed for the background electrons. A (Sagdeev-type) pseudopotential formalism is employed to derive an energy-balance like equation. The range of allowed values of the soliton speed (Mach number), wherein solitary waves may exist, is determined. The Mach number range (allowed soliton speed values) becomes narrower under the combined effect of the electron beam and of the superthermal electrons, and may even be reduced to nil (predicting no solitary wave existence) for high enough beam density and low enough kappa (significant superthermality). For fixed values of all other parameters (Mach number, electron beam-to-ion density ratio and electron beam velocity), both soliton amplitude and (electric potential perturbation) profile steepness increase as kappa decreases. The combined occurrence of small-amplitude negative potential structures and larger amplitude positive ones is pointed out, while the dependence of either type on the plasma parameters is investigated.

Adsorption-controlled molecular-beam epitaxial growth of BiFeO3

BiFeO3 thin films have been deposited on (111) SrTiO3 single crystal substrates by reactive molecular-beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth overpressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry. Four-circle x-ray diffraction reveals phase-pure, untwinned, epitaxial, (0001)-oriented films with rocking curve full width at half maximum values as narrow as 25 arc sec (0.007 degrees). Second harmonic generation polar plots combined with diffraction establish the crystallographic point group of these untwinned epitaxial films to be 3m at room temperature. (C) 2007 American Institute of Physics.

Ferroelectric nanostructures fabricated by focused-ion-beam milling in epitaxial BiFeO3 thin films

We studied a suitable route to fabricate ferroelectric islands by focused-ion-beam milling in bismuth ferrite epitaxial thin films. Piezoresponse force microscopy shows that the damage induced by the milling process is extended to 1 mu m away from the edge of the focused-ion-beam patterned islands. After a combined vacuum and oxygen atmosphere annealing procedure, ferroelectricity is fully recovered in structures with sizes down to 500 nm, while for 250 nm islands the defects at the interfaces induce polarization direction pinning.