[en] In this article we report the damage and annealing behavior as well as lattice site location of Fe atoms in GaN. The Fe ions were homogeneously implanted in GaN films with an energy of 150 keV at room temperature. A two-step annealing (650 degree C 15 min and then 1000 degree C 2 min) was performed to remove the implantation-induced damage and to drive the dopants into the lattice site. The structure of GaN films before and after the implantation as well as at each stage of the annealing was characterized by Rutherford backscattering/channeling combined with particle induced x-ray emission and high resolution x-ray diffraction. The Fe⁺ implanted GaN films exhibits an expanded lattice. After the two-step annealing, the lattice distortion does not fully recover. Angular scans along both [0001] and [10 bar 11] directions show that the Fe atoms occupy the lattice site of Ga atoms in the case of low dose implantation after annealing. However, for the high dose implanted GaN, about 75% of the implanted Fe atoms substitutes Ga atoms, showing that the damage is not annealed out and the dopants are not completely activated. [copyright] 2001 American Institute of Physics

[en] After completing their introductory studies on thermodynamics at the university level, typically in a second-year university course, most students show a number of misconceptions. In this work, we identify some of those erroneous ideas and try to explain their origins. We also give a suggestion to attack the problem through a systematic and detailed study of various thermodynamic cycles. In the meantime, we derive some useful relations.

[en] We investigate a class of cylindrically symmetric inhomogeneous Λ-dust spacetimes which have a regular axis and some zero expansion component. For $\mathrm{\Lambda <!--\; ??\; -->}\ne <!--\; ???\; -->0$, we obtain new exact solutions to the Einstein equations and show that they are unique, within that class. For $\mathrm{\Lambda <!--\; ??\; -->}=0$, we recover the Senovilla–Vera metric and show that it can be locally matched to an Einstein–Rosen type of exterior. Finally, we explore some consequences of the matching, such as trapped surface formation and gravitational radiation in the exterior. (paper)

[en] Dynamical models of prototype gravastars are constructed and studied. The models are the Visser–Wiltshire three-layer gravastars, in which an infinitely thin spherical shell of a perfect fluid with the equation of state p = (1−γ)σ divides the whole spacetime into two regions, where the internal region is de Sitter, and the external one is Schwarzschild. When γ<1 and Λ≠0, it is found that in some cases the models represent stable gravastars, and in some cases they represent 'bounded excursion' stable gravastars, where the thin shell is oscillating between two finite radii, while in some other cases they collapse until the formation of black holes occurs. However, when γ≥1, even with Λ≠0, only black holes are found. In the phase space, the region for both stable gravastars and 'bounded excursion' gravastars is very small in comparison to that for black holes, although it is not completely empty

[en] Dynamical models of prototype gravastars were constructed in order to study their stability. The models are the Visser–Wiltshire three-layer gravastars, in which an infinitely thin spherical shell of stiff fluid divides the whole spacetime into two regions, where the internal region is de Sitter, and the external one is Schwarzschild. It is found that in some cases the models represent the 'bounded excursion' stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes occurs. In the phase space, the region for the 'bounded excursion' gravastars is very small in comparison to that of black holes, but not empty. Therefore, although the possibility of the existence of gravastars cannot be excluded from such dynamical models, our results indicate that, even if gravastars do indeed exist, that does not exclude the possibility of the existence of black holes