Scalar-Tensor theories from Λ(ϕ)\Lambda(\phi) Plebanski gravity

We study a modification of the Plebanski action, which generically corresponds to a bi-metric theory of gravity, and identify a subclass which is equivalent to the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories. In this manner, scalar-tensor theories are displayed as constrained BF theories. We find that in this subclass, there is no need to impose reality of the Urbantke metrics, as also the theory with real bivectors is a scalar-tensor theory with a real Lorentzian metric. Furthermore, while under the former reality conditions instabilities can arise from a wrong sign of the scalar mode kinetic term, we show that such problems do not appear if the bivectors are required to be real. Finally, we discuss how matter can be coupled to these theories. The phenomenology of scalar field dark matter arises naturally within this framework.Comment: 21 page

La constitution algérienne de 1989: une passerelle entre le socialisme et l'islamisme

The riots of October 1988, the most violent uprising since independence against FLN-rule, forced president Chadli Bendjedid to accelerate and to extend the constitutional reforms announced earlier. An adaption of the constitutional law to the ongoing economic liberalization-process had become a necessity, but the popular pressure now not only asked economic changes, but also profound political reform.The new constitutional text was rapidly elaborated by a small circle of persons around the President and then submitted directly to a popular referendum. In contradiction with the procedure fixed by the previous constitution, the National Assembly was not involved nor even consulted. The constitution of 1989 generates an entirely new political regime. The word "socialism", basis of the official doctrine since independence and largely confirmed by the provisions of the constitution of 1976, is banned completely. The new constitution also provides for the political responsibility of the Head of the Government and the members of the Government to the National People's Assembly, and not any more to the Presidentonly. In the chapter on fundamental freedoms and the rights of man, it is explicitly provided that the State guarantees the right to form political associations. This new timorous formulation entails the end of the one-party system and the FLN's exclusive hold on power.Some basic principles remain: Algeria is still considered a popular democratic state. Islam is the state religion and the official language is Arabic. No reference is made to the Berber language or culture. New is that the exercise of the guaranteed fundamental freedoms and rights can not be submitted any more to the imperatives of a socialist revolution. It is also stated that judges only obey to the law, they are not submitted any more to the revolutionary legality. A Constitutional Council is created to ensure that the constitution is respected but citizens have no right to submit a case, only the President and the President of the Assembly have.The tasks of the army are limited to safeguard the national independence and sovereignty; the army has no duties any more to safeguard the socialist revolution.The introduction of a responsible Government affects the presidential powers only in a minor way. The President presides over the Council of ministers, where bills are discussed. The President can ask the Assembly for a second reading of a law and this new vote requires a two-thirds majority. Only the President has the initiative for a constitutional revision. The President chairs a number of other councils. Finally the declaration of the state of emergency is depending only on the decision of the President; this attributes him large exceptional powers. Thus, the constitution of 1989 confirms a strong presidential regime but on the other hand it has introduced a real multi-party system in Algeria. More than 20 political parties are recognised. During the local elections of1990 the ruling FLN was defeated in most places by a massive victory of the islamic fundamentalist party, the FIS. A new electorial law, voted by the - still exclusive FLN - National Assembly beginning 1991, had to ensure a better result for the FLN during the forthcoming first free national elections. In June 1991 violent and even armed protest, organised by the fundamentalists against the law forced president Bendjedid to postpone elections, to declare the state of emergency but also to promise early presidential elections. Meanwhile manyfundamentalists, and between them the main party-leaders, were arrested. The army played a crucial role in reestablishing public order and as a consequence gained more importance, but there were no signs that it exceeded its authority. Under present difficulties one wonders whether the constitution of 1989 will help to create a representative democratic multi-partyism, with an equitable liberal economy, whether it will help to open the way for a regime dominated by islamic fundamentalists

Killing spinor space-times and constant-eigenvalue Killing tensors

A class of Petrov type D Killing spinor space-times is presented, having the peculiar property that their conformal representants can only admit Killing tensors with constant eigenvalues.Comment: 11 pages, submitted to CQ

Constructive factorization of LPDO in two variables

We study conditions under which a partial differential operator of arbitrary order $n$ in two variables or ordinary linear differential operator admits a factorization with a first-order factor on the left. The factorization process consists of solving, recursively, systems of linear equations, subject to certain differential compatibility conditions. In the generic case of partial differential operators one does not have to solve a differential equation. In special degenerate cases, such as ordinary differential, the problem is finally reduced to the solution of some Riccati equation(s). The conditions of factorization are given explicitly for second- and, and an outline is given for the higher-order case.Comment: 16 pages, to be published in Journal "Theor. Math. Phys." (2005

Influence of hydrogen on the structural stability of annealed ultrathin Si/Ge amorphous layers

Semiconductor structures based on Si and Ge are generally submitted to hydrogenation because H passivates the dangling bonds of Si and Ge. By this way the devices prepared from those semiconductors, e.g., solar cells, have much better electrical properties. However, H stability is still a critical issue. In fact, there is wide evidence that H is very unstable against illumination as well as heat treatment. It has been seen that H out effuses from the samples under such treatments. As this causes unsaturation of the dangling bonds the electrical properties worsen significantly. In this work we will show that in the case of ultrathin Si/Ge amorphous layers the H thermal instability also affects the structural stability even up to the micrometric scale depending on the H content. Such type of structure can also be used to prepare SiGe alloys by mixing the layers with heat treatments. The samples were amorphous multilayers (MLs) of alternating ultrathin (3 nm) layers of Si and Ge deposited by sputtering on (100) oriented Si substrate. The total thickness of the MLs was 300 nm. The samples were hydrogenated by introducing H in the sputter chamber with flow rates varying from 0.8 to 6 ml/min. The MLs underwent different heat treatments, from the one at 350 ?C for 1 h up to the one at 250 ?C for 0.5 h + 450 ?C for 5 h. The samples were analysed by AFM, TEM, energy filtering TEM and Small-Angle X-Ray Diffraction (SAXRD). AFM showed that upon annealing the structure of the samples degrades with formation of surface bumps whose size increases by increasing the annealing temperature and/or time, for the same H content, or by increasing the H content for the same annealing conditions. For high H content and/or annealing conditions AFM showed that the bumps have blown up giving rise to craters. This suggests that H was released from its dangling bonds to Si and Ge and formed H bubbles in the MLs because of the energy supplied by the annealing. Additional energy for the break of the Si-H and Ge-H bonds could be the one supplied by the recombination of thermally generated carriers associated with the band gap fluctuations caused by the not uniform distribution of H in the MLs. The first sites of H accumulation are very likely nanocavities certainly present in the amorphous MLs. By TEM it has been seen that layer intermixing occurred which could be the first step of H bubbles formation. SAXRD measurements as well as TEM energy filtering maps for Si and Ge showed that Si and Ge interdiffusion took place in an asymmetric way as Si was seen to diffuse to the Ge layers whereas Ge did not diffuse to the Si layers. This might be due to the higher density of free dangling bonds in the Ge layers created by annealing because the binding energy of the Ge-H bond is smaller than the one of the Si-H bond

Partially ionized plasmas in electromagnetic fields

The interaction of partially ionized plasmas with an electromagnetic field is investigated using quantum statistical methods. A general statistical expression for the current density of a plasma in an electromagnetic field is presented and considered in the high field regime. Expressions for the collisional absorption are derived and discussed. Further, partially ionized plasmas are considered. Plasma Bloch equations for the description of bound-free transitions are given and the absorption coefficient as well as rate coefficients for multiphoton ionization are derived and numerical results are presented.Comment: 18 pages, 8 figures, accepted for publication in J. Phys.: Conf. Se