Galois covers of the open p-adic disc

This paper investigates Galois branched covers of the open $p$-adic disc and their reductions to characteristic $p$. Using the field of norms functor of Fontaine and Wintenberger, we show that the special fiber of a Galois cover is determined by arithmetic and geometric properties of the generic fiber and its characteristic zero specializations. As applications, we derive a criterion for good reduction in the abelian case, and give an arithmetic reformulation of the local Oort Conjecture concerning the liftability of cyclic covers of germs of curves.Comment: 19 pages; substantial organizational and expository changes; this is the final version corresponding to the official publication in Manuscripta Mathematica; abstract update

Small Scale Detonation Studies: Direct impulse measurements for detonations and deflagrations

This report is an account of research carried out from January to June 2000 on the feasibility of detonation initiation and impulse generation for small-scale pulse detonation engines. The initial work was focussed on the direct measurement of impulse using the ballistic pendulum technique for single detonations initiated in a tube with one end open to the atmosphere through a thin diaphragm. Three tubes were used: (1) 38-mm diameter by 1.5 m long. (2) 75-mm diameter by 0.6 m long. (3) 75-mm diameter by 1 m long. At the closed end of the tube, combustion was initiated by a low energy, less than 50 mJ, capacitor discharge system. A fast flame or detonation was created by transition to detonation. The effect of spirals and orifice plates was examined on propane- and ethylene-oxygen-nitrogen mixtures with varying initial pressure, equivalence ratio, and dilution amounts. A simple model for the impulse in prompt detonations was developed and calibrated. The results of our experiments were compared with this model

Crystallographic and Mössbauer study of zinc blende type FeS

Until a few years ago two main phases of FeS were known : a hexagonal phase which is antiferromagnetic and semi metallic, and a tetragonal phase which is non magnetic. We present here the results of a crystallographic and Mössbauer study of the new cubic zinc blende type phase which was discovered recently. This phase is unstable at room temperature. When the temperature is decreased it undergoes a first order crystallographic transition (cubic → orthorhombic) at about 234 K. The Mössbauer effect has shown that at all temperatures the new phase is apparently ionic, with high spin Fe++ ions. Above 234 K it is paramagnetic and below 234 K it exhibits a first order magnetic transition to an ordered collinear phase in which the magnetic moments are all parallel to one of the axes of the orthorhombic cell. Various mechanisms have been examined in order to interpret the first order magnetocrystalline transition at 234 K : standard exchange magnetostriction ; Jahn-Teller effect involving the Γ3 orbital doublet, in association with exchange magnétostriction or generalized exchange and so on, but we could not arrive at a definite conclusion. As expected the volume per FeS formula increases in going from the semi metallic hexagonal phase to the non magnetic tetragonal phase, and from the tetragonal phase to the ionic high spin cubic phase. The properties of the tetragonal phase, which had been interpreted as covalent, could perhaps also be described in terms of low spin ferrous ions (strong crystalline field case).Jusqu'à présent, on connaissait essentiellement deux phases de FeS : une phase hexagonale, antiferromagnétique et semi-métallique et une phase tétragonale non magnétique. Nous présentons ici l'étude cristallographique et par effet Mössbauer de la nouvelle phase cubique de type blende qui a été découverte il y a quelques années. Cette phase est instable à l'ambiante. Quand la température décroît, elle subit une transition cristallographique du premier ordre aux environs de 234 K, passant de la structure blende cubique à une structure orthorhombique. L'effet Mössbauer a montré qu'à toute température la nouvelle phase est apparemment ionique et qu'elle contient des ions ferreux de haut spin. Au-dessus de 234 K, elle est paramagnétique et au-dessous elle subit une transition magnétique du premier ordre vers une phase ordonnée colinéaire où tous les moments magnétiques sont parallèles à l'un des côtés de la maille orthorhombique. Divers mécanismes ont été envisagés en vue d'expliquer la transition magnétocristalline du premier ordre à 234 K : magnétostriction d'échange classique ; effet Jahn-Teller mettant en jeu le doublet orbital Γ 3, associé à de la magnétostriction d'échange ou de l'échange généralisé, etc., mais nous n'avons pas pu arriver à une conclusion définie. Comme on pouvait le prévoir, le volume par motif FeS augmente lorsqu'on passe de la phase hexagonale semi-métallique à la phase tétragonale non magnétique et de celle-ci, à la phase cubique ionique de haut spin. Les propriétés de la phase tétragonale, que l'on a attribuées à de la covalence, pourraient peut-être aussi être décrites en termes d'ions ferreux bas spin (cas de champ cristallin fort)