Ph. L'Héritier

The Hereditary Virus of Drosophila

Publisher Summary The first problem raised by CO 2 sensitivity in Drosophila concerns the true nature of the determinative agent, whether it is a virus or an extrachromosomal genetic factor. Pathogenicity is surely not a valid criterion for a restrictive definition of viruses. Many virus infections are not accompanied by any pathologic symptoms and, on the other hand, true pathologic conditions can be the direct outcome of genetic factors. Infectiveness—that is, the capacity to spread from cell to cell—is also not an attribute that is restricted solely to viruses. On the other hand, the ability of cells to be freed from integrated virus under special environmental conditions is not a better criterion for distinguishing these biological elements from so called normal cellular components. The characterization of viruses as a taxonomic class might be achieved better on morphological ground. Notwithstanding the lack of any direct information on its intimate structure, there is a good evidence to consider σ as a true virus. On the other hand, σ shares with other insect viruses some common attributes that point to some sort of taxonomic relationship. Turning now to the stabilized condition, it must be admitted that the evidence is less compelling for denying mature virus any role in the viral genetic continuity through fly generations. The chapter concludes with a discussion on the effect of a rise in temperature on the host–virus relationships.

A STRUCTURAL ANALYSIS AND SOME MAGNETIC PROPERTIES OF THE R2FE17HX SERIES

Abstract A structural and magnetic characterization of the compounds R 2 Fe 17 H x through the R series (except for R = Eu, Yb) is presented. The starting alloys crystallize with the rhombohedral ( Th 2 Zn 17 - type R -3 m ) or the hexagonal ( Th 2 Ni 17 - type P 6 3 / mmc ) structure for the light and heavy rare earths, respectively. It is seen that the host alloy symmetry is retained upon hydrogenation. Hydrogen is found to accomodate rare earth-rich octahedral and tetrahedral sites. The maximum hydrogen uptake ranges from 3 to 5 hydrogen atoms per formula unit for the heavier and lighter rare earths, respectively. The highly anisotropic cell expansion upon hydrogenation is analyzed. A filling scheme for hydrogen is proposed. The effect of hydrogenation on intrinsic magnetic parameters such as T C and the total magnetization has been studied. It is correlated to the variation of short Fe-Fe distances within the structure and discussed in terms of variations of the exchange interaction. It is also shown that hydrogenation modifies the thermal variation of the anisotropy constants in some compounds.

Structural and magnetic properties of RE2Fe14BH(D)x; REY, Ce, Er☆

Abstract The structural and magnetic characteristics of RE 2 Fe 14 BH x hydrides with RE ≡ Y, Ce, Er have been studied as a function of x . Both Curie temperature and magnetization are increased on hydriding the ternary bondes. Neutron diffraction experiments performed on deuterated samples have permitted the location of the host sites and the measurement of the magnetic moments of iron and RE. Different anomalies reported in the structural and magnetic behaviour of Ce 2 Fe 14 BH x are related to the change of electronic state on cerium.

Effects of hydrogen absorption on the 3d and 4f anisotropies in RE2Fe14B (RE ≡ Y, Nd, Ho, Tm)

Abstract The effects of hydrogen absorption on the iron and rare earth sublattice anisotropies in RE2Fe14B (RE ≡ Y, Nd, Ho, Tm) have been investigated using the singular point detection technique. Hydrogen is found to substantially depress both the iron and rare earth contributions to the magnetocrystalline anisotropy. However, such a reduction appears to be very large for holmium, and particularly for neodymium, which both give a resultant axial anisotropy. It is less important in the case of thulium which gives a resultant planar anisotropy. The observed hydrogen composition dependence of the holmium anisotropy implies the existence of a particular hydrogen filling sequence.

Crystallographic study, by Rietveld's method, of barium - magnesium - tantalum oxides based ceramics for use as dielectric resonator. X-ray dilatometry at low temperature

Abstract Sintering at high temperature for a long time greatly improves the dielectric loss quality Q of Ba 3 MgTa 2 O 9 dielectric resonator ceramics. Starting material perovskite powder and grinded dielectric resonator ceramic of this compound have been studied by X-ray diffraction and using Rietvelds method. Sintered phase at 1550°C is completely long-range ordered, starting powder calcined at 1200°C is almost completely long-range disordered. The observed ordering is a well known 1 2 ordering on the octahedral sites of some perovskites; Mg-Ta-Ta-Mg octahedra layer sequence is observed along the [111] direction of the pseudo-cubic cell. Electrostatic repulsion between two successive Ta planes causes the extension of the cubic cell which becomes trigonal. Crystallographic symmetry lowering reduces the number of degeneracies of the phonon spectrum and thus reduces dielectric loss. At low temperature below 200 K, dielectric loss of Ba 3 MgTa 2 O 9 ceramic increases for decreasing temperatures in contrast with theoretical models. Dilatometric study of the cell versus temperature seems to show abnormal behaviour of the cell parameters between 150 K and 70 K.

Spin reorientation phenomena in RE2Fe14B (RE Ce, Dy, Er) alloys from 57Fe and 161Dy Mössbauer spectroscopies

Abstract Magnetically aligned polycrystalline samples of Er 2 Fe 14 B, Ce 2 Fe 14 B and Dy 2 Fe 14 B have been studied by Mossbauer spectroscopy ( 57 Fe and 161 Dy) in the temperature range between 4.2 K and 350 K. The results demonstrate a constant collinear spin structure for Ce 2 Fe 14 B and Dy 2 Fe 14 B in this temperature range. A spin reorientation from the basal c-plane to the c - axis is observed for Er 2 Fe 14 B when temperature is increased above 328 ± 3 K.

Structures magnetiques et transitions du premier ordre dans les perovskites metalliques GaMn3(C1−xNx). Relation avec les composes de terres rares a changement de valence

Abstract Strong analogies are shown between the change of electronic structure in metallic perovskite-type compounds of manganese GaMn 3 (C 1−x N x ) and the valence instabilities in rare earth compounds.

166Er Mössbauer spectroscopy in the Er2Fe14B H¢L alloys

Abstract 166Er Mossbauer spectroscopy is investigated in Er2Fe14B and its hydride. The hyperfine interaction parameters are discussed and compared to those reported for other intermetallic Er alloys. A consistent interpretation of the hyperfine data along the RE2Fe14B series (RE = rare earth element) is proposed.

Hydrogen dependence of the intrinsic magnetic properties of R2Fe14B

Abstract Magnetization measurements on Dy2Fe14BHx show that hydrogen increases the magnetization Ms, the Curie temperature Tc and the magneto-crystalline anisotropy. On saturation, at x = 4.7, Ms increased from 9 to 15 μB per formula unit at 4.2 K, Tc increased from 595 to 661 K and the phenomenological fourth-order anisotropy constant K2 was doubled throughout the range 4.2–300 K. A non-collinear spin structure was deduced for the dysprosium sublattices in the hydride. No spin reorientation was observed for Er2Fe14BHx. as x was varied at room temperature.

The non-collinear component in the magnetic structure of Mn4N

The magnetic structure of Mn4N is shown to be not exactly of ferrimagnetic collinear type. Experiments using, separately, non-polarised and polarised neutrons reveal the existence of a small triangular magnetic component on each of the Mn atoms that are at the centres of the faces of the cubes.