J.P. Senateur

Molybdenum disilicide: crystal growth, thermal expansion and resistivity

Abstract Single crystals of MoSi2 with very high resistivity ratios have been grown using a Crochralski technique and a modified Hukin-type crucible. Thermal expansion coefficient and resistivity have been measured as a function of temperature and are found anisotropic. The resistivity ratio ϱ(293 K)/ϱ(4.2 K) = 1760 is extremely high in this class of compounds.

Etude par spectrométrie Mössbauer des carbures de Fer Fe3C et Fe5C2

Abstract Mossbauer spectra of Fe 3 C and Fe 5 C 2 have been obtained from 4°K up to the Curie temperature of each carbide. The asymmetry parameter of the Fe I atom of Fe 3 C is close to 1. The splitting of site II of Fe 5 C 2 is observed. A comparison of the quadrupole splittings of Fe 3 C and Fe 5 C 2 , in the paramagnetic state, leads us to attribute sites I and II of Fe 5 C 2 to Fe I and Fe II , respectively. In the case where there is only a spin direction for Fe I , this direction is located in the planes or near the planes ( a , c ) or ( a , b ).

Optical properties of WSi2 and MoSi2 single crystals as measured by spectroscopic ellipsometry and reflectometry

Abstract The dielectric functions of monocrystalline MoSi 2 and WSi 2 and their reflectivity have been investigated. A comparison is established between these two silicides and several differences are underlined. Absorption seen from the dielectric function and the reflectivity are suggested to come from sets of d orbitals bands below and above the Fermi level. In the Infra-red, both materials exhibit different transparency windows, at 0.46 and 0.6 eV respectively, attributed to partly covalent Si-bonding. These silicides are potential intrinsic solar absorbers.

Electronic properties of CoSi2 studied by reflectivity and spectroscopic ellipsometry

Abstract A detailed comparison of the optical properties of pure Co and of CoSi 2 is made, based on spectroscopic ellipsometry and reflectivity measurements. We show that the hybridization between Si 3p and Co 3d levels in the silicide leads to the appearance of a strong absorption around 2.5 eV associated with the shifted metal d-states. A relatively sharp drop of reflectivity is observed around 1 eV for CoSi 2 and discussed.

Etude par spectrometrie mössbauer de ZnMn3 et de la transition antiferro-ferromagnetique de GaMn3 C dopes au fer 57

Abstract Mossbauer measurements have been performed on 57 Fe-doped GaMn 3 C and ZnMn 3 C. A strong variation of hyperfine field parameters at the antiferro-ferromagnetic transition of GaMn 3 C is related to the variation of the transferred fields and to the electronic anisotropy. The direction of spins is [111] for ferro and antiferromagnetic GaMn 3 C and [001] for ferromagnetic ZnMn 3 C.

Low temperature thermal conductivity of pure and doped single crystals of semiconducting SmS

Abstract We measured the thermal conductivity of pure SmS and of SmS doped with P, As and Se between 1.5 and 350 K. The lattice thermal conductivity of pure samples and of SmSSe obeys a T− 3 2 law for temperatures T greater than 150 K, and is very sensitive to point defects in the lattice. The highest values are measured on samples close to the stoichiometric composition. P and As dopants act as phonon scattering centers and considerably reduce the low temperature lattice conductivity.

Magnetoresistance measurements of V3Si evidence for precursor effects of the martensitic transformation

Abstract We report magnetoresistivity measurements on transforming and non-transforming V3Si. The martensitic structural transformation (Tm ≊ 21 K) is induced by annealing. The transforming sample shows large deviations from the Kohlers rule [Δϱ/ϱ = ƒ(H/ϱ)] , at temperatures much higher than Tm. They are attributed to precursor effects of the transition, previously observed by other techniques, and are interpreted in terms of the development of different relaxation times for the scattering of the electrons.

Preferential substitution of 57Fe in Co1−χBχ glasses

Abstract The substitution of iron for cobalt in crystalline Co3B and Co78B22 and Co72Si2B26 glasses has been studied by Mossbauer spectroscopy. In both crystalline and amorphous states, iron preferentially replaces those cobalt atoms which have two boron nearest neighbours.

On the specific heat jump at Tc of the V3Si crystals

Abstract Parameters deduced from new specific heat measurements on transforming V3Si single crystals are compared with those previously given for non transforming samples within the microscopic electronic model of Labbe-Friedel. As a result, a new approach to the martensitic transition is suggested to explain the discrepancy relative to the above parameter which differentiates the two classes of V3Si samples.

Properties of Nb3Ge tapes for high field magnets

Abstract A laboratory process for long Nb 3 Ge tapes fabrication by chemical vapor deposition (CVD) has been set up. The Nb 3 Ge tapes which were fabricated offer the possibility of high current and high field operation at 4.2 K since the values of critical current densities, J c , measured in high magnetic fields at 20T and 4.2K exceed 5 × 10 4 A cm −2 which is the generally accepted criterion for producing a superconducting magnet.