Effets de taille quantiques dans le pouvoir thermoelectrique de couches minces de Bi et de BixSb1−x

The existence of quantum size effects has been observed by thermoelectric power measurements in Bi and Bi_xSb_1-x thin films. The observed discontinuities at 90 K in the curves α vs film-thickness can be understood when the quantization of energy levels in the conduction band is taken into account.     

Thermoreflectance spectra of Cd3As2

The near normal incidence thermoreflectance spectra of Cd₃As₂ have been investigated at room temperature and liquid nitrogen temperature in the spectral range 2.5–6.2 eV.     

Magnetoplasma reflectivity studies on Cd3As2

Magnetoplasma reflectivity measurements were performed on Cd₃As₂ in order to obtain more experimental details about the structure of the plasma reflectivity edge. The experimental results can be explained by assuming the existence of a surface layer with a carrier concentration-gradient which is responsible for optical interference effects.on leave from Dept. of Physics, University of Technology, 5600 MB, Eindhoven, the Netherlandson leave from University of Würzburg, D8700, Würzburg, FRG     

Etude des proprietes de conductivite ionique des phases appartenant au systeme PbF2-InF3

A detailed study of the transport properties of the disordered fluorite-type solid solution Pb_1-xIn_xF_2+x (0 < × < 0.25) and of the ordered fluorite-type related Pb₂InF₇ (x = 0.333) and Pb₇In₅F₂₉ (x = 0.417) has been carried out. For x = 0.25 the conductivity is shown to decrease when disordering diminishes as a consequence of annealing. The disordered Pb_0.88In_0.12F_2.12 phase is a fairly good anionic conductor.     

Fundamental reflectivity spectra of monocrystalline and polycrystalline bulk Cd3As2

Fundamental reflectivity spectra of mono and polycrystalline bulk samples of Cd₃As₂ in the energy range 0.8 – 5.9 eV are measured. The results are compared with existing experimental data and are interpreted on the basis of Lin-Chung model.     

The role of vacancies in the band structure of Cd3As2

Using the pseudopotential method we have studied the effect of the vacant Cd-atom sites on the band structure of Cd₃As₂. The previous calculations of Lin-Chung were based on a hypothetical flourite structure and did not take account of the distribution of vacancies nor of spin-orbit coupling We have first extended Lin-Chung's model to include spin-orbit coupling and then used the results as a starting point in calculations in which the vacancy potentials are treated by the method of perturbations. The change involved in the behaviour of the electronic bands in passing from the hypothetical fluorite structure to the real structure has been discussed.     

Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4

Wide line and pulsed NMR studies are reported for MgH₂; and Mg₂NiH₄ hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH₂, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgH_x, (x<0.04).Mg₂NiH₄ shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T₁ and T₂ relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10^?8 < D < 4.6 × 10^?8 cm²/s.     

Relation entre structure et conductivite ionique basse temperature de Ag3PO4

The ionic conductivity of Ag₃PO₄ versus the temperature was determined. The cristal structure was reinvestigated at different temperatures (170 K, 290 K and 570 K) showing a normal increase of the unit cell with the temperature but a contraction of PO₄ tetraedra. This leads to Ag-0 bonds somewhat looser and too more space for the Ag⁺ cation to move.     

Negative magnetoresistance in Dirac semimetal Cd3As2 with in-plane magnetic field perpendicular to current

Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic theory. Chiral anomaly induced negative magnetoresistance(negative MR) under parallel magnetic field and current has been used as a probable evidence of Weyl fermions in recent years. Here we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd3As2nanowires. The negati...     

Mesure des largeurs et des deplacements des raies de la bande 0 → 2 de CO autoperturbe et perturbe par N2, O2, H2, HCl,NO et CO2

Linewidths of CO pressure-broadened by itself and by foreign gases have been measured in the 0 → 2 band. An experimental slit correction was used. The very small lineshifts of compressed CO have also been estimated using two different methods of measurement. An order-of-magnitude estimate is given for lineshifts due to foreign gases.     

Van der Pauw resistivity measurements on evaporated thin films of cadmium arsenide, Cd3As2

Cadmium arsenide is a II-V semiconductor, exhibiting n-type intrinsic conductivity with high mobility and narrow bandgap. It is deposited by thermal evaporation, and has shown the Schottky and Poole-Frenkel effects at high electric fields, but requires further electrical characterisation. This has now been extended to low-field van der Pauw lateral resistivity measurements on films of thickness up to 1.5 μm. Resistivity was observed to decrease with increasing film thickness up to 0.5 μm from about 3 × 10⁻³ Ω m to 10⁻⁵ Ω m, where the crystalline granular size increases with film thickness. This decrease in resistivity was attributed to a decrease in grain boundary scattering and increased mobility. Substrate temperature during deposition also influenced the resistivity, which decreased from around 10⁻⁴ Ω m to (10⁻⁵ to 10⁻⁶) Ω m for an increase in substrate deposition temperature from 300 K to 423 K. This behaviour appears to result from varying grain sizes and ratios of crystalline to amorphous material. Resistivity decreased with deposition rate, reaching a minimum value at about 1.5 nm s⁻¹, before slowly increasing again at higher rates. It was concluded that this resulted from a dependence of the film stoichiometry on deposition rate. The dependence of resistivity on temperature indicates that intercrystalline barriers dominate the conductivity at higher temperatures, with a hopping conduction process at low temperatures.     

Optical band-gap of Zn3As2

Absorption measurements of single Zn₃As₂ crystals were made at temperatures 5, 80 and 300 K. Free-carrier absorption is interpreted in the simple classical model. Interband absorption shows contributions from Urbach-like excitations. The direct optical gap has been estimated as 0.99 eV at 300 K, 1.09 eV at 80 K and 1.11 eV at 5 K. The linear dependence of band-gap on temperature was found in the range 80–300 K with dE_g/dT = ? 4.55 × 10^?4eVK^?1.     

Proprietes electriques des phases de basse temperature M1, T et M2 dans VO2 dope au chrome et a l'aluminium

The electrical conductivity and thermopower have been measured on Cr and Al doped VO₂ single crystals. The insulating M₁, T and M₂ phases have a similar behaviour with an activation energy E_a ?0.40 eV. The conductivity halves abruptly at the T→M₂ transition but no discontinuity is observed for the thermopower. The ansiotropy of the thermopower may be due to the anisotropic mobility of the holes in the lower quasi one dimensional Hubbard band.     

The Raman scattering of Zn3As2

The Raman scattering of Zn₃As₂ crystal is presented within 50–420 cm⁻¹ energy range. The group theory method are used to determine symmetry of the allowed lattice modes, and further to discuss the results obtained.     

Etude des transitions de phases des composes Rb2KMO3F3, Cs2KMO3F3 et Cs2RbMO3F3 (M = Mo,W)

Crystallographic, micro-D.T.A., dielectric and pyroelectric measurements have been performed for the Rb₂KMO₃F₃, Cs₂KMO₃F₃ and Cs₂RbMO₃F₃ (M = Mo, W) compounds. Two types of transitions have been detected : the first one, for Rb₂KMoO₃F₃ at low temperature, is due to the disappearance of the superstructure detected for Rb₃MoO₃F₃α, the second one is of ferroelectric - paraelectric type (Rb₂KMO₃F₃ and Cs₂RbMO₃F₃ ; M = Mo, W). The transition temperatures have been compared vs the size of the alcaline ions and the nature of the M-O bonds. The increase of the transition temperatures from the A₂A'MO₃F₃ to the A₃MO₃F₃ compounds is likely due to the simultaneous presence of the A⁺ cations in the former ones in 6 and 12 coordination sites.     

Transition de phase dans le nitrate de calcium Ca(NO3)2

The phase transition in calcium nitrate at 14° C has been investigated. Results of second harmonic generation (S.H.G.), dielectric measurements and other investigations are presented. S.H.G. experiments indicate absence of inversion center below 14° C. The phase transition is clearly distinguished by the disappearance of the SHG signal at 14° C. Dielectric measurements show a sharp maximum of ε′ at 14° C. Ferroelectric hysteresis loop is not observed below this temperature. Physical properties of calcium nitrate and cadmium nitrate are also compared.     

Caracterisation d'une variete amorphie de FeF3 : Etude thermique,magnetique et Mossbauer

Amorphous FeF₃ has been prepared by fast vaporisation of the bulk material and condensation : it has been characterized by X-ray diffraction and differential thermal analysis. Strong antiferromagnetic interactions, but low spin-freezing temperature are observed. The magnetic arrangment is speromagnetic. The spin glass behaviour is questionable; the Mössbauer study reveals an electric field gradient distribution which, in average, has no definite sign.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Structure electronique des oxydes de cobalt CoO et Co3O4

The total density of occupied states in the valence band of CoO and Co₃O₄ is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2_p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2_p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the d_n ?1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co₃O₄. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co₃O₄. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.     

Diffraction de neutrons dans CoCl2, 6D2O determination de ses structures cristallographique et magnetique

We have performed neutron diffraction experiments on a CoCl₂, 6D₂O monocrystal. We show that a crystallographic transition from the C 2/m monoclinic structure to the P $\text{1}$ triclinic structure occurs at T = Tmt = 73.5K., together with a twinning of the monocrystal. We describe the two possible twins (four types of domains) related to the loss of the initial monoclinic symmetry, say of the elements 2 and m : either m becomes twin plane (Albite twinning) or 2 becomes twin axis (Pericline twinning). Each domain has an antiferromagnetic structure P 2a T, with an easy magnetization axis tilted from the ac plane by an angle φ = (30 ± 4)°, its component on this plane making an angle φ = (3.5 ± 2)° with the c axis.