Transmission optique,en lumiere polarisee,de couches tres minces de sodium,sous incidence oblique

The transmission factors of sodium in thin films have been determined for incidence angles of 0°, 45°, 70°; the electric field E being either parallel or perpendicular to the incidence plane. These factors present important differences according to polarization.     

Orientation-selective excitation and dissociation in multilayer benzene

A newly constructed rotatable detector for desorption ion is described. The high resolution total yield spectra of desorbing ions from condensed benzene, along with the corresponding photoabsorption spectra, have been measured with various polarization angles of incident synchrotron radiation (SR) in the C 1s region. Polarization analysis of photoabsorption measurement showed that molecular planes of the adsorbed benzene molecules were randomly oriented. However, enhanced ion yields were observed at some resonances corresponding to in-plane transition, i.e. σ-symmetry, when the electric field vector (E) of SR became close to parallel to the surface normal. The results suggest that the probability of C–D dissociation and ion-desorption sequence becomes higher value in the molecular orientation perpendicular to the surface. The result also shows considerably high desorption probabilities in grazing incidence angles of less than 10°. This suggests that the contribution of a secondary effect resulting from bulk electrons is suppressed because the penetration depth of soft X-rays becomes small.     

Evolution des proprietes optiques des couches minces de sodium en fonction de leur structure

With the aim of determining the origin of abnormal absorption bands displayed by alkali metals when in bulk state or in the form of thin films, we studied the transmission - in light polarized in a straight line and under oblique incidence - of very thin, granular or continuous, films of sodium.In the case of granular films, abnormal absorption bands were observed. One of these bands only occurs when the electric field of the incident wave is parallel to the plane of incidence. This plane only subsists with continuous films. These results can be explained by the hypothesis of collective oscillations of electrons.     

Dependence of electron spin g-factor on magnetic field in quantum wells

The dependence of electron spin g-factor on magnetic field has been investigated in GaAs/AlGaAs quantum wells. We have estimated the electron g-factor from spin precession frequency in time-resolved photoluminescence measurements under a magnetic field in different configurations; the magnetic field perpendicular (g_⊥) and parallel (g_∥) to the quantum confinement direction. When the angle between the magnetic field and the confinement direction is 45°, we have found that g-factor varies depending on the direction of magnetic field and the circular polarization type of excitation light (σ⁺ or σ^?). These dependences of g-factor exhibit main features of Overhauser effect that nuclear spins react back on electron spin precession. The value of g_⊥ and g_∥ corrected for the nuclear effects agree well with the results of four-band k·p perturbation calculations.     

光束通过在静电场作用下的α-碘酸锂单晶产生的垂直于c向的散射带

激光束通过c向静电场作用下的α-碘酸锂单晶时,观察到与c向正交而且张角很大的散射带,其强度在透射光斑的两侧并不对称。当入射光的电向量垂直于c向时,散射光的电向量则平行于c向;而前者平行于c向时,后者则垂直于c向。如果入射光的电向量既不垂直也不平行于c向,则出现电向量分别平行和垂直于c向的两条散射带,在特定条件下它们相重合。本工作详细研究了散射带的偏振态、强度分布的类准周期结构及其弛豫行为等。我们认为这种散射带很可能是由于晶体中载流子沿c向运动产生的某种效应所引起。同时用显微镜观察了解电场作用下α-腆酸锂单 关键词：     

Magnetic properties and structures of PrPt and NdPt

The magnetic properties of the CrB-type orthorhombic phase of PrPt and NdPt are presented. Below their ordering temperature which is 15 and 23 K respectively, these compounds exhibit a ferromagnetic behaviour. The magnetic structures determined by neutron diffraction are collinear. While in PrPt, the magnetic moments are parallel to c, in NdPt, the moments lie in the (a, c) plane and make an angle of 22° with a. These properties and especially the peculiar moment direction in NdPt are discussed in terms of crystalline electric field effects on the rare earth ion.     

Magnetoelectricity in the rare earth hydroxides ROOH (R=Er,Tb, Dy)

The existence of the magnetoelectric effect in the antiferromagnetic monoclinic rare earth oxide hydroxides ROOH (R = Er, Tb, Dy) has been confirmed by the observation of the electrically induced magnetic moment in these materials. Both parallel and perpendicular ME susceptibilities were measured as a function of temperature in powder specimens. The transition temperatures were found to be 4.1±0.1 ErOOH, 10.0±0.2 (TbOOH), and 7.2±0.1° K (DyOOH) from the ME measurements. The maximum values of the measured ME susceptibilities were (in gaussian units) 4.5× 10^-4, 3.8×10^-4, and 9.8×10^-5 for R=Er, Tb, and Dy respectively. TbOOH is an example of a ME material whose magnetic unit cell is double its chemical cell.     

Courants critiques d'alliages supraconducteurs de concentration modulee spatialement I — Effet de la periode

We discuss the effect of the period of the sinusoidal variations of concentration of superconducting $\text{Pb}$Bi films, on the critical current densities J_c as a function of an applied magnetic field H, perpendicular to J and parallel to the film plane. The existence of peaks on the J_c(H) curves suggests a matching effect between the vortex lattice and the modulation of concentration.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

First-order phase transition from an antiferromagnetic ferroelectric to a cycloidal multiferroic with weak ferromagnetism during the joint action of applied magnetic and electric fields

The thermodynamics of the phase transition in a perovskite-like multiferroic, in which an antiferromagnetic ferroelectric transforms into a new magnetic state where a spiral spin structure and weak ferromagnetism can coexist in applied magnetic field H, is described. This state forms as a result of a first-order phase transition at a certain temperature (below Néel temperature T _N ), where a helicoidal magnetic structure appears due to the Dzyaloshinskii-Moriya effect. In this case, the axes of electric polarization and the helicoid of magnetic moments are mutually perpendicular and lie in the ab plane, which is normal to principal axis c. Additional electric polarization p, which decreases the total polarization of the ferroelectric P, appears in the ab plane. The effect of applied magnetic and electric fields on the properties of a multiferroic with a helicoidal magnetic structure is described. An alternating electric field is shown to cause a field-linear change in magnetic moment m, whose sign is opposite to the sign of the change of electric field E. The detected hysteretic phenomena that determine the temperature ranges of overheating and supercooling of each phase are explained. A comparison with the experimental data is performed.     

States of cobalt fluoride in a strong magnetic field

The behavior of the magnetic subsystem of cobalt fluoride is investigated in a strong magnetic field oriented in an arbitrary direction in space. In the case where the magnetic field is out of the planes passing through the easiest magnetization axis A and the axis [100] ‖ X or through the easiest magnetization axis A and the axis [010] ‖ Y, it follows from the derived system of equations that the antiferromagnetic vector l does not change direction to be align with the basal plane, provided the magnetic field has a nonzero component along the A axis. It is demonstrated that the antiferromagnetic vector l becomes parallel to the basal plane only when the magnetic field is perpendicular to the A axis. The case of the magnetic field directed parallel to the [110] axis is examined thoroughly. The critical value of the magnetic field is determined at which the antiferromagnetic vector l becomes parallel to the basal plane and perpendicular to the external magnetic field H for H _⊥ → ∞.     

High pressure tuning of optical transitions in Mg[Pt(CN)4]·7H2O

Mg[Pt(CN)₄]·7H₂O belongs to the class of tetracyanoplatinates(II) which crystallize in columnar structures. In different M_x[Pt(CN)₄]·yH₂O (MCP) single crystals the in-chain Pt-Pt-distance R varies between 3.67 Å (NaCP) and 3.15 Å (MgCP). Two optical transitions can be observed in polarized emission with the electric field vector E either parallel or perpendicular to the columnar (c)-axis. Polarized emission spectra of MgCP are recorded under hydrostatic pressure up to p ≈ 18 kbar (at 295 K). The transition energy v?₆ can be tuned from 17,600 cm^-1 to about 12,000 cm^-1 (2.18-1.48 eV). The pressure induced red shift for the two transitions is: E 6 c: dv?₆/dp = -320±20 cm^-1/kbar, E ⊥ c: dv?_⊥/dp = -270±20 cm^-1/kbar. These values are discussed in the context of the known functional relationship (for ambient conditions) between v? and R.     

Reassignment of the “Forbidden” character in the Ã1B3u-X̃1Ag transitions of pyrazine-d0, 15N and d4

Contours of bands polarized in the plane of the pyrazine (1,4-diazine) molecule are analyzed in greater detail than before (J. Mol. Spectrosc.11, 257 (1963); 21, 66 (1966)). Largely on the basis of simulations of the pyrazine-¹⁵N and d₄ bands, it is concluded that the vibronic transition moment lies not along the line of the nitrogen atoms but perpendicular to that line. The reassignment requires the vibration responsible for the bands to be ν_10a and the electronic transition(s) which loans intensity to be ¹B_2u. It is confirmed from the derived changes of inertial constants that the two nitrogen atoms lie closer together in the ecited state than in the ground state.     

Variation du champ seuil avec une tension uniaxiale dans FeCl2 A 4,2°K

In FeCl₂, at low temperature, the Fe²⁺ ions present an antiferromagnetic order characterised by an easy magnetization axis perpendicular to the ion-containing planes. There exists a value h_s of a magnetic field parallel to the axis, which transforms the system from an antiferromagnetic order into a saturated paramagnetic order. There we present the variations of h_s at 4·2°K as well as the deformations of the sample, when we impose upon it a strain parallel to the axis without allowing its lateral dilatation. These results, when added to those already obtained with χ_i in similar experiments enable us to determine the variations of the crystalline anisotropy and anti-ferromagnetic exchange (between Fe²⁺ ions belonging to two consecutive planes) in terms of the deformations. Our results concerning h_s are different from the ones obtained when exerting a hydrostatic strain. We compare the two cases by using the relations between elastic constants of FeCl₂ obtained at 300°K and recorded in the Appendix.     

Ultrasonic attenuation determination of Hc1 and Hc2 for ErRh4B4 at 1.5 K

The attenuation coefficient of longitudinal sound waves propagating in ErRh₄B₄ has been measured as a function of applied magnetic field where the propagation direction (q) of the sound waves was oriented either parallel or perpendicular to H_applied. For both orientations there is evidence of a type II-1 superconducting transition at H_c1 for T ≈ 1.5 K. In addition, when q ⊥ H an increase in attenuation is evident at H_c2, which does not appear when q 6 H, consistent with theories developed by Tachiki et al. utilizing supercurrent screening of the internal magnetic fields.     

The magnetooptical properties of InSe near the band gap

We have studied the magnetooptical properties of InSe near the band gap by wavelength modulation spectroscopy at 10 K with the electric vectorE perpendicular to theC-axis of the crystal. For the first time, Landau levels have been observed from which we obtain the reduced effective mass perpendicular to $\overrightarrow{C}$, μ_⊥ = (0.090 ± 0.010)m₀. The n = 1, 2 and 3 exciton lines along with a diamagnetic shift of the n = 1 level have been resolved from which we evaluate the Rydberg constant R = 15.0 ± 0.5 meV, the diamagnetic shift σ₁ = 14 ± 2 meV/(kG)² and ε_⊥ε_∥ = 81 ± 8. Evidence of the presence of the γ and ε polytypes in the sample due to stacking faults is given.     

Dielectric dispersion and its temperature variation in calcite single crystals

The variation of dielectric constant and loss of calcite single crystals with frequency and temperature over wide ranges (10² to 2.4×10¹⁰c/s and ?193 to 450 °C) in two orientations of the crystal, namely, the electric field (i) parallel to the optic axis and (ii) perpendicular to the optic axis, has been studied and the results reported. The dielectric constant at 25 °C in both orientations (E‖, 7.8 andE⊥, 8.2) is frequency-independent. The dielectric loss shows a minimum in the 10⁵ to 10⁸ c/s region. Dielectric constant as a function of temperature exhibits two distinct regions: frequency-independent (?193 to 50 °C) and frequency-dependent (50 to 450 °C). The temperature variation of specific conductivity can be represented by a sum of two exponentials, e.g., \(\sigma = Ae^{ - E_1 /kT} + Be^{ - E_2 /kT}\) with the usual notation; the constants have different values in the two orientations. The intrinsic conductivity with larger activation energy (E‖ 0.91 eV andE⊥ 1.05 eV) is attributed to the movement of lattice vacancies in the crystal. The 15% larger value for the intrinsic activation energy alongE⊥ optic axis over the other orientation may be due to a probably large electrostatic interaction between the charge carriers and the lattice ions in this orientation.     

Hot́ hole anisotropic effect in silicon and germanium

Anisotropic effect of the hole drift velocity in silicon and germanium has been investigated with the time of flight technique by applying the electric field parallel to the <100 and <111 crystallographic axis. The measurements were performed for electric fields ranging from 10 to 3 × 10⁴V/cm and temperatures from 40 to 200°K. The results indicate that the anisotropic effect v_d<100/v_d<111 increases with decreasing temperature and increasing electric field, and reaches a saturation value at high electric fields (? 10⁴V/cm). The maximum anisotropic effect for Ge is 1.25 at 40°K and for Si is 1.2 at 45°K. A qualitative analysis of the experimental data indicates that the anisotropic effect is due to the warped heavy-valence-band shape.     

Effect of structural water on the critical characteristics of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) in a humid argon atmosphere and subsequent annealing (930°C) on the critical parameters of a highly textured YBa₂Cu₃O_6.9 has been studied. During annealing at T = 200°C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at T = 930°C and subsequent oxidation, the superconducting characteristics (j _c , B _1c , and F _p ) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the c axis (⊥ c). The optimum conditions of double annealing substantially increasing the critical current density (j _c ≥ 10⁴ A/cm²) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to ～3 T, the critical current density j _c is isotropic despite the conservation of high texture in the samples.     

Anisotropy of the magnetoresistive properties of granular high-temperature superconductors resulting from magnetic flux compression in the intergrain medium

To elucidate the origin of the well-known anisotropy of the magnetoresistive properties of granular high-temperature superconductors (HTSs), which is related to the mutual orientation of magnetic field H and transport current j, we investigate the hysteretic dependences of magnetoresistance R(H) of the yttrium HTS sample at the perpendicular (H ⊥ j) and parallel (H || j) configurations. The hysteretic R(H) dependences are analyzed using the concept of the effective field in the intergrain boundaries through which superconducting current carriers tunnel. The effective degree of magnetic flux compression in the intergrain medium at the perpendicular configuration was found to be twice as much as at the parallel one. This approach explains well the anisotropy of the magnetoresistive properties of granular HTSs, which was previously reported by many authors, and the temperature dependences of the resistance in the resistive transition region.