Structural study on the crystallization behavior of Sb3Te2 alloy for phase change memory

The atomic crystal structure of the Sb₃Te₂ binary alloy has been investigated with high‐resolution transmission electron microscopy (HR‐TEM) and fast Fourier transform patterns. As a result, there is inconsistency between the previous theoretical model and the experimental result. But, from the calculations of lattice parameters and the number of layers in the unit cell, it is found that the Sb₃Te₂ alloy can be crystallized into the Sb₂Te structure with P m1 space group and the difference between stacking sequences of the Sb₃Te₂ and the Sb₂Te structures has been discussed with the proposed atomic arrangement model of unit cells. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Δn) and the dielectric constant (epsilon_⊥) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T_PN) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T_PN, indicating the presence of polarised domains. Our measurements show that ΔT_PN(= T_PN(E)-T_NI(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E². It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.     

Colloidal inclusions in smectic films with spontaneous bend

Inclusions in free-standing smectic films are simple model systems for two-dimensional anisotropic dispersions. From theory and experiment, different topologies of elastic distortions of the embedding liquid crystal are known. Quadrupolar and different dipolar defect configurations in the vicinity of the inclusion are possible, and these configurations determine the type of interactions between the inclusions. The quadrupolar configuration is often energetically preferred. We show, however, that dipolar director configurations around inclusions can be energetically favourable over quadrupolar arrangements in chiral smectics, as a consequence of a spontaneous-bend term in the elastic-energy formulation. As the inclusion size influences the selection of the deformation types, the corresponding spontaneous-bend constant can be estimated for the strong anchoring limit if the c -director fields around inclusions of different diameters are taken into account.     

Order-disorder transformation in the binary MPt3 and ternary CuMPt6 (M=3d metals) alloys

The order-disorder(O-D) transformation in the binary MPt₃ and the ternary CuMPt₆ (M=3d transition metals) alloys were studied. The O-D transformation temperatures (T_c) were calculated by combining the method of pseudopotentials with the static concentration waves. The method originally proposed by Khachaturyan [Progress in Materials Science, vol. 22, Pergamon Press Ltd., Oxford, 1978, pp. 47-49; Phys. Met. Metallography 13 (1962) 493; Sov. Phys. Solid State 5 (1963) 16; Sov. Phys. Solid State 5 (1963) 548] was applied to the case of ternary CuMPt₆ alloys by using the concept of a pseudoatom. It is found that this model predicts results that are consistent with the experimental data.     

Polarizabilities and hyperfine structure constants of the low-lying levels of barium

The results of ab initio calculation of energies, hyperfine structure constants and static polarizabilities for several low-lying levels of barium are reported. The effective Hamiltonian for the valence electrons has been constructed in the frame of CI+MBPT method and solutions of many electron equation were found. Using the wave functions obtained the hyperfine structure constants and static polarizabilities were calculated. Received: 22 June 1998 / Revised: 2 September 1998 / Accepted: 15 September 1998     

Optimal wavelet filter for suppression of coherent noise with an application to seismic data

Wavelet analysis is combined with the Karhunen-Loève (KL) transform into an innovative hybrid method for locally filtering coherent noise. In applying our method, the original time series is first decomposed with wavelet transform, the scales more contaminated with noise are reduced by an attenuation factor A_f, and the signal is reconstructed using the inverse wavelet transform. Then the KL transform is applied to the reconstructed signal and the behavior of the first energy modes is analyzed as a function of A_f. The point corresponding to a minimum in the first mode is identified with the maximum extraction of the coherent noise. Our methodology is applied with success to seismic data with the aim of locally extracting the relevant coherent noise, namely the ground roll noise. The procedure can be easily extended to other situations where an undesirable signal is associated with a specific set of energy modes.     

Magnetic trapping of strongly-magnetized Rydberg atoms

Effective magnetic moments of drift Rydberg atoms in strong magnetic fields are obtained for different energy and angular-momentum states. Classical two-body trajectory calculations and quantum-mechanical one-body calculations are employed. For heavy atoms such as rubidium, the trapping dynamics can largely be explained by the net magnetic moment due to the cyclotron and the magnetron motion of the Rydberg electron. In light Rydberg atoms such as hydrogen, the intrinsic two-body nature of the dynamics becomes manifest in that the ionic motion significantly contributes to the effective magnetic moment. Also, light drift Rydberg atoms exhibit an anisotropic response to field-inhomogeneities parallel and transverse to the magnetic-field lines. The results are relevant to magnetic trapping of Rydberg atoms in strong-magnetic-field atom traps.     

Structure and topography modifications of austenitic steel surfaces after friction in sliding contact

Friction experiments between two austenitic stainless steel (AISI 304L) surfaces in sliding contact were carried out under very low loads in two liquid environments, namely demineralized water and methanol, in order to study the correlation between surface damage (wear and surface topography) and structural modifications (phase formation and microstructure). The particularity of our approach was to perform the tests under Hertzian pressures, which were several orders of magnitude lower than the elastic limit of stainless steel. The structural modifications produced during friction were analysed by X-ray diffraction and transmission electron microscopy and the surface topography was studied by scanning electron microscopy and three-dimensional (3D) profiling. Whatever the experimental conditions investigated, the morphology of the damage observed on both surfaces consisted of very fine, smooth and parallel grooves typical of an abrasive wear process of a ductile material caused by the ploughing action of hard particles. From the beginning of the tests, the transformation of austenite into martensite was observed in the superficial layers and the dominant presence of martensite was identified in the wear debris. These results suggest that, under our experimental conditions, abrasion is the dominant mechanism of material removal. Received: 12 March 2002 / Accepted: 3 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/4949-6692, E-mail: jean.paul.riviere@univ-poitiers.fr     

Theoretical model of the transition between C1 and C2 chevron structures in smectic liquid crystals

We present a study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. The coexistence of C1 and C2 chevron structures within a single cell causes zigzag defects to occur and may affect the optical characteristics of the cell. By standard Euler-Lagrange minimisation of the total energy of the system, we obtain analytical expressions for the equilibrium director cone angle in the two chevron states. These in turn allow us to compare the total energies of the states and determine the globally stable chevron profile. We show that analytical predictions for the critical transition temperature, which depends on anchoring strength and pretilt angle, are in good agreement with those obtained numerically.