Structural origin of magnetic anisotropy in Co-Pt alloy films probed by polarized XAFS

Polarized X-ray absorption fine structure (XAFS) measurements at the Co K and Pt L₃ edges show that the perpendicular magnetic anisotropy found in epitaxial fcc CoPt₃ (111) films stems from the existence of anisotropic local ordering. Such ordering, induced during the codeposition process and dependent on the growth temperature, is characterized by preferential CoCo pairs in the film plane, balanced by preferential CoPt pairs out of the plane, resulting from some Co 2D-segregation. Polarized XAFS at the Pt edge reveals similar anisotropic local ordering in epitaxial hcp Co₃Pt (0001) films exhibiting a larger magnetocrystalline anisotropy compared to that of bulk hcp Co. Besides, a polarization dependence of the Co XANES profile is observed only for the Co₃Pt films exhibiting hcp symmetry. Received: 29 April 1998 / Revised: 27 July 1998 / Accepted: 31 August 1998     

Perturbative and nonperturbative processes in adiabatic population transfer

With the help of superadiabatic techniques for quantum systems depending slowly on time, we demonstrate how the total transition amplitude, tracked in time in the usual adiabatic basis, can be decomposed into a perturbative part consisting of terms proportional to powers of the adiabaticity parameter, and a nonperturbative component. The interference of both components underlies the oscillations that accompany transitions in the adiabatic basis. Whereas for traditionally considered systems the final nonadiabatic transition probability is determined by the nonperturbative part alone, this is no longer correct for models describing stimulated Raman adiabatic passage (STIRAP). We explain the recently discovered breakdown of the Dykhne-Davis-Pechukas formula on general grounds, and provide simple, but accurate approximations for transition amplitudes in STIRAP systems. Received: 22 January 1998 / Revised: 17 March 1998 / Accepted: 31 March 1998     

Cooperative diffusion in weakly charged polyelectrolyte gels

A general expression for the cooperative diffusion constant of weakly charged gels is derived as a function of the thermodynamic parameters such as polyelectrolyte concentration, salt concentration, ionic strength, and the degree of crosslinking. In the low concentration range it decreases with the monomer concentration. Received: 30 January 1998 / Revised: 4 May 1998 / Accepted: 6 May 1998     

Wavenumber dependence of structural relaxation in a molecular liquid

Structural relaxation in liquid orthoterphenyl is studied by means of coherent neutron time-of-flight and backscattering spectroscopy over a large temperature range. Not only amplitude and relaxation time but also the spectral line shape show a significant variation with wavenumber. These variations are correlated with the static structure factor, in qualitative accord with mode-coupling results for simple liquids. Even far above the melting point, relaxation remains non-exponential. Received: 5 January 1998 / Revised: 3 April 1998 / Accepted: 19 June 1998     

Theory of site-disordered magnets

In realistic spinglasses, such as , and , magnetic atoms are located at random positions. Their couplings are determined by their relative positions. For such systems a field theory is formulated. In certain limits it reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the Viana-Bray model. The model has a percolation transition, while for RKKY couplings the “concentration scaling” occurs. Within the Gaussian approximation the Ginzburg-Landau expansion is considered in the clusterglass phase, that is to say, for not too small concentrations. Near special points, the prefactor of the cubic term, or the one of the replica-symmetry-breaking quartic term, may go through zero. Around such points new spin glass phases are found. Received: 27 April 1998 / Received in final form: 27 July 1998 / Accepted: 13 August 1998     

Polarisation effect of laser field in inelastic electron - hydrogen collisions

We study the influence of the laser polarization on the electron impact excitation of atomic hydrogen. Our method takes into account the “dressing” of the target states by including the laser-atom interaction to first order time-dependent perturbation theory, while the interaction of the laser field with the incident electron is treated to all orders by using the non relativist Volkov function. The interaction of the fast projectile with the target atom is treated in the first Born approximation. The calculations are performed via two distinct computations. The first one is based on a direct calculation, the second based on a Sturmian approach. Important differences appear between the angular distributions depending on the polarization chosen. Received : 17 february 1998 / Revised : 20 july 1998 / Accepted : 2 september 1998     

Charge transfers in complex transition metal alloys ()

We introduce a new Non-Orthogonal Tight-Binding model, for complex alloys, in which electronic structure is characterized by charge transfers. We give the analytic calculation of a charge transfer, in which overlapping two-center terms are rigorously taken into account. Then, we apply numerically this result to an approximant phase of a quasicrystal of alloy. This model is more particularly adapted to transition metals, and gives realistic densities of states. Received: 7 July 1997 / Revised: 10 March 1998 / Accepted: 3 April 1998     

Quantum-Monte-Carlo simulations of polyethylene

The diffusion Quantum-Monte-Carlo method of solving the Schr?dinger equation is applied to the vibrational ground state of a polyethylene molecule. The results for the ground state energy show good agreement with normal mode analysis. In addition to stretching, bending and torsional interaction van-der-Waals interaction is applied to a single chain showing a decrease of the energy of 5%. The decrease for a polyethylene system of 5 chains with 10 atoms per molecule at the positions of a unit cell is determined to be 4.8% per molecule. Finally first steps towards simulating excited states were performed. Received: 9 February 1998 / Revised: 2 April 1998 / Accepted: 23 April 1998     

Influence of hydrodynamics on the dynamics of a homopolymer

We present an analytical approach of the dynamics of a polymer when it is quenched from a solvent into a good or bad solvent. The dynamics is studied by means of a Langevin equation, first in the absence of hydrodynamic effect, then taking into account the hydrodynamic interactions with the solvent. The variation of the radius of gyration is studied as a function of time. In both cases, for the first stage of collapse or swelling, the evolution is described by a power law with a characteristic time proportional to N ^4/3 (N), where N is the number of monomers, without (with) hydrodynamic interactions. At larger times, scaling laws are derived for the diffusive relaxation time. Received: 10 March 1998 / Received in final form: 15 September 1998 / Accepted: 25 September 1998     

Nuclear fusion in charge-asymmetric muonic molecules

Nuclear fusion reactions in hydrogen-lithium muonic molecules, (where h=p,d,t) are considered and fusion rates from rotational states J=0 of the molecules are presented. Results obtained depend on the isotopic composition of the molecules and range between and . The upper limit for fusion rates from rotational states J=0 of hydrogen-helium muonic molecules, and , equal , is also found. Received: 4 December 1997 / Revised: 30 April 1998 / Accepted: 7 May 1998     

A Fourier optics model of two-beam scanning laser interferometers

The article illustrates the use of Fourier optics to describe the operation of two-beam scanning laser interferometers. It deals with the effect of diffraction on the spatial periodicity of a monochromatic and coherent beam. Particular attention is given to the analysis of systematic errors in high-accuracy laser metrology. The article reviews the special case of plane wave and Gaussian illuminations, examines how beam truncation affects the period of traveling fringes and presents a general relation between the relative wavelength deviation and the impulse standard deviation of the photons. Received: 18 February 1998 / Revised: 21 October 1998 / Accepted: 16 November 1998     

Investigations on the soft-mode mechanism of the ferroelectric phase transition in

Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B / B g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles strongly that of monoclinic . Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June 1998     

Bending of charged flexible membranes due to the presence of macroions

We investigate the bending of flexible charged membranes due to the presence of rigid rodlike macroions in the framework of the Debye-Hückel approximation. When the macroions are fixed in space at some distance from the bilayer the membrane bends towards them; we calculate the exact deformation profile. On the other hand a macroion which is adsorbed on the membrane causes a deflection of the bilayer. Finally, we consider swollen lamellar polyanion/charged-lipid complexes where the macroions are intercalated between charged lipid bilayers. We predict the occurrence of a double adsorption (pinching effect) of the macroion for sufficiently flexible membranes. Received: 9 February 1998 / Revised: 9 June 1998 / Accepted: 2 July 1998     

Splitting of vibrational mode of CO isotopic species in the unstable trapping site in argon matrix

FTIR spectra of the antisymmetric stretch and symmetric bending vibrational modes of isotopic species of CO₂ trapped in argon matrix are recorded at 5 K with a resolution of . A splitting of the mode is observed for the different species in the trapping site termed unstable. Considering this splitting and the width of the line observed for this unstable site, one expects this site to be larger than the stable one. A theoretical model is elaborated to interpret this splitting. Calculations performed at harmonic level of approximation for vibrational modes in a single substitutional site (S1) and a double substitutional one (S2) in a fcc distorted argon lattice to minimize the free energy of the molecule-matrix system, show the splitting to occur in the larger site. The latter result leads us to invert former attribution of the sites: the stable site is a single substitutional one, whereas the unstable site is a double substitutional one. Received: 27 January 1998 / Revised: 30 July 1998 / Accepted: 17 August 1998     

A matrix study of the transfer of polarized radiation in plasmas. Application to neon-like germanium lasers

In order to solve the radiative-transfer equation for polarized beams propagating in plasmas a matrix approach is applied. The solution is the four-components Stokes vector, and the effect of the medium on the state of the radiation is represented by an amplification operator. Our approach is applied to the neon-like germanium 23.6 nm line, when a right-circularly polarized beam is injected into an amplifying plasma. The conditions governing the recovery of the initial polarization state are investigated over the entire spectrum of the output. Received: 10 October 1997 / Revised: 12 January 1998 / Accepted: 27 January 1998     

Longitudinal Stern-Gerlach effect for slow cesium atoms

The mechanical Stern-Gerlach effect is investigated in the case of a slow atomic cloud falling through an inhomogeneous magnetic field featuring a strong longitudinal gradient. The resulting Zeeman sublevel state selection is demonstrated under various experimental conditions. Longitudinal spatial separations are in agreement with numerical simulations that take into account the gravitational acceleration and both the transverse and axial magnetic forces. Since separations greater than 20 cm are obtained, potential applications in atom optics are outlined. Received: 16 February 1998 / Received in final form: 1 April 1998 / Accepted: 6 April 1998     

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