3-d Model for strain ordering in steel: I Static effects

We present a spring-defect model in 3-dimensions to describe the connection between elastic distortion and interstitial carbon ordering associated with phase transition from a body centred cubic (BCC) to body centered tetragonal (BCT) structure in BCC metals such as α-iron. The presence or the absence of the carbon is modelled in terms of a pseudo spinŝ=+1or -l.An Ising interaction between carbon atoms is recovered after eliminating the lattice degrees of freedom, which is longranged. The coupling between the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and the variation of the lattice parameter with carbon concentration. The mean field results for the paraelastic to ferroelastic transition, lattice parameter and static compliance are presented. The significant feature of this calculation is not only a derivation of the defect-defect interaction, but also an explicit calculation of the strain dipole tensor associated with each defect, from a microscopic model.     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

End-chain segment ordering in lamellar sublayers of a diblock copolymer

The segmental ordering within a lamellar polystyrene-polydimethylsiloxane diblock (PS-PDMS) was examined by NMR. A non-uniform stretching of the PDMS chains was evidenced. Although chain segments displaying isotropic reorientational motions are present along the chains, relaxation measurements on the free PDMS extremities demonstrate that these latter are oriented and submitted to a non-zero constraint. This specific result allows to precise some features of the order distribution within the lamellae. Received: 27 March 2003 / Accepted: 9 May 2003 / Published online: 4 June 2003 RID="a" ID="a"e-mail: deloche@lps.u-psud.fr     

Effects of charge-density-wave depinning on the low frequency shear compliance of NbSe3

We report on measurements of the voltage dependence of the complex shear compliance of two crystals of the charge-density-wave (CDW) conductor NbSe₃, for torsional frequencies between 1 Hz and 71 Hz. For both samples, there is a frequency independent, ∼1% increase of the magnitude of the compliance when the voltage exceeds the threshold for CDW depinning, but the internal friction has a striking sample dependence which we do not understand. For one sample, there is a frequency independent decrease in internal friction with CDW depinning, suggesting that the elastic changes are not relaxational and might reflect changes in the screening of the crystal strain by the CDW. For the second sample, the sign and magnitude of the change in internal friction is strongly frequency dependent, which we associate with a change in screening due to the finite electron diffusion time. The second sample also exhibits a frequency dependent peak in internal friction near threshold that may reflect relaxation of the CDW phase. Received 14 August 2001 and Received in final form 3 October 2001     

The flexoelectric effect in nematic liquid crystals: A statistical- mechanical approach

The flexoelectric coefficients e₁ and e₃ for polar liquid crystals, such as 4-n-pentyl- 4^′-cyanobiphenyl, are investigated theoretically by means of an integral equation approach, which takes into account translational, orientational correlations as well as their coupling. e₁ and e₃ are evaluated from microscopic expressions derived on the basis of a density-functional method. Received 27 April 2001 and Received in final form 27 August 2001     

Behavior of intruder based states in light Bi and Tl isotopes: the study of 187Bi α decay

The excitation energies of the single-particle normal and intruder levels in both ¹⁸³Tl and ¹⁸⁷Bi were measured for the first time via the α decay of ¹⁸⁷Bi produced in the ⁹⁷Mo(⁹²Mo,pn)¹⁸⁷Bi reaction. The previously unobserved ¹⁸⁷Bi ground state (h_9/2) to ¹⁸³Tl ground state (s_1/2) α transition was identified, establishing the ¹⁸⁷Bi intruder state excitation energy to be 112(21) keV, 70 keV less than that of the same level in ¹⁸⁹Bi. Received: 10 September 1998 / Revised version: 18 December 1998     

Advanced memory effects in the aging of a polymer glass

A new kind of memory effect on low frequency dielectric measurements on plexiglass (PMMA) is described. These measurements show that cooling and heating the sample at constant rate give an hysteretic dependence on temperature of the dielectric constant ε. A temporary stop of cooling produces a downward relaxation of ε. Two main features are observed (i) when cooling is resumed ε goes back to the values obtained without the cooling stop (i.e. the low temperature state is independent of the cooling history) (ii) upon reheating ε keeps the memory of all the cooling stops (Advanced memory). The dependence of this effect on frequency and on the cooling rate is analyzed. The memory deletion is studied too. Finally the results are compared with those of similar experiments done in spin glasses and with the famous experiments of Kovacs. Received 24 September 2001 and Received in final form 20 November 2001     

Nuclear effects and higher twists in F3 structure function

We analyze the CCFR collaboration iron target data on the xF₃ structure function making particular emphasis on the extraction of the higher twist contributions from the data. Corrections for nuclear effects are applied in order to extract data on the structure function of the isoscalar nucleon. Our analysis confirms the observation made earlier, that the higher twist terms depend strongly on the level to which QCD perturbation theory analysis is applied. We discuss the impact of nuclear effects on the higher twist term as well as on the QCD scale parameter extracted from the fit to data. Received: 4 August 2000 / Accepted: 17 September 2000     

First observation of excited states in 184Pb: spectroscopy beyond the neutron mid-shell

Excited states have been identified for the first time in ¹⁸⁴Pb, the first even-even Pb isotope beyond the 82 < N < 126 mid-shell, using the recoil-decay tagging (RDT) technique. A collective band built on the first-excited 2⁺ state has been observed. This resembles those seen in ^186,188Pb and the Hg isotones, and can thus be associated with a prolate-deformed shape. Variable moment of inertia (VMI) fits of the prolate 0⁺ level energies in ^184,186,188Pb indicate that the minimum appears at N= 103, the same neutron number at which the corresponding minimum in Hg isotopes is observed. Received: 19 May 1998 / Revised version: 1 July 1998     

Alpha-preformation in even-even alpha emitters: An alpha-decay without tunnelling approach

An ‘alpha-decay without tunnelling’ approach shows that alpha-preformation probability in trans-lead even-even alpha-emitters lies in the range of 0·17–0·08, indicating a clear shell-closure effect atN = 126 and exhibits appreciably enhanced values in open-shell nuclei and executes a zig-zag nature with increasing neutron number in an isotopic series. All these findings are in fair qualitative agreement with experimental observations. Experimental results are found to overestimate this parameter for these heavy deformed nuclei.     

Comparison of XYZ Model Fitting Functions for 111Cd in In3La

The XYZ model describes the interaction between nuclear probes and an electric field gradient that fluctuates among three orthogonal directions. The model presents a means to calculate the perturbation function that represents spectra obtained using perturbed angular correlation spectroscopy. Three analytic approximations of the perturbation function have been developed previously, and they are evaluated in the present paper in the context of Cd jumping among In-lattice sites in In₃La.     

EPR study of Dy 3 + ions in DyBa 2 Cu 3 O 6 + x

Concentrated polycrystalline DyBa₂Cu₃O_6+x compounds are studied by X-band EPR spectroscopy. A broad resonance line due to the highly anisotropic EPR spectrum of Dy³⁺ ions is identified on several specimens at low temperatures. Powder simulation of the EPR spectra complies with the ground Kramers doublet predicted by crystal field analysis of Dy³⁺ ions. Calculations of the second and fourth moments of the resonance lines due to the dipole-dipole interactions of Dy³⁺ indicate the presence of substantial exchange narrowing of the dipolar-broadened EPR linewidth. Received 3 June 2001     

Bianchi Type-III Cosmic Strings and Domain Walls Cosmological Model in General Relativity

Bianchi type-III space time is considered in the presence of cosmic strings and thick domain walls source in the frame work of general relativity. Exact cosmological models using various cases of ρ=α λ and p=γ ρ are presented. It is observed that the behavior of these models (with cosmic strings and domain walls), based on their physical and kinematical properties, is found to be identical.     

Aging effects and dynamic scaling in the 3D Edwards-Anderson spin glasses: a comparison with experiments

We present a detailed study of the scaling behavior of correlations functions and AC susceptibility relaxations in the aging regime in three-dimensional spin glasses. The agreement between simulations and experiments is excellent confirming the validity of the full aging scenario with weak sub-aging effects. Received 21 December 2000 and Received in final form 22 February 2001     

Quantum chemical construction of a reduced reaction Hamiltonian and T 1 -relaxation and pure T 2 -dephasing rates for the proton transfer in 3-chlorotropolone

Separating multidimensional problems into that of a relevant system which is coupled to a bath of harmonic oscillators is a common concept in condensed phase theory. Focusing on the specific problem of intramolecular proton transfer in an isolated tropolone derivative, we consider the reactive proton moving in the plane of the molecule as the system and the remaining substrate normal modes as the bath. An all-Cartesian system-plus-substrate Hamiltonian is constructed employing density functional theory. It is then used to determine the temperature-dependent effective reduced reaction Hamiltonian and the state-to-state dissipation rates induced via the system-substrate coupling up to the bi-quadratic order. The important substrate modes for the T₁-relaxation and the pure T₂-dephasing rates, which are either intra- or inter-well in nature, are identified numerically and analyzed physically with molecular details. Received 19 November 2001 and Received in final form 19 February 2002     

Ageing of natural rubber under stress

We report a dynamical-mechanical study of stress relaxation at small deformation in a natural (polyisoprene) rubber well above its glass transition temperature . We find that an almost complete relaxation of stress takes place over very long relaxation periods, even though the elastic network is retained. The relaxation rate and the long-time equilibrium modulus are sensitive functions of temperature which do not follow time-temperature superposition. Many characteristic features of non-ergodic ageing response are apparent at both short and very long times. We interpret the observed behaviour in terms of the nature of rubber cross-links, capable of isomerisation under stress, and relate the results to recent models of slow glassy rheology. Received 22 November 1999 and Received in final form 18 January 2000     

SU(3) coupling schemes for odd-odd nuclei in the interacting boson – fermion – fermion model with both odd proton and odd neutron in natural parity orbits

The SU^BFF(3) dynamical symmetry limits of interacting boson – fermion – fermion model are identified and they are appropriate for heavy deformed odd – odd nuclei for configurations with both the odd proton and odd neutron occupying all the natural parity orbits in the corresponding valence shells. There are three symmetry limits and their correspondence with two quasi-particle (proton-neutron) Nilsson configurations is established; one of the limits mixes both Nilsson n_z's and Λ's and other two limits mix only Nilsson Λ's. The ¹⁹¹Ir (d,t) ¹⁹⁰Ir single nucleon transfer spectroscopic strengths are well described by one of the symmetry limits that mixes only Nilsson Λ's. Received: 22 June 1998     

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO₂) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO₂ and α-AlPO₄ (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO₂.     

Enthalpy recovery of a glass-forming liquid constrained in a nanoporous matrix: Negative pressure effects

The T _g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic tension in the liquid in the pores, it does not develop until near the reduced T _g of the constrained material --well below the bulk T _g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence of an intrinsic size effect as the cause of the reduced T _g. Received 3 September 2001     

Light scattering by longitudinal acoustic modes in supercooled molecular liquids II: microscopic derivation of the phenomenological equations

The constitutive equations for the orientational dynamics of a liquid formed of linear molecules are derived microscopically. The resulting generalised Langevin equations coincide with the phenomenological approach of Dreyfus et al. [1]. Formally exact expressions are given for the phenomenological coefficients and various constraints are shown to be consequences of this microscopic approach. Received 14 August 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: franosch@hmi.de