Spin polarizabilities of the nucleon from polarized low-energy Compton scattering

As a guideline for forthcoming experiments, we present predictions from Chiral Effective Field Theory for polarized cross-sections in low-energy Compton scattering for photon energies below 170 MeV, both on the proton and on the neutron. Special interest is put on the role of the nucleon spin polarizabilities which can be examined especially well in polarized Compton scattering. We present a model-independent way to extract their energy dependence and static values from experiment, interpreting our findings also in terms of the low-energy effective degrees of freedom inside the nucleon: The polarizabilities are dominated by chiral dynamics from the pion cloud, except for resonant multipoles, where contributions of the -resonance turn out to be crucial. We therefore include it as an explicit degree of freedom. We also identify some experimental settings which are particularly sensitive to the spin polarizabilities.Received: 19 August 2003, Revised: 30 October 2003, Published online: 27 April 2004PACS: 13.40.-f Electromagnetic processes and properties - 13.60.Fz Elastic and Compton scattering - 14.20.Dh Protons and neutronsH.W. Grießhammer: Permanent address: Technische Universität MünchenT.R. Hemmert: Permanent address: Technische Universität München     

Study by X-ray diffraction and Raman spectroscopy of a Dy@C $_{\mathsf{82}}$ single crystal

We report the first X-ray diffraction and Raman spectroscopy study of a single crystal of the rare-earth endohedral fullerene Dy@C₈₂. The lattice is found to be body-centered cubic (a = 25.79 Å, space group which is at variance with previous reports and confirms that several types of structures can be stabilized in Dy@C₈₂. X-ray diffraction/diffuse scattering methods reveal no low-temperature change down to 12 K for the present structure. The Raman spectroscopy data are comparable to those of other M@C₈₂ endohedral compounds. However, the Dy oxidation state and the force constant of the low-frequency metal-cage stretching mode do not follow the simple relationship observed before.Received: 29 August 2003, Published online: 15 October 2003PACS: 61.10.Nz X-ray diffraction - 61.48.+c Fullerenes and fullerene-related materials - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

Island growth in submonolayer deposition with aggregation and fragmentation

Island growth is studied in the case of island aggregation and break-up during submonolayer deposition. It is demonstrated that the island size distributions are of the scaling form and the mean island size has a power-law behaviour corresponding to hyperthermal deposition conditions. The corresponding scaling exponents are analytically derived and compared with the simulations by the revised particle coalescence method developed here. The scaling exponents are found to depend only on the homogeneity exponents of aggregation and fragmentation kernels.Received: 4 September 2003, Published online: 30 January 2004PACS: 68.55.-a Thin film structure and morphology - 68.35.Fx Diffusion; interface formation - 36.40.Sx Diffusion and dynamics of clustersPresent addressM. Rusanen: Institut Français du Pétrole, Groupe de Modélisation Moléculaire, BP 311, 92852 Rueil-Malmaison, France;Present address J. Asikainen: HRS F27, ETH Zentrum, 8092 Zürich, Switzerland     

Single-crystal structural study of the pressure-temperature-induced dimerization of C<Subscript>60</Subscript>

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C₆₀ single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C₆₀ dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C₆₀-C₆₀ dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.Received: 3 October 2003, Published online: 19 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 61.43.Bn Structural modeling: serial-addition models, computer simulation - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

Confinement of molecular liquids: Consequences on thermodynamic,static and dynamical properties of benzene and toluene

We relate the dynamical behavior of molecular liquids confined in mesoscopic cylindrical pores to the thermodynamic properties, heat capacity and density and to the static structure by combining different experimental methods (H-NMR, calorimetry, elastic and inelastic neutron scattering, numerical simulations). The crystallization process is greatly reduced or avoided by confinement under standard cooling conditions, instead a glass transition temperature T _g at the 1000s time scale can be observed. The pore averaged local structure of the confined liquid is not noticeably affected when excluded-volume corrections are carefully applied, but follows the density changes reflected by the Bragg peak intensities of the porous matrices. The pore size dependence of T _g is dominated by two factors, surface interaction and finite-size effect. For the smallest pores ( , being the van der Waals radius of a molecule), one observes an increase of T _g and a broadening of the transition region, related to the interaction with the surface that induces a slowing-down of the molecules close to the wall. This is confirmed by neutron scattering experiments and molecular-dynamics simulations at shorter time scales and higher temperatures, which indicate a remaining fraction of frozen molecules. For larger pore sizes, taking the decrease of density under confinement conditions into account, a decrease of T _g is observed. This could be related to finite-size effects onto the putative cooperativity length that is often invoked to explain glass formation. However, no quantitative determination of this length (not to mention its T-dependence) can be extracted, since the interaction with the wall itself introduces an additional length that adds to the complexity of the problem.Received: 1 January 2003, Published online: 30 October 2003PACS: 64.70.Pf Glass transitions - 65.20. + w Thermal properties of liquids: heat capacity, thermal expansion, etc. - 61.12.-q Neutron diffraction and scatteringD. Morineau: Present address: Groupe Matiére Condensée et Matériaux, CNRS-UMR 6626, Bâtiment 11A, Université de Rennes 1, F-35042 Rennes, France.V. Teboul: Present address: Laboratoire des Propriétés Optiques des Matériaux et Applications, CNRS-UMR 6136, Université dAngers, F-49045 Angers, France.Y. Xia: Present address: School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.     

Cylindrical molecular brushes under poor solvent conditions: microscopic observation and scaling analysis

Axial contraction of cylindrical molecular brushes of polymethylmethacrylate was observed by static light scattering and scanning force microscopy. Single brush molecules were visualized as worm-like particles whose length was almost three times shorter than the contour length of the backbone. A somewhat larger length was measured by light scattering in a good solvent. A scaling approach has been used to analyze the driving forces for the axial contraction and the conformation of the molecular brushes.Received: 27 November 2003, Published online: 25 March 2004PACS: 36.20.-r Macromolecules and polymer molecules - 36.20.Ey Conformation (statistics and dynamics) - 61.16.Ch Scanning probe microscopy: scanning tunneling, atomic force, scanning optical, magnetic force, etc.S.S. Sheiko: Current address: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290, USAS.A. Prokhorova: Current address: University of Freiburg, IMTEK - Institute for Microsystem Technology, Georges-Koehler-Allee 103,S.A. Prokhorova: 79110 Freiburg, GermanyM. Möller: Present adress: Institut für Technische Chemie und Makromolekulare Chemie, Rhein-Westfälische Technische Hochschule Aachen, Veltmanplatz 8, 52062 Aachen, Germany     

Role of tensile stress in actin gels and a symmetry-breaking instability

It has been observed experimentally that the actin gel grown from spherical beads coated with polymerization enzymes spontaneously breaks the symmetry of its spherical shape, and yields a comet pushing the bead forward. We propose a mechano-chemical coupling mechanism for the initialization of this symmetry breaking. Key assumptions are that the dissociation of the gel takes place mostly in the region of the external surface, and that the rates of the dissociation depend on the tensile stress in the gel. We analyze a simplified two-dimensional model with a circular substrate. Our analysis shows that the symmetric steady state is always unstable against the inhomogeneous modulation of the thickness of the gel layer, for any radius of the circular substrate. We argue that this model represents the essential feature of three-dimensional systems for a certain range of characteristic lengths of the modulation. The characteristic time of the symmetry-breaking process in our model depends linearly on the radius of curvature of the substrate surface, which is consistent with experimental results, using spherical latex beads as substrate. Our analysis of the symmetry-breaking phenomenon demonstrates aspects of mechano-chemical couplings that should be working in vivo as well as in vitro.Received: 16 July 2003, Published online: 23 March 2004PACS: 87.17.Jj Cell locomotion; chemotaxis and related directed motion - 87.15.Rn Reactions and kinetics; polymerization - 62.40. + i Anelasticity, internal friction, stress relaxation, and mechanical resonancesK. Sekimoto: Present address: Université Louis Pasteur, 3 rue de lUniversité, 67084 Strasbourg, France.J. Prost: Also at ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France.F. Jülicher: Present address: Max Planck Institut für Physik komplexer Systeme, Nöthnitzer Str. 38, 01187 Dresden, Germany.A. Bernheim-Grosswasser: Present address: Chemical Engineering Department, Ben-Gurion University, P.O. Box 653, 84105 Beer-Sheva, Israel.     

Comparison of the dynamics of n-hexane in ZSM-5 and 5A zeolite structures

The translational and rotational dynamics of n-hexane adsorbed in ZSM-5 and 5A zeolites has been studied by neutron scattering and deuterium solid-state NMR, at various temperatures. The dynamics of n-hexane is quite different in the two zeolites. In the ZSM-5 structure, the molecule sits in channel segments, the energy barrier between adjacent adsorption sites is small and fast anisotropic motions are observed. In the 5A zeolite, the molecule is adsorbed in -cages; the barrier between adjacent cages is larger so that the molecule spends a longer time exploring the volume of an -cage, leading to a more isotropic motion. The diffusion coefficient of the molecule is reduced by more than 4 orders of magnitude in 5A zeolite compared with ZSM-5.Received: 1 January 2003, Published online: 14 October 2003PACS: 82.75.Jn Measurements and modeling of molecule migration in zeolites - 61.12.Ex Neutron scattering techniques (including small-angle scattering) - 76.60.-k Nuclear magnetic resonance and relaxation     

Local moment model for the Invar effect

It has been shown by small-angle neutron scattering that the magnetic structure of Invar alloys in the ground state is characterized by the occurrence of static magnetic fluctuations on the scale of 10–12 Å. Centres of such fluctuations are iron atoms surrounded by the same sort of atoms in the nearest coordination sphere. These fluctuations are due to antiferromagnetic iron pair interactions. The magnetic part of thermal expansion coefficient of Fe-Ni alloys is evaluated within the framework of the molecular field approximation. The Invar effect is brought about by a mixed exchange interaction and by a strong dependence of the exchange integral J_FeFe on the interatomic distance.     

Integrable scattering theories with unstable particles

We formulate a new bootstrap principle which allows for the construction of particle spectra involving unstable as well as stable particles. We comment on the general Lie algebraic structure which underlies theories with unstable particles and propose several new scattering matrices. We find a new Lie algebraic decoupling rule, which predicts the renormalization group flow in dependence of the relative ordering of the resonance parameters. The proposals are exemplified for some concrete theories which involve unstable particles, such as the homogeneous sine-Gordon models and their generalizations. The new decoupling rule can be validated by means of our new bootstrap principle and also via the renormalization group flow, which we obtain from a thermodynamic Bethe ansatz analysis.O.A. Castro-Alvaredo: Present addressLaboratoire de Physique, Ecole Normale Supérieure de Lyon, Allée dItalie, 69364 Lyon Cedex, FranceJ. Dreißig: Present address Universität Potsdam, Institut für Physik, Am Neuen Palais 10, 14469 Potsdam, GermanyA. Fring: Present address Centre for Mathematical Science, City University, Northampton Square, London EC1V 0HB, UK     

Encapsulation of filaments of a self-assembling bicopper complex in polymer nanowires

We report on the preparation conditions and the characterization by calorimetry and small-angle neutron scattering of a molecular composite material obtained via the heterogeneous nucleation of the fibrils of a thermoreversible gel. This physical process allows encapsulation of monomolecular filaments of a self-assembling bicopper complex into nanosized polymer fibrils. Due to the existence of 1-D arrangements of copper atoms, this material may possess unusual magnetic properties (spin ladders). Received 20 January 1999 and Received in final form 14 May 1999     

Atomic and superatomic structures of solid solutions of zirconium and calcium oxides

Solid solutions of the ZrO₂-CaO system with 7, 9.6, and 20 mol % of calcium oxide are analyzed by X-ray and neutron diffraction and small-angle neutron scattering. Samples are synthesized by the sol-gel method and annealed at the temperature T = 600°C for 4 h. All samples have a tetragonal structure (P4₂/nmc space group). The size of crystallites is 10–20 nm. Small-angle neutron scattering curves are characterized by a peak near the momentum transfer q = 0.5 nm⁻¹. The existence of the peak is caused by correlation between solid nanoparticles in the samples. Density correlation functions are numerically computed from the experimental small-angle scattering cross-sections by the inverse Fourier transform. Correlation lengths, concentrations of scattering nanoparticles, and specific surface areas of the samples are determined.     

Two-dimensional spatially ordered system of nickel nanowires probed by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic nickel nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. The small-angle diffraction pattern exhibits many diffraction peaks, which corresponds to the scattering from highly correlated hexagonal structure of pores and magnetic nanowires. Magnetic contribution to the scattering has complex behavior and cannot be explained without taking into account stray fields located between magnetized nanowires.     

On the structure and conformation of polymer chains in latex particles. I. Small-angle neutron scattering characterization of polystyrene latexes of small diameter

Polystyrene latexes with 40-60 nm diameter and molecular weights ranging from 6 × 10⁴ to 6 × 10⁶ g/mol were synthesized by a two-step equilibrium swelling method, with deuterated polymer forming either the first or second step. Below about 1 × 10⁶ g/mol, small-angle neutron scattering gave zero-angle scattering intensities much higher than expected on the basis of gel permeation chromatography molecular weights. Several models were examined, the leading model based on a core-shell latex structure. The development of such structure was found to depend on the ratio of the radius of gyration of the polymer chain to that of the diameter of the latex particle, reaching a maximum in the range where the polymer chain dimensions are about half that of the latex particle. For the highest molecular weights, normal scattering intensities were found. These results lead to the finding that the polymer chains were compressed in the latex particles with constraining in the range of one to four, for this molecular weight range.     

Molecular dynamics simulation analysis of small-angle neutron scattering by a solution of stearic acid in benzene

Data of small-angle neutron scattering by a diluted solution of stearic acid in deuterated benzene have been analyzed using the results of molecular dynamics simulation. The molecular dynamics simulation approach has been used to calculate the time-averaged distribution of the neutron scattering length density at the interface between the acid molecule and the solvent. It has been shown that the organization of the solvent at the interface with the acid molecule leads to a modulation of the neutron scattering length density and makes a significant contribution to the scattering. This contribution should be taken into account when interpreting the experimental small-angle neutron scattering curves for both the considered system and its analogues.     

Fragment charge correlations and spinodal decomposition in finite nuclear systems

Enhanced production of events with almost equal-sized fragments is experimentally revealed by charge correlations in the multifragmentation of a finite nuclear system selected in ¹²⁹Xe central collisions on ^nat Sn. The evolution of their weight with the incident energy: 32, 39, 45, 50 AMeV, is measured. Dynamical stochastic mean-field simulations performed at 32 AMeV, in which spinodal instabilities are responsible for multifragmentation, exhibit a similar enhancement of this kind of events. The above experimental observation evidences the spinodal decomposition of hot finite nuclear matter as the origin of multifragmentation in the Fermi energy regime.Received: 22 November 2002, Revised: 22 April 2003, Published online: 30 September 2003PACS: 25.70.Pq Multifragment emission and correlations - 24.60.Ky Fluctuation phenomenaG. Tabacaru: Present address: Cyclotron Institute, Texas A&M University, College station, Texas 77845, USA.N. Le Neindre: Permanent address: Institut de Physique Nucléaire, IN2P3-CNRS, F-91406 Orsay cedex, France.     

On pionic hydrogen. Quantum field theoretic,relativistic covariant and model-independent approach

We consider pionic hydrogen , the bound state. Within the quantum field theoretic and relativistic covariant approach we calculate the shift and width of the energy level of the ground state of pionic hydrogen caused by strong low-energy interactions treated perturbatively. The generalization of the Deser-Goldberger-Baumann-Thirring (DGBT) formulas (S. Deser, M.L. Goldberger, K. Baumann, W. Thirring, Phys. Rev. 96, 774 (1954)) is given. The generalized DGBT formulas for the energy level displacement of the ground state of pionic hydrogen contain the non-perturbative and model-independent correction of about , caused by the relativistic covariant smearing of the wave function of the ground state around the origin. This non-perturbative correction is very important for the precise extraction of the S-wave scattering lengths of the scattering from the experimental data on the energy level displacements in pionic hydrogen by the PSI Collaboration. In addition, the shift of the energy level of the ground state of pionic hydrogen is improved by the second-order correction of strong low-energy interactions which is about 0.1%. This testifies the applicability of the perturbative treatment of strong low-energy interactions to the analysis of pionic hydrogen. We show that the width of the energy level of the ground state of pionic hydrogen is valid to all orders of the perturbation theory in strong low-energy interactions.Received: 28 April 2003, Revised: 26 June 2003, Published online: 2 December 2003PACS: 11.10.Ef Lagrangian and Hamiltonian approach - 11.55.Ds Exact S matrices - 13.75.Gx Pion-baryon interactions - 21.10.-k Properties of nuclei; nuclear energy levels - 36.10.-k Exotic atoms and molecules (containing mesons, muons, and other unusual particles)A.N. Ivanov: Permanent address: State Polytechnical University, Department of Nuclear Physics, 195251 St. Petersburg, Russian Federation;N.I. Troitskaya: Permanent address: State Polytechnical University, Department of Nuclear Physics, 195251 St. Petersburg, Russian Federation.     

Magnetic anisotropy of ferromagnetic layered manganites

The magnetic anisotropy of the ferromagnetic layered manganites La is investigated. For x = 0.4, the easy-plane anisotropy comes out predominantly from dipolar interaction but it is partly cancelled out by a significant uniaxial magnetocrystalline anisotropy. This latter one is discussed on the basis of a simple single ion model and compared to existing experimental data in ferromagnetic manganites.Received: 29 April 2003, Published online: 23 July 2003PACS: 75.10.Dg Crystal-field theory and spin Hamiltonians - 75.30.Gw Magnetic anisotropy - 75.30.Vn Colossal magnetoresistance M. Velázquez: Present address: Centre Interdisciplinaire de Recherche Ions-Lasers, UMR 6637 CNRS/CEA/ISMRA, groupe "Matériaux et Instrumentation Laser", 6 Boulevard du Maréchal Juin, 14050 Caen Cedex 4, France     

Measuring a colloidal particle's interaction with a flat surface under nonequilibrium conditions

A new and general approach is proposed to analyze the dynamics of a colloidal particle interacting with a nearby wall. This analysis can be used to determine the acting forces even when the system is non-stationary. As an illustration, we use total internal reflection microscopy to investigate the forces acting on a polystyrene sulfate latex particle as it is receding from a charged glass surface. Received 10 October 2002 Published online: 16 April 2003 RID="a" ID="a"Present address: Department of Polymer Physics, BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany RID="b" ID="b"Present address: Arryx. Inc., Chicago, IL 60601, USA