Low-temperature properties of some disordered systems from the statistical properties of nearly degenerate two-level excitations

The thermal fluctuations that exist at very low temperature in disordered systems are often attributed to the existence of some two-level excitations. In this paper, we revisit this question via the explicit studies of the following 1D models (i) a particle in 1D random potentials (ii) the random field Ising chain with continuous disorder distribution. In both cases, we define precisely the two-level excitations and their statistical properties, and we show that their contributions to various observables are in full agreement at low temperature with the the rigorous results obtained independently. The statistical properties of these two-level excitations moreover yield simple identities at order T in temperature for some generating functions of thermal cumulants. For the random-field Ising chain, in the regime where the Imry-Ma length is large, we obtain that the specific heat is dominated by small non-universal excitations, that depend on the details of the disorder distribution, whereas the magnetic susceptibility and the Edwards-Anderson order parameter are dominated by universal large excitations, whose statistical properties only depend on the variance of the initial disorder via the Imry-Ma length.Received: 21 July 2004, Published online: 5 November 2004PACS: 75.40.Cx Static properties - 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion     

Dynamics of concentrated colloidal suspensions during drying --aging,rejuvenation and overaging

We report on the slow dynamics of concentrated colloidal suspensions during drying and rewetting under conditions of reversible concentration changes without coalescence or aggregation. We used multispeckle diffusing-wave spectroscopy to monitor the slow dynamics of particles. We found that the relaxation of the suspensions exhibits successively slowing-down, acceleration and a stationary regime during drying at constant rates. Under rewetting conditions, we observed slowing-down and a stationary regime. The characteristic time of the stationary regime is inversely proportional to the rate of concentration change and identical for both drying and rewetting. We explain these regimes as aging (overaging), rejuvenation and plastic flow of the suspensions induced by a deviatoric stress (a combination of compressive and elongational stresses) which is induced by the uniaxial compressive strain generated by evaporation.Received: 3 June 2004, Published online: 3 August 2004PACS: 82.70.Dd Colloids - 47.55.Mh Flows through porous media - 42.25.Dd Wave propagation in random media     

Heterogeneous nature of the dynamics and glass transition in thin polymer films

Recent experiments have demonstrated that the dynamics in liquids close to and below the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. We use here a model proposed recently for explaining these features, and show that the heterogeneous nature of the dynamics has important consequences when considering the dynamics of thin films. We show how the dominant relaxation time in a thin film is changed as compared to the bulk, as a function of the thickness, the interaction energy with the substrate, and the temperature. The corresponding time scales cover the so-called VFT (or WLF) regime and vary between 10^-8 s to 10⁴ s typically. In the absence of interaction, our model allows for interpreting suspended films experiments, in the case of small polymers for which the data do not depend on the polymer weight. The interaction leads to an increase of for an interaction per monomer of the order of the thermal energy T. This increase saturates at the value corresponding to strongly interacting films for adsorption energies slightly larger and still of order T. In particular, we predict that the shift can be non-monotonous as a function of the film thickness, in the case of intermediate interaction strength.Received: 1 July 2004, Published online: 26 October 2004PACS: 64.70.Pf Glass transitions - 61.41. + e Polymers, elastomers, and plastics - 68.15. + e Liquid thin films     

Spin-Peierls lattice fluctuations and disorders in CuGeO$_\mathsf{3}$ and its solid solutions

The inorganic quasi-one dimensional (1D) S = 1/2 antiferromagnetic (AF) system CuGeO₃ undergoes a 2nd order spin-Peierls (SP) phase transition at T _SP = 14.2 K. In this study we present an X-ray synchrotron radiation investigation which confirms that the SP instability is announced by an important regime of pretransitional structural fluctuations which have been detected until 36 K. Furthermore we show that these fluctuations are 1D above 24 K, a feature expected for a structural instability triggered by the Cu^{2 +} chains of spin 1/2. By extrapolating the thermal dependence of the correlation length in the chain direction, we show that formation of singlet dimers begins at about 50 K, a temperature that we identify as the mean field temperature of the SP chain. The critical nature of the pretransitional fluctuations does not change when low amounts (<1%) of non-magnetic dopants substitute either the Cu site (case of Zn and Mg) or the Ge site (case of Si and Al) of CuGeO₃. However, the spatial extension of the fluctuations is considerably reduced when the magnetic dopant Ni substitutes the Cu site. In the SP ground state of doped materials we have been able to detect, in addition to the superlattice SP reflections previously observed, a very weak anisotropic diffuse scattering. We give evidences that this scattering originates from dopant-induced quasi-1D domains in which the dimerisation is perturbed. If we assume that each domain is limited by a soliton-antisoliton pair, pinned either on the substituent of the Cu site or by the deformation field induced by the substituent of the Ge site, we deduce that the soliton and antisoliton are separated by a distance of about Å, and that the soliton half width amounts to about Å. With these numbers we are able to account for the rate of decrease of T _SP as a function of the dopant concentration, and to deduce the critical concentration above which the long-range SP order vanishes. The overall size of the perturbed domains thus obtained, Å, is comparable with the size of the magnetic inhomogeneities determined by muon spin spectroscopy in the AF phase of doped CuGeO₃.Received: 22 December 2003, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 61.72.Dd Experimental determination of defects by diffraction and scattering - 75.45. + j Macroscopic quantum phenomena in magnetic systems     

The pion mass in finite volume

We determine the relative pion mass shift due to the finite spatial extent L of the box by means of two-flavor chiral perturbation theory and the one-particle Lüscher formula. We use as input the expression for the infinite volume forward scattering amplitude up to next-to-next-to-leading order and can therefore control the convergence of the chiral series. A comparison to the full leading order chiral expression for the pion mass in finite volume allows us to check the size of subleading terms in the large-L expansion.Received: 19 November 2003, Published online: 18 February 2004     

Optical evidence for the proximity to a spin-density-wave metallic state in $\mathsf{Na_{0.7}CoO_2}$

We present the optical properties of Na_0.7CoO₂ single crystals, measured over a broad spectral range as a function of temperature (T). The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate ( ) and effective mass (m ^*) of the itinerant charge carriers. We find that at low temperatures and for . This suggests that Na_0.7CoO₂ is at the verge of a spin-density-wave metallic phase.Received: 6 August 2004, Published online: 9 September 2004PACS: 78.20.-e Optical properties of bulk materials and thin films - 74.70.Dd Ternary, quaternary and multinary compounds (including Chevrel phases, borocarbides, etc.) - 75.30.Fv Spin-density waves     

Kinetic energy distributions in pionic hydrogen

A framework for analyzing the Doppler broadening of the X-ray lines in pionic hydrogen is proposed. It is shown that the kinetic energy distributions at the instant of the (n = 2-4) radiative transitions are related to each other. In order to establish the connection, the collisional processes for pionic hydrogen scattering from hydrogen atoms and molecules have been calculated in different models. The proposed method can be used to determine reliably the strong interaction width of the ground state of pionic hydrogen from the X-ray line profiles measured recently at the Paul-Scherrer-Institut.Received: 6 August 2004, Published online: 28 September 2004PACS: 34.50.-s Scattering of atoms and molecules - 36.10.Gv Mesonic atoms and molecules, hyperonic atoms and molecules     

Size and viscoelasticity of spatially confined multilamellar vesicles

We studied viscoelastic properties and scaling behavior of multilamellar vesicles (MLVs) confined between two parallel plates as a function of the shear rate and sample thickness (gap size between parallel plates). The rheological properties are classified into two regimes; the shear-thinning regime at high shear rates and the shear-thickening regime at low shear rates. In the former, the MLV radius results from the mechanical balance between the effective surface tension σ_eff and viscous stress force. The MLV radius is independent of the gap size. σ_eff estimated by van der Linden model is 2.1 ±0.15 ×10^-4 Nm^-1 corresponding to the same value obtained by SANS measurement. Power law exponents for the steady state viscosity and yield stress against pre-shear rate ( , ) well agree with prediction based on the layering of membranes. Therefore, viscoelastic properties in this regime could be modeled by assuming that the dynamics of MLVs are driven by layering of MLV polydomains, which could be accompanied by the viscous dissipation, i.e., the stress relaxation on the MLV, induced by continuous sequence of yields of MLVs. The flow curve is empirically explained by the assumption of a relaxation time for the MLV shape. In the latter, however, scaling laws observed in the shear-thinning regime break down. The MLV radius increases when the gap size is reduced below the threshold value and MLV is no longer formed at very small gap sizes. Different dynamics from the shear-thinning regime seem to dominate the viscoelasticity.     

Forward $\pi^\circ$ production and associated transverse energy flow in deep-inelastic scattering at HERA

Deep-inelastic positron-proton interactions at low values of Bjorken-x down to which give rise to high transverse momentum -mesons are studied with the H1 experiment at HERA. The inclusive cross section for -mesons produced at small angles with respect to the proton remnant (the forward region) is presented as a function of the transverse momentum and energy of the and of the four-momentum transfer Q ² and Bjorken-x. Measurements are also presented of the transverse energy flow in events containing a forward -meson. Hadronic final state calculations based on QCD models implementing different parton evolution schemes are confronted with the data.Received: 8 April 2004, Revised: 7 July 2004, Published online: 30 July 2004Dedicated to the memory of Krzysztof Rybicki     

Concerning the Landau pole in 3-3-1 models

Some 3-3-1 models predict the existence of a non-perturbative regime at the TeV scale. We study in these models and their supersymmetric extensions, the energy at which the non-perturbative limit and a Landau-like pole arise. An order of magnitude for the mass of the extra neutral vector boson, , present in these models is also obtained.Received: 12 October 2004, Revised: 1 November 2004, Published online: 21 December 2004PACS: 12.60.Cn, 12.60.Jv     

The total virtual photoabsorption cross section,deeply virtual Compton scattering and vector-meson production

Based on the generic two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low , we stress the intimate direct connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA. Received: 16 February 2004, Revised: 16 July 2004, Published online: 2 September 2004 Supported by Deutsche Forschungsgemeinschaft, contract number schi 189/6-2. An erratum to this article is available at .     

Unusual behaviour of poly(ethylene-oxide) in aqueous mixtures

The model system of poly(ethylene-oxide) or PEO, where the changing hydrogen-bond connectivity of the water has large effect on the conformation of the polymer chain, in mixtures of water and acetonitrile, is experimentally studied. The results show the existence of a threshold water content in the system at which the 3d connectivity of the water network begins. Unusual expansion of the polymer chain, an effect larger than that observed in either of the pure solvents, is seen. Upon addition of small amounts of a monovalent salt, binding of ion to polymer takes place in pure acetonitrile solutions. Salt ions begin to co-ordinate with water molecules at the same solvent ratio as the threshold for water network formation. Ions now no longer complex to PEO; instead, hydrogen bonding of water to the polymer strongly dictates conformation in this regime.Received: 10 September 2004, Published online: 3 November 2004PACS: 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 61.12.Ex Neutron scattering techniques (including small-angle scattering) - 36.20.Ey Conformation (statistics and dynamics)     

Theoretical notes on dense polymers in two dimensions

Two models of dense two-dimensional (2d) polymers are considered: (1) when chain intersections in 2d are totally forbidden, and (2) when they are allowed to some extent. It is shown that both polymer chain statistics and dynamics are entirely different for the two models. In the first case studied by Duplantier in 1986 polymer chains are essentially segregated and are characterized by non-classical exponents. The contact line between segregated chains is fractal which leads to an unusual demixing behavior in 2d blends. In the second case (crossings are allowed) polymer coils are overlapping and show mean-field statistics with logarithmic corrections. The correlation function of concentration fluctuations in this system is predicted to exhibit a universal long range power tail (1/r ⁴) which is due to non-mean-field effects. The dynamical behavior of the two models is even more drastically different: The first model is characterized by a relatively fast dynamics with conformational relaxation time (i.e. t _N is slightly shorter than the Rouse time ). On the other hand an exponentially slow dynamics is predicted for model 2 (with 3d entanglements).Received: 3 September 2003PACS: 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 67.70. + n Films (including physical adsorption)     

Long relaxation times and tilt sensitivity in Rayleigh Bénard turbulence

We study the influence of a small tilt angle ( rd) on the Nusselt number in a 1/2 aspect ratio Rayleigh-Bénard cell, at high Rayleigh number (5 x 10¹¹ < Ra < 4 x 10¹²). The small decrease observed is interpreted as revealing a two rolls structure of the flow. Transitions between different global flows are also observed, on very long times, comparable to the diffusion time on the whole cell. The consequence is that the Nusselt number observed in most high Ra experiments should significantly depend on initial conditions.Received: 19 May 2004, Published online: 12 August 2004PACS: 92.60.Ek Convection, turbulence, and diffusion - 47.27.Te Convection and heat transfer - 44.25. + f Natural convection     

Implications of constraints on mass parameters in the Higgs sector of the nonlinear supersymmetric SU(5) model

By using the non-relativistic effective Lagrangian approach to bound states, a complete expression for the isospin-breaking corrections to the energy levels and the decay widths of kaonic hydrogen is obtained up-to-and-including in QCD. It is demonstrated that, although the leading-order corrections at emerging due to the unitarity cusp are huge, they can be expressed solely in terms of the KN S-wave scattering lengths. Consequently, at leading order, it is possible to derive parameter-free modified Deser-type relations, which can be used to extract the scattering lengths from the hadronic atom data.A. Rusetsky: On leave of absence from HEPI, Tbilisi State University, University st. 9, 380086 Tbilisi, Georgia     

Chemical structure and internal diffusion within polymer chains in the melt

The single chain dynamics of polydimethylsiloxane in the melt is studied by means of quasielastic neutron scattering. For this polymer the wave vector range 0.03 Å^–1q0.30 Å^–1 covers the regime of universal modes as well as local diffusive processes. A model is described which incorporates the specific chemical structure of the macromolecule and allows to interpret our data in the full wave vector regime. The only parameter which enters the model, the monomer diffusion constantD _m, is found to be (1.2±0.2)·10^–5 cm²s^–1.     

Near-threshold production of a0(980)-mesons in the reaction \rm {pp \rightarrow d K^ + \bar{K^0}}

Using an effective Lagrangian approach as well as the Quark-Gluon Strings Model, we analyze near-threshold production of a ₀(980)-mesons in the reaction as well as the background of non-resonant -pair production. We argue that the reaction at an energy release MeV is dominated by the intermediate production of the a ₀(980)-resonance. At larger energies the non-resonant -pair production --where the kaons are produced in a relative P-wave-- becomes important. The effects of final-state interactions are evaluated in a unitarized scattering length approach and found to be in the order of a 20% suppression close to threshold. Thus, in present experiments at the Cooler Synchrotron COSY-Jülich for MeV the a ₀ ⁺ signal can reliably be separated from the non-resonant background.Received: 26 February 2004, Published online: 21 September 2004PACS: 25.10. + s Nuclear reactions involving few-nucleon systems - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV)     

Growth of resistance with density of scatterers in one dimensional wave propagation

We study wave propagation in a one-dimensional disordered array of scattering potentials. We consider three different ensembles of scatterer configurations: anN-ensemble with a fixed numberN of scatterers, anL-ensemble with a varying number of scatterers distributed over a fixed lengthL, and anNL-ensemble where bothN andL are fixed. The latter ensemble allows a detailed study of the mean resistance and its variance for a fixed lengthL as the number of scatterersN increases. We find that the Landauer result, which predicts an exponential increase of the mean resistance withN, is valid only in the low-density regime. At high density the mean resistance grows exponentially with N and the concept of optical potential applies. In the crossover regime we find an interesting resonance.     

A new analysis of $\pi K$ scattering from Roy and Steiner type equations

With the aim of generating new constraints on the OZI suppressed couplings of chiral perturbation theory a set of six equations of the Roy and Steiner type for the S- and P-waves of the scattering amplitudes is derived. The range of validity and the multiplicity of the solutions are discussed. Precise numerical solutions are obtained in the range GeV which make use as input, for the first time, of the most accurate experimental data available at GeV for both and amplitudes. Our main result is the determination of a narrow allowed region for the two S-wave scattering lengths. Present experimental data below 1 GeV are found to be in generally poor agreement with our results. A set of threshold expansion parameters, as well as sub-threshold parameters are computed. For the latter, a matching with the SU(3) chiral expansion at NLO is performed.Received: 28 October 2003, Revised: 12 December 2003, Published online: 13 February 2004Work supported in part by the EU RTN contract HPRN-CT-2002-00311 (EURIDICE) and by IFCPAR contract 2504-1.