Dynamics of selfavoiding tethered membranes. I. Model A dynamics (Rouse model)

The dynamical scaling properties of selfavoiding polymerized membranes with internal dimension D are studied using model A dynamics. It is shown that the theory is renormalizable to all orders in perturbation theory and that the dynamical scaling exponent z is given by . This result applies especially to membranes (D=2) but also to polymers (D=1). Received: 5 September 1997 / Accepted: 17 November 1997     

Zero temperature phase transitions in spin-ladders: Phase diagram and dynamical studies of Cu2(C5H12N2)2Cl4>

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields H_c1 and H_c2. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical points in one-dimension, are extracted from (a) the thermodynamic quantities (magnetization) and (b) the dynamical functions (NMR relaxation). A simple mapping of strongly coupled spin ladders in a magnetic field on the exactly solvable XXZ model enables to make detailed fits and gives an overall understanding of a broad class of quantum magnets in their gapless phase (between H_c1 and H_c2). In this phase, the low temperature divergence of the NMR relaxation demonstrates its Luttinger liquid nature as well as the novel quantum critical regime at higher temperature. The general behavior close these quantum critical points can be tied to known models of quantum magnetism. Received: 13 March 1998 / Received in final form and Accepted: 21 July 1998     

Dynamics of selfavoiding tethered membranes. II. Inclusion of hydrodynamic interaction (Zimm model)

The dynamical scaling properties of selfavoiding polymerized membranes with internal dimension D embedded into d dimensions are studied including hydrodynamical interactions. It is shown that the theory is renormalizable to all orders in perturbation theory and that the dynamical scaling exponent z is given by z=d. The crossover to the region, where the membrane is crumpled swollen but the hydrodynamic interaction irrelevant is discussed. The results apply as well to polymers (D=1) as to membranes (D=2). Received: 5 September 1997 / Accepted: 17 November 1997     

Spherical averaged jellium model with norm-conserving pseudopotentials

The effect of non-local norm-conserving pseudo-potentials on the static and dynamic properties of Na_n and Li_n cluster with n=6,8 is investigated in the frame of self-consistent LDA calculations with spherically averaged ionic density (SAPS model). A comparison with previous calculations which use local pseudo-potentials as well with uniform averaged non-local pseudo-jellium calculation has been carried out. A better quantitative agreement with experiments has been found in the calculation of the photoresponse cross-section with respect to either simple jellium or pseudo-jellium model, even in very small clusters, where deviations from sphericity are not negligible. Received: 3 March 1998 / Received in final form and Accepted: 2 June 1998     

Effects of finite thickness on interfacial widths in confined thin films of coexisting phases

The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w₀) is broadened due to capillary waves, which lead to fluctuations, of correlation length of the local interface positions in the directions parallel to the confining walls. We postulate that acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial width w varies as . In the limit of large D this yields or respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model. Received: 19 February 1998 / Received in final form: 2 September 1998 / Accepted: 8 September 1998     

An universal relation between fractal and Euclidean (topological) dimensions of random systems

It is shown that a dimension-invariant form for fractal dimension D of random systems (where d is Euclidean dimension of the embedding space) is in good agreement with results of numerical simulations performed by different authors for critical (p=p _c ) and subcritical (p<p _c ) percolation, for lattice animals, and for different aggregation processes. Received: 9 July 1998 / Revised and Accepted: 12 July 1998     

A phenomenology of boundary lubrication: the lumped junction model

We propose a phenomenological model of boundary lubricated junctions consisting of a few layers of small molecules which describes the rheological properties of these sytems both in the static, frozen, and sliding, molten, states as well as the dynamical transition between them. Two dynamical regimes can be distinguished, according to the level of internal damping of the junction, which depends on its thickness and on the normal load. In the overdamped regime, under driving at constant velocity v through an external spring, the motion evolves continuously from “atomic stick-slip” to modulated sliding. Underdamped systems exhibit, under given external stress, a range of dynamic bistability where the sheared static state coexists with a steadily sliding one. The frictional dynamics under shear driving is analyzed in detail, it provides a complete account of the qualitative dynamical scenarios observed by Israelashvili et al., and yields semiquantitative agreement with experimental data. A few complementary experimental tests of the model are suggested. Received: 18 December 1997 / Received in final form and accepted: 26 March 1998     

Competing bulk and surface fields in d=2 Ising films near bulk criticality: effects of fluctuations

The two-dimensional Ising films with bulk H and surface H₁ fields of opposite sign are studied above and close to bulk criticality by the density matrix renormalization group method. This technique, applied recently to d=2 Ising films, allows for very accurate results for the adsorption as a function of the reduced deviation from the critical temperature .For strong H₁ three distinct classes of shapes of ,determined by the value of the parameter ,where L is the width of the film, are found in agreement with earlier predictions [A. Macioek, A. Ciach, R. Evans, J. Chem. Phys. 108, 9765 (1998)]. For strong and for weak bulk fields is a monotonic function, increasing for strong H and decreasing for weak H, in agreement with scaling analysis and earlier mean-field results. For H between these extreme cases assumes a maximum for and for a depletion occurs, as in recent experiments for critical adsorption in porous materials. For a limited range of H a qualitatively new behavior of is found. In addition to a maximum, a minimum of for appears, which in the mean-field analysis was absent. Received: 11 February 1998 / Received in final form: 16 February 1998 / Accepted: 17 March 1998     

Investigation of magneto-optical effect in transverse magnetic field in He

The non-linear Voigt effect has been studied in He discharge under resonance laser interaction with the He transition. The range of non-linear signal existence was determined. The contribution of the lower and upper states to the overall signal was analyzed. The cross-section for depolarizing collisions with ground state He atoms was estimated for the 3³ D _2,3 He state. Received: 16 June 1998 / Received in final form: 14 December 1998     

Collapse transitions of a periodic hydrophilic hydrophobic chain

We study a single self avoiding hydrophilic hydrophobic polymer chain, through Monte-Carlo lattice simulations. The affinity of monomer i for water is characterized by a (scalar) charge , and the monomer-water interaction is short-ranged. Assuming incompressibility yields an effective short ranged interaction between monomer pairs (i,j), proportional to . In this article, we take (resp. ()) for hydrophilic (resp. hydrophobic) monomers and consider a chain with (i) an equal number of hydro-philic and -phobic monomers (ii) a periodic distribution of the along the chain, with periodicity 2p. The simulations are done for various chain lengths N, in d=2 (square lattice) and d=3 (cubic lattice). There is a critical value p _c (d,N) of the periodicity, which distinguishes between different low temperature structures. For p >p _c , the ground state corresponds to a macroscopic phase separation between a dense hydrophobic core and hydrophilic loops. For p <p _c (but not too small), one gets a microscopic (finite scale) phase separation, and the ground state corresponds to a chain or network of hydrophobic droplets, coated by hydrophilic monomers. We restrict our study to two extreme cases, and to illustrate the physics of the various phase transitions. A tentative variational approach is also presented. Received: 10 March 1998 / Received in final form: 25 June 1998 / Accepted: 1st July 1998     

Well-defined Co clusters embedded in an Ag matrix: A model system for the giant magnetoresistance in granular films

Well-defined granular Co/Ag films have been prepared by the co-deposition of in-beam prepared Co clusters and Ag atoms. In this way we were able to study the giant magnetoresistance (GMR) as a function of mean Co cluster size for a fixed Co cluster volume fraction v_cl as well as a function of v_cl for a fixed . Mean Co cluster size has been varied between and 6.9 nm, Co cluster volume fraction between 5 and 43%. The GMR was measured in-situ at T=4.2 K in magnetic fields 1.2 T. The analysis of the GMR data obtained from these studies clearly shows that spin-dependent scattering at the Co-cluster/Ag-matrix interface is the only relevant scattering mechanism causing the GMR in our well-defined samples. Received: 21 April 1998 / Received in final form: 17 May 1998 / Accepted: 18 May 1998     

Molecular dissociative sequential excitation and ionization of strontium vapor

We report new measurements on laser-assisted molecular dissociative sequential excitation and ionization spectra of strontium vapor. A single Nd:YAG pumped dye laser in conjunction with an atomic beam apparatus have been used to investigate the even parity 5snd ^3,1 D ₂ Rydberg series resulting from the 5s5p ¹ P ₁ resonance level. The relative intensities changes among the 5snd ³ D ₂ and ¹ D ₂ Rydberg series are attributed to the interactions with the 4d6s ^1,3 D ₂ perturbers, respectively. The interchannel interactions between the 5snd ¹ D ₂ series and the 4d6s ¹ D ₂ level and among the 5snd ³ D _1,2,3 series and the 4d6s ³ D _1,2,3 intruders have been separately parameterized using the two-channel quantum defect theory. Received: 10 September 1998 / Received in final form: 16 December 1998     

Sub-Doppler fluorescence on the atomic <Emphasis Type="Italic">D</Emphasis><Subscript>2</Subscript> line of a submicron rubidium-vapor layer

An extremely thin cell (ETC) with the thickness of a Rb atomic vapor layer in the range of 100–300 nm was fabricated. It is demonstrated that a simple laser-diode technique with a single resonant light beam is sufficient to observe separately all of the atomic hyperfine transitions of the D ₂ line of Rb (780 nm) and also allows us to measure the relative transition probabilities of the hyperfine transitions. The onset of collisional self-broadening of the hyperfine transitions as the number density of atoms increases was studied. The detrimental role of the atoms with slow longitudinal velocity in the sub-Doppler response of the Rb ETC is demonstrated by studies in which the cell is tilted from normal incidence of the laser beam. It is also shown that using an ETC allows us to resolve in a moderate external magnetic field the Zeeman splitting of the hyperfine transitions of the ⁸⁷Rb D ₁ transition F _g=1F _e=1,2. Received: 19 February 2003 / Revised version: 4 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +374/32-31172, E-mail: david@ipr.sci.am     

Cold events in thermal-neutron-induced fission of heavy nuclei

An interpretation of the cold fission events in thermal-neutron-induced fission of heavy nuclei is given. The descent from the saddle point is considered as a dynamical process with reversible coupling between collective and intrinsic degrees of freedom. The distribution function for the collective variables is expressed as a product of two terms: the adiabatical and the dynamical factors. A simple model for symmetric fission to study the mass distribution is presented. As example, the calculations are performed for the nucleus ²⁶⁴Fm. Gross features of the cold fission are discussed as well as the dependence of the theoretical mass distribution on the parameters of the model. Received: 29 April 1998 / Revised version: 11 September 1998     

How to get access to long range states of highly excited molecules

Due to competition between ionic and covalent dissociation, the excited potential curves of the alkali dimers display long range structures, leading in some cases to secondary wells. We discuss the possibility of populating the well (with a depth of 1614.8 cm^-1, located at R _e = 30.5a ₀ ) that is present in the Na₂ potential curve and propose detection schemes. Received: 28 May 1998 / Revised: 5 August 1998 / Accepted: 24 September 1998     

Auger electron spectroscopy of the kinetics of evaporation of palladium beaded films from sapphire substrate

) surface of sapphire were investigated by Auger electron spectroscopy in the temperature range of 1053–1083 K. It was found that the decrease of the effective thickness during heat treatments under 10^-8 mbar was caused by evaporation from the surface of the substrate. Using the model developed by Kaganovskii and Beke we have established that the process is controlled by surface diffusion. It was shown that the evaporation rate of palladium adatoms from an alumina surface is given by I_o=1.45×10³⁶exp{-(393±46 kJ/mol)/RT}ms, and, on the basis of an estimation of the surface diffusion length, the effective surface diffusion coefficient D_s was evaluated: Received: 30 March 1998/Accepted: 28 May 1998     

Predictions of geometrical structures and ionization potentials for small barium clusters Ba

The geometrical structure of ground state Ba_n clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba₇ to Ba₁₃ appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba₁₃ which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998     

Cooling mechanisms in potassium magneto-optical traps

We report on a theoretical and experimental investigation of ³⁹K magneto-optical trapping. The small hyperfine splitting characterizing the upper level of the cooling transition affects the cooling mechanism. In order to model the atom-laser interaction, the whole level structure of the D₂ line has to be taken into account. Two different regimes have been recognized, one optimizing the loading of the trap, the second minimizing the temperature of the atoms. We investigated these two regimes experimentally and found results in agreement with the theoretical predictions. Received: 6 March 1998 / Received in final form: 13 May 1998 / Accepted: 13 May 1998     

Polymer chain in a quenched random medium: slow dynamics and ergodicity breaking

The Langevin dynamics of a self-interacting chain embedded in a quenched random medium is investigated by making use of the generating functional method and one-loop (Hartree) approximation. We have shown how this intrinsic disorder causes different dynamical regimes. Namely, within the Rouse characteristic time interval the anomalous diffusion shows up. The corresponding subdiffusional dynamical exponents have been explicitly calculated and thoroughly discussed. For the larger time interval the disorder drives the center of mass of the chain to a trap or frozen state provided that the Harris parameter, (Δ/b ^d)N ^{2 - νd}≥1, where Δ is a disorder strength, b is a Kuhnian segment length, N is a chain length and ν is the Flory exponent. We have derived the general equation for the non-ergodicity function f (p) which characterizes the amplitude of frozen Rouse modes with an index p = 2πj/N. The numerical solution of this equation has been implemented and shown that the different Rouse modes freeze up at the same critical disorder strength Δ _c ∼ N ^{- γ} where the exponent γ ≈ 0.25 and does not depend from the solvent quality. Received 17 December 2002 Published online 23 May 2003 RID="a" ID="a"e-mail: vilgis@mpip-mainz.mpg.de     

Eu3+ luminescence and Gd3+-Eu3+ energy transfer in K5Li2GdF10:Eu3+

Polycrystalline samples of europium-doped K₅Li₂GdF₁₀ have been obtained by a slow cooling of melted compound and investigated using spectroscopy methods. Luminescence from the ⁵ D ₂ level of Eu³⁺ is found to be weak. Intense visible emission upon excitation into the ⁵ D ₂ or higher energy levels has been attributed to overlapping transitions from long-lived ⁵ D ₁ and ⁵ D ₀ levels. A strong increase of the ⁵ D ₀ emission at the expense of the ⁵ D ₁ emission occurs between 5 K and 25 K without significant change of the ⁵ D ₁ lifetime. To account for this, it is supposed that both the radiative and the nonradiative transition rates are temperature-dependent. Efficient energy transfer from the ⁶ G _J levels of Gd³⁺ to Eu³⁺ ions has been evidenced by excitation spectra in the VUV region and VUV-excited luminescence. It has been concluded that the cross relaxation contributes to the energy-transfer process. Received: 8 May 2001 / Accepted: 11 May 2001 / Published online: 25 July 2001