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The dynamics of polymer chains in melts, solutions, and networks was studied by the aid of NMR relaxation spectroscopy. Proton data of the spin-lattice relaxation times in the laboratory and rotating frames, T\ and T\" respectively, and of transverse relaxation curves are reported. Frequency, temperature, concentration, molecular weight, and cross-link density dependences have been investigated. Ti was measured in a frequency range of 103 to 3 X 10s Hz predominantly using the field-cycling technique. The study refers to polyisoprene, polyisobutylene, poly(tetrahydrofuran), polystyrene, polyethylene oxide), polyethylene, and poly(dimethylsiloxane). The range of molecular weights was 103-106. The power laws for the time dependence of the mean-square displacement,

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We have studied the nonlinear current-voltage characteristic of a two-dimensional lattice Coulomb gas by Monte Carlo simulation. We present three different determinations of the power-law exponent a(T) of the nonlinear current-voltage characteristic, VϳI a(T)ϩ1 . The determinations rely on both equilibrium and nonequilibrium simulations. We find good agreement between the different determinations, and our results also agree closely with experimental results for Hg-Xe thin-film superconductors and for certain single crystal thin-film high-temperature superconductors.

We have studied the linear resistance of a three dimensional lattice Superconductor model in the London limit London lattice model by Monte Carlo simulation of the vortex loop dynamics. We find excellent finite size scaling at the phase transition. We determine the dynamical exponent z = 1.51 for the isotropic London lattice model.The fluctuation regime in high T c superconductors (HTCS) is expected to be sufficiently wide that critical fluctuations are observable [1,2]. In particular the conductivity is supposed to scale as σ ∝ ξ 2−d+z [1,2], where ξ is the correlation length and d is the dimension of the system. This scaling relation has been applied in recent experiments on YBCO in zero magnetic field [3]. From which the value z ≈ 2.6 and ν ≈ 1.2 (ν is the correlation length exponent) was extracted. Accordingly an accurate determination of z and ν in models of high T c superconductors is of great interest. The phenomenology of superconductors is described by the Ginzburg-Landau (GL) model. The model is to complicated to allow all degrees of freedom to be included in calculations. Among the standard approximations of the GL model one can mention: the XY [4,5], Villain [6][7][8], and the lattice superconductor model in the London limit [9][10][11][12][13][14][15][16].In the present paper we determine z in the zero field London lattice model (LLM). The exponent z is known to be close to 3/2 in the 3 dimensional XY -model, corresponding to model (E) [17].It is of interest to know whether the London model in which the spin wave degrees of freedom are integrated out is characterised by the same exponent. Equilibrium properties of the XY and the LLM for λ = ∞ are known to be the same since they are connected through the Villain duality transformation [6]. However, the dynamical properties might not be the same. It is seen in other systems where the spin degrees of freedom have an effect on the dynamics of the topological defects [18]. However, as we show below, in fact the LLM has z = 1.5. This result is reassuring given that the model is used to study the dynamics of vortex systems in the relation to HTCS [19].Since the magnetic field H mag = 0 we can limit our study to the isotropic system. We derive an expression for the resistance R, based on the Nyquist formula [20] for voltage fluctuations. From the Nyquist formula we derive a simple finite size scaling relation for the resistivity at the critical temperature T c and determine the critical dynamical exponent z.The LLM describes the vortex loop fluctuations of a bulk superconductor. The model originates from a Ginzburg -Landau description with no amplitude fluctuations and the spin waves integrated out within a Villain approximation. On a cubic lattice a vortex loop consists of four line elements forming a closed loop.The LLM is defined by the partition function Z on a cubic lattice of side length L using periodic boundary conditions:(1)where H is the Hamiltonian, the link variables q α represent the vortex line elements. The are three kinds of q α , one for ea...

Some errors were found in the equations. They result from a change in the definition of E ␥ in the final version of the manuscript. All calculations were done with a correct set of equations and all results and conclusions remain unchanged. The correct equations readand E ␥ =−2k ␥ ͱ 4 ͉E c E c ͉ with k ␥ = ͱ 2␥ / ͱ 4 ␤ 1 ␤ 2 ͑first paragraph on page 3͒.

A review of the superconducting magnetic properties of doped fullerenes is presented. Experimental results on the main superconducting properties, such as the critical fields and the characteristic lengths, are critically discussed. Different methods to evaluate the lower critical field are discussed. Finally, experimental data on properties connected to flux pinning, such as the critical current density, the irreversibility line and the pinning force, are summarized and discussed.

The thermally activated resistivity, r,, and the negative Hall resistivity, ry are explained as two consequences of the same effect, namely the unbinding of vortex pairs m the vicinity of T,. Both r, and rw exhibit a thermally activated behaviour. The activation energy depends logarithmically on the magnetic field. Our explanation suggests rwrh with a = 1 in accordance with recent measurements.