Arne T. Skjeltorp

Conductivity Enhancement in Carbon Nanocone Adhesive by Electric Field Induced Formation of Aligned Assemblies

We show how an alternating electric field can be used to assemble carbon nanocones (CNCs) and align these assemblies into microscopic wires in a commercial two-component adhesive. The wires form continuous pathways that may electrically connect the alignment electrodes, which leads to directional conductivity (∼10(-3) S/m) on a macroscopic scale. This procedure leads to conductivity enhancement of at least 2-3 orders of magnitude in the case where the CNC fraction (∼0.2 vol %) is 1 order of magnitude below the percolation threshold (∼2 vol %). The alignment and conductivity are maintained on curing that joins the alignment electrodes permanently together. If the aligned CNC wires are damaged before curing, they can be realigned by an extended alignment period. This concept has implications in areas such as electronic packaging technology.

LATERAL FORCE ON A MAGNET PLACED ABOVE A PLANAR YBA2CU3OX SUPERCONDUCTOR

The lateral force interaction between a permanent bar magnet and a large slab of high Tc superconductor has been investigated at 77 K, under conditions of a constant vertical separation of 2 mm. The restoring force as a function of lateral displacement rises very steeply, and reaches 90% of its saturation value, 5.1 mN, after 1.8 mm. The profile of the force‐displacement curve is in qualitative agreement with the existing theory. A significant quantitative discrepancy is interpreted as due to a theoretical penetration depth exceeding the sample thickness. The lateral magnetic stiffness or spring constant, is found to be independent of displacement away from lateral equilibrium.

Vibrations of a magnet levitated over a flat superconductor

Abstract Based on a dipole-dipole interaction model, we discuss the levitation force and related vibration problems to understand the effects of flux pinning in type-II superconductors, and the applications of levitation in superconducting systems.

Forces, Growth and Form in Soft Condensed Matter: At the Interface between Physics and Biology

- Preface. Organizing committee and participants. - The physico-chemical basis of self-assembling structures J. Israelachvili, I. Ladyzinski. - Supramolecular assembly of biological molecules C.R. Safinya. - Simple examples of cell motility J.-F. Joanny. - Statistical physics of unzipping DNA D.R. Nelson. Can theory predict two-state protein folding rates? An experimental perspective B. Gillesbie, K.W. Plaxco. - Copolymers with long-range correlations: sequence design near a surface N.Yu. Starovoitova, P.G. Khalatur, A.R. Khokhlov. - Novel approach to the study of rotational and translational diffusion in crystals A.V. Belushkin. - The bacterial flagellar motor R.M. Berry. - Self-assembly and the dynamics of magnetic holes A.T. Skjeltorp, J. Akselvoll, K. De Lange Kristiansen, G. Helgesen, R. Toussaint, E.G. Flekkoy, J. Cernak. - Structures in molecular networks K. Sneppen, S. Maslov, K.A. Eriksen. - Oscillating gene expressions in regulatory networks M.H. Jensen, K. Sneppen, T. Giana. Transport properties of segmented polymers and non-spherical nanoparticles studied by Brownian dynamics simulation S.N. Naess, A. Elgsaeter. - Cytokinesis: the initial linear phase crosses over to a multiplicity of non-linear endings D. Biron, P. Libros, D. Sagi, D. Mirelman, E. Moses. Information dynamics in living systems J. Breivik. - Index.

Magnetic phase diagram of gadolinium gallium garnet

Abstract To determine a possible phase transition in gadolinium gallium garnet (GGG), we have measured the ac susceptibility down to mK temperatures. In zero field, no long range order is found down to our lowest attainable temperature of 25 mK. However, the results show evidence for field induced antiferromagnetic phases.

Interaction model for magnetic holes in a ferrofluid layer

Nonmagnetic spheres confined in a ferrofluid layer (magnetic holes) present dipolar interactions when an external magnetic field is exerted. The interaction potential of a microsphere pair is derived analytically, with precise care for the boundary conditions along the glass plates confining the system. Considering external fields consisting of a constant normal component and a high frequency rotating in-plane component, this interaction potential is averaged over time to exhibit the average interparticular forces acting when the imposed frequency exceeds the inverse of the viscous relaxation time of the system. The existence of an equilibrium configuration without contact between the particles is demonstrated for a whole range of exciting fields, and the equilibrium separation distance depending on the structure of the external field is established. The stability of the system under out-of-plane buckling is also studied. The dynamics of such a particle pair is simulated and validated by experiments.

Tendril perversion—a physical implication of the topological conservation law

Tendril perversion—a phenomenon ruled by the topological conservation law—is presented. A contemporary, quantitative analysis of the phenomenon is confronted with its qualitative, intuitive analysis carried out by Charles Darwin. The linking number, twist and writhe are defined. The topological conservation law is introduced. The Gauss formula for calculating the linking number and the Calugareanu formula for calculating writhe are derived and discussed using physical arguments.

Lifting forces acting on a cylindrical magnet above a superconducting plane

Abstract Based on the London theory, we have calculated and measured the lifting force acting on a cylindrical magnet placed above a superconductor. Comparison between the theoretical expectation and experimental data shows a good agreement.

Braid Description of Few Body Dynamics

A few body system of magnetic holes set in motion by external ac magnetic fields is studied both experimentally and numerically. Notions from braid theory are used to describe the dynamics ina very compact manner. The time histroy of n magnetic holes moving in a plane is represented by an n-strand braid which enables us to extrat periodic orbits and to map out phase phase portraits. The transitions between different periodic modes of motion are either continuous or noncontinuous followed by strong hysteresis.

Potential and force between a magnet and a bulk Y1Ba2Cu3O7-δ superconductor studied by a mechanical pendulum

The interaction between a magnet and a high Tc superconductor (HTSC) is investigated by a novel method. A permanent magnet mounted at one end of a stiff rod hanging by a knifes edge forms a mechanical pendulum. When a HTSC is placed below the magnet the frequency v and damping factor Gamma will be changed due to the interaction between the magnet and the HTSC. The experiments show that, as the separation between the magnet and the HTSC decreases, the flux lines start to penetrate the sample via intergranules and grain boundaries, which results in a horizontal restoring force even for the field range B<<Bc1. The present experimental results are consistent with the theoretical model of weak links for bulk HTSC. The power-law-like potential Veff, and force, Feff, are introduced to describe the interaction. The behaviour of Veff and Feff obtained from the present measurements is in disagreement with the dipole-image model even for the field range B<<Bc1.