2. GRAVITY 7. MICROSCOPIC PHYSICS 13. TOPOLOGY OF DEFECTS 18. ANOMALOUS NON-CONSERVATION OF FERMIONIC CHARGE 22. EDGE STATES AND FERMION ZERO MODES ON SOLITON 26. LANDAU CRITICAL VELOCITY 29. CASIMIR EFFECT AND VACUUM ENERGY

The Universe in a Helium Droplet

Progress in preparation, processing and applications of polyaniline

Most plastics are good insulators. But conducting polymers also form the basis of a new field of ‘plastic electronics’. Some of these materials show exceptionally high conductivities, almost as high as metals. But their properties deviate from true metallic behaviour in several important ways. Now a conducting plastic with resistivity properties much more like those of true metals has been synthesized. The properties of this polyaniline compound may bring practical plastic electronics a little closer. True metallic conductivity in a much-studied conducting polymer (polyaniline) is demonstrated, but synthesized by a route that minimizes the density of structural defects believed responsible for the earlier deviations from classical metallic behaviour. Despite nearly three decades of materials development, the transport properties in the ‘metallic state’ of the so-called conducting polymers are still not typical of conventional metals1,2,3,4,5,6,7. The hallmark of metallic resistivity—a monotonic decrease in resistivity with temperature—has not been obtained at temperatures over the full range below room temperature; and a frequency dependent conductivity, σ(ω), typical of metals has also not been observed. In contrast, the low-temperature behaviour of ‘metallic’ polymers has, in all previous cases, exhibited an increase in resistivity as temperature is further decreased, as a result of disorder-induced localization of the charge carriers1,2,3,4. This disorder-induced localization also changes the infrared response such that σ(ω) deviates from the prediction of Drude theory5,6,7. Here we report classic metallic transport data obtained from truly metallic polymers. With polyaniline samples prepared using self-stabilized dispersion polymerization8, we find that for samples having room-temperature conductivities in excess of 1,000 S cm-1, the resistivity decreases monotonically as the temperature is lowered down to 5 K, and that the infrared spectra are characteristic of the conventional Drude model even at the lowest frequencies measured.

Metallic transport in polyaniline

Conducting polymers (CPs) have received much attention in both fundamental and practical studies because they have electrical and electrochemical properties similar to those of both traditional semiconductors and metals. CPs possess excellent characteristics such as mild synthesis and processing conditions, chemical and structural diversity, tunable conductivity, and structural flexibility. Advances in nanotechnology have allowed the fabrication of versatile CP nanomaterials with improved performance for various applications including electronics, optoelectronics, sensors, and energy devices. The aim of this review is to explore the conductivity mechanisms and electrical and electrochemical properties of CPs and to discuss the factors that significantly affect these properties. The size and morphology of the materials are also discussed as key parameters that affect their major properties. Finally, the latest trends in research on electrochemical capacitors and sensors are introduced through an in-depth discussion of the most remarkable studies reported since 2003.

Electrical and Electrochemical Properties of Conducting Polymers.

The first measurements on vortices in rotating superfluid $^{3}\mathrm{He}$ have been conducted in the Low Temperature Laboratory at Helsinki University of Technology during the past five years. These experiments have revealed unique vortex phenomena that are not observed in any other known superfluids. In this review, the concept of broken symmetry is applied to investigate the quantized vortex lines in superfluid $^{3}\mathrm{He}$. In the superfluid $A$ phase, vorticity can be supported by a continuous winding of the order parameter; this gives rise to continuous "coreless" vortices with two flow quanta. Novel vortices with a half-integer number of circulation quanta may also exist in $^{3}\mathrm{He}$-$A$ due to a combined symmetry of the superfluid state. In the superfluid $B$ phase, the vortices have a complicated core structure. The vortex-core matter is ferromagnetic and superfluid, and it displays broken parity. The ferromagnetism of the core is observed in NMR experiments due to a gyromagnetic effect. The calculated core structures exhibit an experimentally observed first-order phase transition. This vortex-core transition in rotating $^{3}\mathrm{He}$-$B$ may be understood in terms of a change in the topology for flaring-out of the vortex singularity into higher dimensions; the topological identification further suggests that the phase transition manifests a spontaneous bifurcation of vorticity---involving half-quantum vortices in $^{3}\mathrm{He}$-$B$. These recent advances of interest in quantum liquids are also of general relevance to a wide range of fields beyond low-temperature physics.

Quantized vortices in superfluid He-3

The temperature ( $T$) dependent dc conductivity ( ${\ensuremath{\sigma}}_{\mathrm{DC}}$) (down to 20 mK) and dielectric function at optical frequencies (0.002--6 eV) and 6.5 GHz are used to probe the inhomogeneous disorder-driven insulator-metal transition in conducting polymers. A correlation between large low $T$ ${\ensuremath{\sigma}}_{\mathrm{DC}}$ and the presence to low $T$ of a small fraction of the carrier density delocalized with long transport times ( $g{10}^{\ensuremath{-}13}\mathrm{s}$) indicates that metallic ${\ensuremath{\sigma}}_{\mathrm{DC}}$ is due to only a small fraction of the charge carriers. The achievable ${\ensuremath{\sigma}}_{\mathrm{DC}}$ for these systems when the entire charge carrier density participates is estimated to surpass that of copper.

http://www.phys.ufl.edu/%7Etanner/PDFS/Kohlman97prl-Polymer.pdf

Limits for Metallic Conductivity in Conducting Polymers

We describe a technique, using thin lines of triplet-state $\mathrm{He}_{2}^{*}$ molecular tracers created by femtosecond-laser field ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid $^{4}\mathrm{He}$, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.

Visualization of the normal-fluid turbulence in counterflowing superfluid He 4

We demonstrate that an unusually small pipe flow apparatus using both liquid helium and room temperature gases can span an enormous range of Reynolds numbers. This paper describes the construction and operation of the apparatus in some detail. A wide range of Reynolds numbers is an advantage in any experiment seeking to establish scaling laws. This experiment also adds to evidence already in hand that the normal phase of liquid helium is a Navier–Stokes fluid. Finally, we explore recent questions concerning the influence of molecular motions on the transition to turbulence (Muriel 1998) and are unable to observe any influence.

Pipe flow measurements over a wide range of Reynolds numbers using liquid helium and various gases

We have studied the interaction of metastable 4He2* excimer molecules with quantized vortices in superfluid 4He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96±6 nm at a pressure of 0.1 bar and 27±5 nm at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium.

Excimers He2* as tracers of quantum turbulence in 4He in the t = 0 limit.

Flow measurements in ultrapure $^{4}\mathrm{He}$ through a micron-size orifice at millikelvin temperatures show, for the first time, the transition from thermal to quantum nucleation of nanometer-size vortices below a crossover temperature of 0.147 K. These observations establish the close relationship between this type of critical flow and negative-ion motion in superfluid $^{4}\mathrm{He}$ and strongly suggest that the underlying mechanism, the nucleation of vortex half-rings, is identical.

Gary G. Ihas

Papers

Limits for Metallic Conductivity in Conducting Polymers

Visualization of the normal-fluid turbulence in counterflowing superfluid He 4

Pipe flow measurements over a wide range of Reynolds numbers using liquid helium and various gases

Excimers He2* as tracers of quantum turbulence in 4He in the t = 0 limit.

Quantum nucleation of vortices in the flow of superfluid 4He through an orifice