Wilfried Schoepe

High-θ polyacetylene: DC conductivity between 14 mK and 300 K

Highly stretch-oriented polyacetylene (6.5:1) yields conductivities σ of typically 20 000–100 000 Ω−1cm−1 at room temperature when highly doped with iodine. Between T = 300 K and T = 14 mK, σ decreases monotonically by about a factor of 5 for fresh samples. Above 400 mK the temperature dependence for fresh samples is fitted by the SHENG formula and can be interpreted within a phenomenological model. On fresh samples, MONTGOMERY measurements of the conductivities parallel (σi) and perpendicular (σσ) to the stretching axis show a temperature independent anisotropy A=σi/σσ of about 25 indicating a common limiting mechanism for both, σi and σσ. Deliberate oxygen ageing drastically changes σ(T) and results in a temperature dependence of A.

Intermittent Switching Between Potential Flow and Turbulence in Superfluid Helium at mK Temperatures

Superfluid flow around an oscillating microsphere is investigated at temperatures down to 25 mK. Stable laminar flow below a critical velocity and turbulence at large drives are found to be separated below 0.5 K by an intermediate range of driving forces where the flow is unstable, intermittently switching between laminar and turbulent phases. We have recorded time series of this switching phenomenon and have made a statistical analysis of the switching probability. The mean lifetime of the turbulent phases grows with increasing drive and becomes infinite at a critical value. Stability of the laminar phases above the critical velocity is limited by natural background radioactivity or cosmic rays.

Negative ion motion in normal and superfluid 3He

We report the first measurements of negative ion motion in the superfluid phases of 3He and in the normal phase below 17 mK. Refrigeration was achieved with nuclear demagnetization of copper and we used a pulsed NMR platinum powder thermometer immersed in the liquid. In the A phase the longitudinal resonance frequency provided an additional high-resolution thermometer. In the normal phase we observed a strictly temperature-independent mobility. In the superfluid phases we found two velocity regimes. For small applied electric fields the velocity is a linear function of the field and the corresponding mobility increases monotonically toward lower temperatures. At high electric fields the velocity is a nonlinear function of the field as a result of the pair-breaking effect of the moving ion. Available theoretical calculations are only in partial agreement with our results.

Variable-range hopping conduction in doped germanium at very low temperatures and high magnetic fields

The conductivity of doped Ge below the metal-insulator transition is measured at temperatures between 4 K and 40 mK and in magnetic fields up to 7 Tesla. In zero field the resistivity exponent diverges asT−1/2. In weak fields the magnetoresistance increases asB2 and becomes exponentially large in strong fields and at low temperatures. The results can be described quantitatively in terms of variable-range hopping between localized states having a Coulomb gap in the density of states at the Fermi level. The magnetoresistance is calculated for arbitrary fields by means of a quasi-classical method. A fit to the data gives the radius of the localized states and the density of states. The sample is found to be very close to the metal-insulator transition. A small increase of the binding energy is observed in strong fields.

Tunneling from Electronic Bubble States in Liquid Helium Through the Liquid-Vapor Interface

Ions have been extensively used in liquid helium to study both the superfluid and normal states. If the experimental cell is only partially filled with liquid, then the ions must pass through the phase boundary between liquid and vapor in order to reach a collector located in the vapor. The transport of charges across a phase boundary is of general interest in a variety of biological, chemical, and physical problems, helium being presumably one of the simplest cases. It has been observed that positive ions are unable to penetrate the liquid-vapor interface. However, negative ions (electronic bubbles) pass into the vapor quite easily, providing the temperature is not too low (T > 2° K). As the temperature is lowered, negative ions experience a rapidly increasing difficulty in penetrating the free surface, resulting in a vanishing current at moderate electric fields below 1°K.

Universal Critical Velocity for the Onset of Turbulence of Oscillatory Superfluid Flow

AbstractThe critical velocity vc for the onset of turbulent drag of small spheres oscillating in superfluid 4He is frequency dependent (ω/2π from 100 Hz to 700 Hz) and is described by

Positive ion mobility in normal and superfluid 3He

v_{c}=2.6\sqrt{\kappa \omega}

Viscosity and Mean Free Path of Very Diluted Solutions of 3He in 4He

, where κ is the circulation quantum. A qualitative analysis based on a recent theory of the onset of superfluid turbulence gives

Energy relaxation of hot electrons and inelastic collision time in thin metal films at low temperatures

v_{c}\approx \sqrt{8\kappa \omega/\beta}

Magnetoresistance of iodine-doped polyacetylene at low temperatures

, where β∼1 depends on the coefficients of mutual friction. This agrees well with the data and implies that vc is a universal critical velocity that is independent of geometry, size, and surface properties of the oscillating body. This is confirmed by comparing our data on spheres with vc obtained with other oscillating structures by other groups. Numerical simulations indicate somewhat larger critical velocity, above which a rapid increase in vortex length is observed.