A.Th.A.M. de Waele

Viscosity of saturated3He-4He mixture below 200 mK

The shear viscosity of saturated3He-4He mixture has been measured at temperatures between 7 mK and 200 mK using a vibrating-wire viscometer and a calibrated pressure cell. The reliability of the vibrating-wire technique was tested by measuring the viscosity of pure4He. The results are internally consistent. A phenomenological expression is given for the viscosity of saturated3He-4He mixture between 7 mK and 80 mK.

The multiple mixing chamber

Abstract This paper deals with the multiple mixing chamber in which He 3 , circulating in a dilution refrigerator, is diluted in steps. The double mixing chamber in the stationary and in the dynamic state is described and the important design features are given. The triple mixing chamber is also discussed. A comparison is made between the performance of dilution refrigerators operating with a single mixing chamber and with a double mixing chamber.

Chapter 6 Superconducting Point Contacts Weakly Connecting Two Superconductors

Publisher Summary This chapter focuses on the superconducting point contacts weakly connecting two superconductors. Special attention has been been paid to the resistive-superconductive region, the current–voltage characteristic and the radiative properties. The point contact will be treated using a two-fluid model, in which the total current is the sum of an ideal Josephson super current and a normal current. The magnetic field dependence of the critical current (the dc Josephson effect) through a double point contact, when the self-induced flux in the enclosed area is taken into account, is studied extensively. Its relation with flux quantization is discussed in this chapter. The properties of double point contacts in the resistive-superconductive region, in relation with both the ac and dc Josephson effects, and device applications such as very sensitive magnetometers and voltmeters are discussed in this chapter. The critical current through a double point contact between two superconductors as a function of the applied magnetic field is also discussed in this chapter.

Thermo-electrostatic effects in superconductors

Abstract Irreversible thermodynamics shows that there is an electrostatic voltage difference in a superconductor if a temperature difference is established across the ends. This voltage difference is due to a difference in the chemical potential μ of the conduction electrons. It is sometimes considered as the analogue of the fountain pressure in helium II. It is equal to the bulk contribution to the temperature dependence of the work function W. An experiment is described in which the W-T dependences of niobium and lead are measured. The order of magnitude of dW/dT in the liquid-helium temperature range is 10 μV/K for Nb and 0.5 μV/K for Pb. The magnitude of the signal is as yet not well understood. Contrary to the fountain pressure in helium II the μ-T dependence cannot simply be written in terms of the entropy per conduction electron. The μ-T dependence seems to be dominated by terms due to the temperature dependence of the lattice constants, the metal-surface dipole layer and the Fermi surface.

Thermodynamics and hydrodynamics of ³He-⁴He mixtures

Publisher Summary The specific heat of liquid 3 He– 4 He mixtures is usually written in terms of the sum of the specific heat of a 3 He-quasiparticle gas and the specific heat of the pure 4 He component. The thermodynamics based on this starting point is derived. Relations of important quantities and their low- and high-temperature limits are given. These are used to derive expressions for the velocity of second sound. This latter quantity is a very important source of information for the Fermi gas properties. Finally, the Fermi gas parameters are summarized in the chapter. The experimental aspects of the 3 He– 4 He hydrodynamics are treated. The appearance of mutual friction that has long been neglected in this field is discussed, together with the properties of the critical velocities. The phenomenological equations of motion are given. The occurrence of mutual friction is a strong indication that 4 He vortices play an important role in 3 He– 4 He hydrodynamics. From the equation of motion of quantized 4 He vortices, the observed cubic velocity dependence of the 4 He chemical potential difference is explained on purely dimensional grounds. A differential equation is given from which the temperature profile in a cylindrical tube in which 3 He flows through superfluid 4 He can be calculated.

Quantum Magnetoconductance of the Two-Dimensional Electron Gas on a Liquid Helium Surface

The longitudinal conductivity σxx of the two-dimensional electron system on a liquid helium surface was measured as function of magnetic field for fields up to 4 Tesla. The temperature was in the range 1.4 < T < 2.0 K, so that scattering of electrons by vapour atoms is dominant. When the quantum limit is reached, which means that the Landau level separation \hbarωc is larger than the thermal energy kbT (\hbarωc/kbT≤4 under the present conditions), a deviation from the classical relation σxx~1/B2 is observed. A discussion is given based on a quantum transport theory.

A stable superconducting double point-contact magnetometer in which the voltage sensitivity is improved by filling the hole with copper

Abstract The construction of reliable double point-contact magnetometers is described. It is shown that the voltage sensitivity can be improved by filling the hole between the superconductors with copper.

Point contacts from BiCaSrCu2Ox

The properties of point-contacts between two pieces of BiCaSrCu2Ox-material were investigated. At 4.2 K critical currents of 200 μA could be obtained. The IcR product was typically 5–10 mV. The derivatives of the I–V characteristics showed local maxima around 25 mV, indicating the possibility of an energy gap of about 13 mV. The shape of the I–V characteristics was to a large extend independent of the contact pressure.

A 3He circulating dilution refrigerator with two mixing chambers

Abstract A dilution refrigerator with two mixing chambers is described. When compared with a dilution refrigerator with only one mixing chamber the temperature range is extended to lower temperatures. The temperature reduction factor can be up to a factor 2.8. The results of an experiment are reported in which a magnetic temperature of 5.5 mK was obtained in the continuous mode and 3.5 mK in the single cycle mode, using a dilution refrigerator that reaches 13 mK in the continuous mode with one mixing chamber.

Tunnelling characteristics of point contacts with high Tc material

Abstract The I – V characteristics of point contacts formed by pressing two pieces of bulk material against each other are considered. One side of the contact consists of a ceramic superconductor. The other side can be either a ceramic superconductor, a normal metal or a classical superconductor. The I – V characteristics of the contacts formed with these different materials often show a remarkable similarity: the I – V characteristics can be very well described by the phenomenological equation IR 0 = V {1 + ( V / V 0 ) α }, where the exponent α is of the order of one and V 0 of the order of 100 mV. The parameters V 0 and α determine the shape of the curve. The temperature dependence of the curves is dominated by the temperature dependence of R 0 . No significant changes are observed at the critical temperature of the superconducting materials. In this paper we show that the observations can be explained by assuming that normal electron tunnelling takes place across a trapezoidal potential barrier.