H. M. Gijsman

Thermodynamic properties of liquid 3He-4He mixtures at zero pressure for temperatures below 250 mK and 3He concentrations below 8%

We calculated the thermodynamic quantities of dilute liquid 3He-4He mixtures, starting from experimental values of the specific heat and the osmotic pressure. The calculations are confined to temperatures below 250 mK and 3He concentrations below 8% at zero pressure. Some results are especially useful for dilution refrigeration. Contrary to the calculations previously performed by Radebaugh, our results are in good agreement with the experimental date on both the osmotic pressure and the osmotic enthalpy.

Quantum Magnetoconductance of a Nondegenerate Two-Dimensional Electron Gas

Magnetoconductance measurements are reported for the low-density, two-dimensional classical electron gas formed on the surface of liquid helium. At strong magnetic fields, large deviations from the classical parabolic behaviour are observed. These deviations are described with a quantum-transport theory of scattering within broadened Landau levels.

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.

Experimental verification of an analytical model for orifice pulse tube refrigeration

Abstract The orifice pulse tube refrigerator has the potential to become a small, reliable cryocooler. In linear approximation the refrigeration produced is due to two independent contributions: an enthalpy flow in the bulk of the gas and an enthalpy flow due to heat exchange with the wall. These effects are treated independently in the literature. For pulse tube operation they can be of the same order of magnitude and have to be combined. Experimental verification is obscured by the regenerator loss, which is difficult to determine accurately. By performing experiments at room temperature this loss can be made negligible. These experiments are in agreement with the theory.

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.

Temperature and electric field dependence of hopping transport in ion-implanted Si:As

Abstract The temperature and electric field dependence of the resistivity of thin layers of As-doped, not intentionally compensated, Si with concentrationsjust below that of the metal-insulator transition are reported. The temperature dependence gives evidence for variable range hopping with a coulomb gap in the density of states at the Fermi-level. From the field dependence of the resistivity, a characteristic length of ≈25 nm can be deduced, which is of the order of magnitude of the hopping length.

Simple activated transport in ion-implanted Si:As at temperatures below 0.5 K

The temperature dependence of the resistance of ion-implanted Si:As, with impurity concentration about 10 % below the metal-insulator transition, is measured in the temperature range 0.1–4.2 K. The temperature dependence below 0.5 K is much stronger than expected from the variable range hopping laws and is well described by purely activated behaviour. This points to the existence of a rigid gap at the Fermi level. Possible origins are discussed.

Negative low-field magnetoresistance of electrons on liquid helium

Abstract A negative low-field magnetoresistance is reported for the nondegenerate two-dimensional electron gas on the surface of bulk liquid helium. The behaviour is observed at temperatures of about 2 K and is interpreted as a manifestation of weak localisation. Preliminary results indicate that the effect becomes larger for lower electron densities, suggesting that electron-electron scattering contributes to the dephasing mechanism.

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.

Electrical transport in as ion-implanted si in the metal-insulator transition range

Abstract Resistance measurements are reported for silicon, ion-implanted with As, for various concentrations within a few percent of the metal insulator transition at temperatures down to 0.1 K. At the lowest temperatures, the temperature dependence of the resistance is dominated by inhomogeneities in the dopant distribution. The observed current dependence of the resistance is discussed.