W. van Haeringen

Calculation of three-dimensional refractive-index field using phase integrals

Abstract A simple method is proposed for the determination of three-dimensional refractive-index distributions of phase objects from interferograms. A given refractive index field has been treated to illustrate the method.

Self-consistent calculations on GaAs-AlxGa1-xAs heterojunctions

The sub-band structure at zero temperature of a two-dimensional electron gas at an AlxGa1-xAs-GaAs heterojunction interface is calculated in the envelope function approach. A model is developed in which the potential energy, the sub-band structure, the areal density of the electron gas as well as depletion lengths can be calculated as a function of material properties and growth parameters only. The authors have also calculated analytically the minimum value of the donor concentration in the AlxGa1-xAs layer for obtaining an electron gas at zero temperature.

Empirical pseudopotential approach to the band structures of diamond and silicon carbide

Abstract Electronic band structures of diamond, silicon and silicon carbide in the sphalerite and wurtzite structure are obtained using a pseudopotential method. The atomic pseudopotentials in diamond and silicon are also used in the polytypes of silicon carbide. The form factors are found by adjustment of the respective band structures to experiments.

Pseudopotential approach to the energy band structure of graphite

Abstract The energy band structure of graphite is calculated with a local centrally symmetrical pseudopotential for carbon. Special attention is devoted to the energy bands along the vertical zone edges, where the valence and conduction bands are known to overlap. An overall band structure is proposed.

The effect of saturation on the polarization parameters of modes in anisotropic gas lasers

Abstract Special attention is devoted to the non-linear medium-induced anisotropic effect on the orientation of the polarization ellipse of a mode in a gas laser. An observed polarization flip with hysteresis is theoretically explained.

On the minimum temperature attainable in dilution refrigerators

Abstract Minimum temperatures attainable in dilution refrigerators of both the double circulation and conventional types are calculated. They are shown to be very sensitive to the way circulation is achieved. For a double circulation refrigerator a model is proposed in which the concentration at injection differs from the equilibrium concentration.

Circular polarization in A j = 1 → j = 0 - transition laser

Abstract The preference for circular polarization of an isotropic laser involving a j = 1 → j = 0 transition is caused by the difference between the relaxation rates of the quadrupole moment and the angular momentum in the degenerate level.

Model calculation of the electron phonon coupling parameter λ

A simple model is described, in which the influence of two parallel Brillouin zone faces on the electron phonon coupling parameter λ can be studied. In this model it is observed that λ, considered as a function of the Fermi energyEitF, has a peculiar structure when the Fermi body just passes through the two Bragg planes. Both an enhancement as well as a reduction of the more familiar density of states effect on λ and therefore on the critical temperatureTitc of superconductors appears to be possible. In this respect approaching a Brillouin zone face turns out to be very favourable.The structure in λ is due to a combination of Umklapp, wave function mixing, and density of states effects. The calculations indicate that it is worthwhile to study these effects in detail in actual systems.

On the choice of tube lengths and diameters in a He3He4 dilution refrigeration system

Abstract A discussion is presented on the problem of choosing the lengths and the diameters of the various tubes in a He 3 ue5f8He 4 dilution refrigeration system, which are directly connected to the mixing chamber(s). The treatment is based on recently obtained explicit solutions of the differential equations describing the temperature distribution along the tubes at temperatures lower than about 0.03 K. A graphical procedure that can be used to obtain the correct tube lengths is outlined.

On the Josephson effect between superconductors in singlet and triplet spin-pairing states

Abstract An expression is derived for the Josephson current between two weakly coupled superconductors of which one or both have pairs in a spin-triplet state. It is shown that there can be no Josephson effect up to second order in the transition matrix elements between a superconductor with spin-triplet pairs and one with spin-singlet pairs if the coupling between the two superconductors can be described with a spin-conserving tunnel hamiltonian. This is shown to offer a possibility to investigate experimentally whether a particular superconductor has spin-triplet pairs by coupling it weakly to a well-known spin-singlet pairing superconductor.