[en] It is shown that the attenuation of longitudinal ultrasound in superconducting niobium cannot be explained by the two-band model. Attempts to fit both the data near Tsub(c) and much below Tsub(c) lead to contradictory assumptions for the model parameters. Experiments, on the other hand, do not at present exclude the existence of multiple gaps if these are close to the BSC-gap. (author)

[en] Based on a detailed modeling of the Fermi surface topology of MgB₂ we calculated the anisotropy of the upper critical field B_c2 within the two gap model. The σ-band is modeled as a distorted cylinder and the π-band as a half-torus, with parameters determined from bandstructure calculations. Our results show that the unusual strong temperature dependence of the B_c2 anisotropy, that has been observed recently, can be understood due to the small c-axis dispersion of the cylindrical Fermi surface sheets and the small interband pairing interaction as obtained from bandstructure calculations. We calculate the magnetic field dependence of the density of states within the vortex state for field in c-axis direction and compare with recent measurements of the specific heat on MgB₂ single crystals

[en] The coupling constants appearing in the two-band model are derived from recent microscopic calculations. The resulting order parameters are close to the BCS value. The model allows the effects of interband impurity scattering to be discussed. (author)

[en] Basic principles of Josephson junction based interferometers are reviewed. Key features of parallel and also serial Superconducting Quantum Interference Filters (SQIFs) are explained in detail. Results of recent mixing experiments with radiofrequency signals between 100 MHZ and 20 GHz are discussed and analyzed with help of a simple model. (orig.)

[en] In the presence of interband scattering of quasiparticles by impurities only a single gap in the excitation spectrum is found. The dependence of this gap and the two order parameters on interband coupling and impurity content is discussed. Graphical representations of the density of states for several sets of parameters are given. For certain ranges of parameters the calculated density of states is qualitatively similar to the experimental results. (author)

[en] On the basis of Gorkov's formulation of superconductivity theory, generalized Eilenberger equations are derived which apply to rotating superfluid ³He in the presence of a magnetic field h and finite superflow v. In analyogy to conventional type II superconductors, the possibility of vortex solutions in discussed. An implicit equation determining the upper critical angular velocity Ω/sub c/2 as a function of temperature T, magnetic field h, and superflow v parallel to the rotation axis is-inferred from the linearized Eilenberger equations. In contrast to the case of slowly rotating ³He-A, the solution of the eigenvalue problem determining the order parameter Δ near the the upper critical angular velocity admits no coreless vortex no coreless solutions. The space-dependent amplitude of the order parameter is analogous to Abrikosov's vortex array solution, while the spin-orbit part is given either by a polar-state type or an Anderson-Brinkman-Morel (ABM)-state-type eigensolution. Among the possible eigensolutions the polar-state type yields for vanishing superflow v the highest critical rotation frequency. For finite superflow v parallel to the rotation axis, however, the ABM-state-type solution is stabilized in comparison to the polar state for Vertical BarvVertical Bar> or approx. =0.2π(Tc/sub c/0/T/sub F/)v/sub f/ at zero temperature

[en] Superconducting pair states for systems with strong spin-orbit interaction and crystallographic anisotropy are usually assumed to transform as the basis functions of the irreducible representations T /SUB n/ of the appropriate point group. Transitions to pair states belonging to different irreducible representations are supposed to occur at different transition temperatures T /SUB c/ (T /SUB n/ ). Here it is shown for p-wave states that, as soon as nonlinear terms in the gap equation or quartic terms in the Ginzburg-landau free energy are taken into account, linear combinations of pair states belonging to different irreducible representations can result. This applies to all those states that are not solutions of the gap equation when the pairing interaction is of the isotropic form V /SUB o/ K.K' used in the theory of superfluid ³He. The spectrum of collective excitations of those states, which are actually solutions of the gap equation, becomes much more complex when spin-orbit interaction and crystal field effects are taken into account by assuming different transition temperatures T /SUB c/ (T /SUB n/ ). In particular, a number of additional modes are found whose frequencies scale as w² alpha ln(T /SUB c/ (T /SUB n/ )/T /SUB c/ (T /SUB m/ )). If the deviation from the isotropic interaction is not very large, these modes will have very low frequencies. They couple to density fluctuations to O(q²) due to particle-hole asymmetry. This can be a large effect in heavy fermion compounds because of the sharp peak in the density of states. These modes could, therefore, be very well responsible for the peaks in the acoustic attenuation observed just below T /SUB c/

[en] It is an experimental fact that there exists a large range of temperatures and pressures where the A and B phases of superfluid ³He coexist and are almost free of thermal excitations. The phase boundary that forms between the two drastically different bulk phases possesses many interesting features. The gap seen by a ballistic excitation traversing the interface is dependent upon the angle of its trajectory. This, together with the complex nature of the triplet Cooper-pair wavefunction interpolating between the A and the B phases provides a rich variety of phenomena and also leads to the formation of bound states of excitations trapped within the interfacial region. In this paper we review the existing description of the A-B phase boundary and discuss work of relevance to the recent experiments carried out by Boyd and Swift. (orig.)

[en] Our previous theory yielded for the Zeeman splitting of the imaginary J = 1 collective mode in ³He-B the result ω = 2Δ+0.25 ΩJ/sub z/ (Ω is the effective Larmor frequency). In this paper we take into account the downward shift of the pair-breaking edge from 2Δ to 2Δ₂-Ω (Δ₂ and Δ₁ are the longitudinal and transverse gap parameters). This leads to a complex Lande factor: the frequencies of the J/sub z/ = +- 1 components become ω = 2Δ+0.39 ΩJ/sub z/, and the linewidths GAMMA of these resonances become finite: GAMMA = 0.18 Ω. The coupling amplitudes of the J/sub z/ = +- 1 components to density are found to be proportional to gap distortion, (Δ₁-Δ₂)/Δapprox.(Ω/Δ)². Our results for the ultrasonic attenuation due to the J/sub z/ = +- 1 modes are capable of explaining the field dependence of the attenuation close to the pair-breaking edge as observed by Dobbs, Saunders, et al. The observed peak is caused by the J/sub z/ = -1 component: its height increases due to gap distortion as the field is increased, and the peak shifts downward in temperature and its width increases with the field due to the complex Lande factor. The J/sub z/ = +1 component gives rise to a corresponding dip relative to the continuum attenuation

[en] The boundary between the A and B phases of superfluid ³He possesses many interesting features because of the drastically different bulk order parameters. We study the transmission and Andreev reflection of ballistic wave packets of excitations incident onto the A-B phase boundary. We find that the A-B phase boundary is magnetically active and is capable of converting an unpolarized incident wave packet into transmitted and Andreev reflected wave pakets which are partially polarized. We also consider equilibrium properties due to bound states and resonances of excitations localized near the phase boundary. These states contribute to magnetic correlations and to mass and spin currents flowing in the plane of the interface