R. De Luca

Irreversible low field flux penetration in sintered superconducting systems

Abstract The irreversible low field magnetic flux penetration in granular superconductors is analyzed by means of a 2D network of Josephson junctions under the conditions of strong grain coupling and non-negligible intergranular regions. Analytical results on simple systems and numerical simulations show that the first penetration critical field is determined by the shielding currents crossing through the junctions. These currents reduce the Josephson potential barriers and cause the instability of the magnetic metastable states. We show that by this approach a critical state model can be derived.

Flux creep in Josephson junction arrays

In order to analyze the diamagnetic properties of weakly coupled structures in high-T/sub c/ ceramic superconductors , the Josephson junction array model is used. It is suggested that the coupling is strong enough to allow magnetic flux trapping inside nonsuperconducting regions surrounded by superconducting loops closed by Josephson junctions. It is noted that the presence of currents flowing through the junctions has to be taken explicitly into account in the Hamiltonian. This description leads to a creep model of the Josephson junctions array. As a result, one observes the following: (1) pinning centers generated by nonsuperconducting regions into the loops, (2) pinning potentials determined by fluxon motion barriers due to the Josephson junctions, (3) absence of degeneracy of the states corresponding to a different number of fluxons in the loops, and (4) a reduction of the barrier height due to measuring currents or to diamagnetic shielding currents. The last effect is equivalent to the Lorentz force effect in type-II semiconductors. Thus, it is believed that this picture greatly modifies the usual superconducting glass model.

Lorentz force on sodium and chlorine ions in a salt water solution flow under a transverse magnetic field

It is shown that, by applying elementary concepts in electromagnetism and electrochemistry to a system consisting of salt water flowing in a thin rectangular pipe at an average velocity vA under the influence of a transverse magnetic field B0, an electromotive force generator can be conceived. In fact, the Lorentz force acting on the sodium and chlorine ions in a water solution gives rise to a so-called Faraday voltage across the two metal electrodes, positioned at the sides of the pipe. The effect is carried along the following chemical reactions at the electrodes: at the cathode, water is reduced (instead of sodium ions) and hydrogen gas is generated; at the anode, chlorine gas is produced. In college physics teaching, this interdisciplinary subject can be adopted to stress analogies and differences between the Hall voltage in conductors and the Faraday voltage in electrolyte solutions.

Josephson equations for the simplest superconducting multilayer system

Abstract We extend Feynmans model of a single Josephson junction to the case of three stacked superconducting layers. In order to derive the correct voltage-phase relation, a self consistent approach to this model is used. By this analysis we notice that, if the intermediate electrode is thin enough to allow the overlap of the superconducting order parameters of the outer layers, an additional term appears in the Josephson current-phase relation. Two representative applications are presented for these types of junctions.

Penetration depth of high-Tc granular superconductors in he low-field reversible regime

Abstract A superconducting granular sample is described by a network of Josephson junctions and inductances associated with the superconducting current branches. By this picture a model of concentric current shells for simulating magnetic field penetration is developed. An analytical calculation of the penetration depth in the low-field reversible regime is performed taking explicitly into account fluxoid quantization for any superconducting loop in the network. The thermodynamic and the experimental reversibility range is finally discussed in terms of the inductive coupling strength among the junctions.

Strongly coupled overdamped pendulums

It is shown, by a first-order perturbation expansion, that the dimensionality of the dynamical equations for the angular variables of two strongly coupled identical overdamped pendulums can be reduced from two to one. The resulting dynamical equation is seen to be similar to the one of a single pendulum with an additional fictitious torque characterized by a second harmonic contribution.

Characteristic low-field magnetic response of granular superconductors

Abstract The low-field magnetic response of a physical system consisting of eight superconducting spherical grains in a cubic arrangement is studied by means of a three-dimensional Josephson junction network. The lower threshold field for this system is numerically studied as a function of the inclination of the externally applied magnetic field H → with respect to the z-axis.

The rotational aspects of the nuclear spectrum within the BRST formalism

Abstract The treatment of collective variables within the BRST formalism is applied to the problem of the coupling between the particles and the rotational degrees of freedom. In zero order the system is described as in the usual version of the unified model, i.e., as the superposition of rotational and intrinsic motions with the usual Coriolis interaction emerging as a leading order coupling term. The higher-order corrections to the collective parameters, as well as the rotational dependence of relevant physical operators, are discussed both for even and odd systems. Several significant differences with the unified model and with the particle-rotor model are emphasized. The Elliott SU(3) model is used as an illustrative example.

Double- and triple-barrier Josephson junctions

A generalization of the semi-classical model describing the Josephson dynamics of tri-layer superconducting systems is given by assuming a constant non-null arbitrary superconducting phase for the inner electrode and the presence of inhomogeneities in the superconducting coupling between electrodes. Extension of the model to triple-barrier Josephson junctions is proposed. Integer and fractional Shapiro steps are predicted and their amplitudes are calculated.

Ratchet potential in superconducting quantum interference devices containing a double-barrier junction

The periodic effective potential of a superconducting quantum interference device containing an underdamped Josephson junction in the first branch and a double-barrier junction in the second is studied. An effective non-sinusoidal expression for the current–phase relation with an additional half harmonic term is used for the double-barrier junction. The system allows voltage rectification under opportunely chosen parameter values.