P. Martinoli

Inductive conductance measurements in two‐dimensional superconducting systems

A two‐coil mutual‐inductance technique for measuring the complex ac response of a two‐dimensional (2‐D) superconductor to a weak ac magnetic field is described. Analytical and numerical methods are presented which allow extraction of the complex ac conductance of the superconductor from the signal voltage induced in the detection coil by the screening currents flowing in the sample. The method is illustrated by measurements of the ac conductance of a square network of aluminum wires from which the penetration depths of both the network and (granular) aluminum are deduced. It is shown that the method provides a powerful tool to observe characteristic features associated with critical phenomena in 2‐D superconducting systems.

A.C. Quantum interference in superconducting films with periodically modulated thickness

Abstract a.c. quantum interference has been observed in type II superconducting films with periodic thickness modulation. Marked supercurrent steps appear at well defined voltages in the current-voltage characteristics when vortex motion is induced by superimposed d.c. and r.f. currents. An essential condition for the observation of the effect is matching of the vortex lattice to the periodic pinning structure due to the thickness modulation.

Josephson oscillation of a moving vortex lattice

Experimental evidence is presented for a supercurrent oscillation arising from vortex motion in the flux-flow regime of superconducting films with periodically modulated thickness. An essential condition for detecting the rf electric field associated with the oscillation is matching of the vortex lattice to the periodic pinning structure represented by the thickness modulation. (AIP)

Two Dimensional Josephson Junction Arrays

Two dimensional Josephson junction arrays (JJAs) offer theopportunity to study a variety of basic physical concepts. Thepresent review focuses on recent experimental work on thedynamics of JJAs, as characterized by ac conductancemeasurements. The review starts with a discussion of basicphysics necessary to describe JJAs. Some experimentalissues, array fabrication and measurement techniques areconsidered next. In a perpendicular magnetic field, a JJA isan experimental realization of the frustrated XY model, withthe frustration parameter f, corresponding to the number offlux quanta in a unit cell of the array, adjusted by themagnetic field. It is thereby possible to investigate thenature of the ground states at arbitrary frustrations. Phasetransitions are the next topic: the vortex unbindingtransition is observed at integer f-values, while, if thejunction coupling energies are appropriately varied across thearray, at half-integer f-values the Ising transition,associated with chiral symmetry breaking, may be observed.Some aspects of vortex dynamics, a subject which is not yetcompletely understood, are then considered. Under certainconditions there is virtually no pinning in JJAs, they aretherefore ideally suited for the study of vortex dynamics. Thenext topic of this review is concerned with the influence ofdisorder on the ground states and on the phase transitions inJJAs. Site percolation in JJAs has provided some insightsinto the physics of disordered systems and allowed to verifysome theoretical predictions on percolation in two dimensions.A quick look at JJAs in the underdamped regime concludesthis review.

Arrays of Josephson Junctions: Model Systems for Two-Dimensional Physics

Planar arrays of Josephson junctions provide excellent model systems to study a variety of phase transitions in two dimensions. Experiments are reported in which the critical behaviour of periodic arrays is studied by measuring the dynamic response of the system to a small oscillating field. Tuning of frustration occurs by adjusting the magnetic flux threading a unit cell of the array, while by varying the driving frequency it is possible to probe the critical behaviour of the system at different length scales. The dynamic response near the phase transition of an un frustrated array is well described by a model incorporating the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory. Some aspects of the dynamic response of frustrated arrays are also discussed.

IcR products of tunnel junctions with depressed order parameter

IcRN products of tunnel junctions with depressed order parameters are calculated. It is shown that in contrast to abrupt, classical tunnel junctions, IcRN products of junctions with depressed order parameter are a function of Ic with IcRN(Ic) ≤ πΔ0/2e.

Coherent flux flow in superconducting films with periodically modulated thickness

Abstract We find an anomalous behaviour of the flux flow characteristics of superconducting films with modulated thickness when the vortex lattice matches the periodic pinning structure. At a matching field there is a voltage rise associated with a decrease in supercurrent which is usually not observed for random pinning. Coherent motion of the vortex lattice is responsible for this behaviour. Structure in the differential flux flow resistivity as a function of current can be related to harmonic components of the periodic pinning potential. A recent theory by Schmid and Hauger may be extended to give a satisfactory description of our experiments.

Superconducting vortices in periodic pinning structures

Abstract The static and dynamic properties of flux line lattices threading pinning structures provided by films of periodically varying thickness are reviewed. The critical current is a maximum when the flux line lattice matches the thickness variations. Critical currents in non-matching configurations are also investigated. In the dynamic flux flow regime oscillating electric fields are predicted and detected. Applied r.f. fields modify the current-voltage characteristics when the frequency with which flux lines cross the thickness modulations is a multiple of their frequencies.

Dimensional crossover and hidden incommensurability in Josephson junction arrays of periodically repeated Sierpinski gaskets

We report a study of overdamped Josephson junction arrays with the geometry of periodically repeated Sierpinski gaskets. These model superconductors share essential geometrical features with truly random (percolative) systems. When exposed to a perpendicular magnetic field B, their euclidian or fractal behavior depends on the relation between the intervortex distance (imposed by B) and the size of a constituent gasket, and was explored with high-resolution measurements of the sample magnetoinductance L(B). In terms of the frustration parameter f expressing (in units of the superconducting flux quantum) the magnetic flux threading an elementary triangular cell of a gasket, the crossover between the two regimes occurs at f_{cN}=1/(2*4^{N}), where N is the gasket order. In the fractal regime (f>f_{cN}) a sequence of equally spaced structures corresponding to the set of states with unit cells not larger than a single gasket is observed at multiples of f_{cN}, as predicted by theory. The fine structure of L(f) radically changes in the euclidian regime (f<f_{cN}), where it is determined by the commensurability of the vortex lattice with the effective potential created by the array. Anomalies observed in both the periodicity and the symmetry of L(f) are attributed to the effect of a hidden incommensurability, which arises from the deformation of the magnetic field distribution caused by the asymmetric diamagnetic response of the superconducting islands forming the arrays.

Inductive conductance measurements of YBaCuO films

Abstract Measurements of the complex ac conductance of sputtered YBaCuo films reveal, in the critical transition region, features strikingly similar to those observed at the phase transition of artificially prepared two-dimensional Josephson junction arrays.