Ch. Leemann

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.

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)

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.

Vortex dynamics in Y Ba2Cu3O7 films

The low frequency complex ac impedance Z of c-axis oriented Y Ba 2 Cu 3 O 7 thin films was measured as a function of the applied magnetic field B . At temperatures not too close to the transition temperature the imaginary part of Z is proportional to B 2 , while the real part consists of a term proportional to B 3 and an additional term proportional to exp (-1/ B ) which appears at higher fields. This behavior is in agreement with a description of vortex dynamics with a Langevin equation of motion.

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.

Dynamics of the phase transition of periodic superconducting networks

The critical behaviour of superconducting networks, as deduced from a study of the complex ac response of the system as a function of temperature, is found consistent with the vortex unbinding idea of the Kosterlitz-Thouless theory. The ac response of networks exposed to a perpendicular magnetic field shows a complex oscillatory dependence on the reduced flux per unit cell f= /0, with structures at integer and half-integer values of f.

Investigation of vortex dynamics in Josephson junction arrays with magnetic flux noise measurements

Abstract We have measured the magnetic flux noise in a two-dimensional Josephson junction array in nominally zero magnetic field, in the vicinity of the superconducting transition. This transition is of the Berezinski–Kosterlitz–Thouless (BKT) type, driven by the unbinding of thermally excited vortex–antivortex pairs. At temperatures just above the BKT transition temperature T c , the flux-noise power spectrum S ( ω ) is white up to a temperature dependent cross-over frequency ω ξ ( T ). We identify ω ξ with the characteristic time related to the scale at which the cross-over from free vortex response to a response dominated by bound pairs occurs. The signature of free vortices is thus white noise while bound vortex pairs give rise to a 1/ ω dependence.

Collective Pinning of a Frozen Vortex Liquid in Ultrathin Superconducting YBa2Cu3O7 Films

The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa2Cu3O7 films was studied by measuring their ac sheet impedance Z over a broad range of frequencies omega. With decreasing temperature the dissipative component of Z exhibits, at a temperature T*(omega) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T*(omega), appears to be frozen at the time scales 1/omega of the experiments.

Penetration depth in YBaCuO films

Abstract With a two coil mutual inductance technique the in plane penetration depth in compact YBaCuO films was measured to be λ (0) ≅ 0.23 μm. In granular YBaCuO films the penetration depth is larger, its value is being determined by the strength of the intergranular Josephson coupling energy.