J. L. Gavilano

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.

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.

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.

Frustration effects in triangular Josephson junction arrays

Using a sensitive inductive technique, we have measured the ac conductance of large two-dimensional triangular arrays of proximity effect Josephson junctions as a function of temperature and frustration parameter f (f = applied magnetic flux per unit cell of the array in units of the flux quantum). The ac conductance is periodic in f (with period one) and exhibits an extremely rich behaviour with structures corresponding to commensurability between array and vortex lattice at various rational values of f . At low temperatures, the strength of the most prominent structures is in satisfactory agreement with existing mean-field predictions. The observed norrowing of the structures at high temperatures is attributed to strong two-dimensional (2D) phase fluctuations.

Complex AC conductance of YBaCuO films

The AC linear response of YBaCuO films prepared by RF magnetron sputtering to a small oscillating field was studied as a function of temperature and transverse magnetic field. The structures observed are quite similar to those found in artificially prepared two-dimensional periodic junction arrays where the transition from a phase-ordered to a phase-disordered state at T/sub c/ is attributed to the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory for phase transitions in two dimensions. The magnetic field data suggest that the typical cell size of the YBCO array is a few mu m, on the order of the grain size. This suggests that the ceramic films can be modeled as two-dimensional networks of Josephson tunnel junctions, the junctions being located at the grain boundaries. >

Thermal vortices in superconducting fractals

A study of the zero-field ac response of a wire network of interconnected 6th-order Sierpinski gaskets as a function of temperature is presented. In a narrow temperature range below the superconducting-to-normal transition thermally nucleated vortices dominate the response. A theoretical model treating the vortices as Brownian particles moving about in the hierarchical potential-energy landscape of the fractal lattice is developed and its predictions are found to agree with the experimental observations.

Influence of Phase Fluctuations in Dynamical Magnetoconductance Measurements of Both Square and Fractal Wire Networks

By using a two coil mutual inductance technique, we have performed dynamical magnetoconductance measurements of both square and fractal superconducting wire networks photolithographically patterned from granular aluminum films. Square networks of periodicity a = 3–8 µm contain 106 nodes connected by wires of width w = 0.7–1.5 µm, whereas fractal network consist of 6th order Sierpinski gaskets connected together periodically, the size of the smallest triangle (0th order) being 5 µm.