The optical switching from shiny metallic to "black" absorbing states of 30 nm Mg2Ni/50 nm TM/10 nm Pd (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Pd) trilayer thin films upon hydrogenation is used to determine the hydrogen absorption/desorption kinetics at moderate H2 pressure and room temperature. A strong correlation is observed between these kinetics and the enthalpies of solution of H in TM. A diffusion model based on chemical potentials of hydrogen in metals has been developed that agrees qualitatively with the experimental data, both from kinetics and hydrogen concentration profile points of view. The influences of the solution enthalpy and the diffusion coefficient of H in TM and that of the TM layer thickness are discussed with respect to this model. Finally, we conclude with potential applications of these trilayers, from optical hydrogen detection to reinvestigation of diffusion coefficients of H in TM.

The hydrogenation of metallic Mg2 Ni films was shown to proceed via a self-organized double layering of transparent Mg2 NiH4 and metallic Mg2 NiH0.3. For stoichiometries departing from Mg2 Ni we conclude from optical reflection, transmission and electrical measurements that the hydrogenation process involves two competing effects: (i) the nucleation of the initial Mg2 NiH4 layer near the substrate and the ensuing growth of this layer and (ii) the slow random nucleation of the same phase within the remaining part of the film. Which process dominates depends critically on the stoichiometry of the parent metal alloy.

We report the magneto-optical observation of a surprising fourfold symmetry of the flux penetration in a superconducting YBa2Cu3O7−δ thin-film disk containing a square array of antidots, leading to an angular variation of the critical current by a factor of nearly 2. This behavior is explained using a vortex channeling model. Potential applications in superconducting devices are discussed.

Using advanced magneto-optics, the field and current distributions in superconducting thin film YBa2⁢Cu3⁢O7 rings in an externally applied magnetic field are studied experimentally. The observations are in close agreement with numerical calculations. During the initial flux penetration and field reversal a highly nonuniform current distribution is observed. In particular, concentric counterrotating current loops occur during field reversal. We explore implications of these results for the determination of critical currents and penetration fields from bulk magnetization measurements.

Penetration of magnetic flux in YBa2Cu3O7 superconducting thin films in an external magnetic field is visualized using a magneto-optic technique. A variety of flux patterns due to non-linear vortex diffusion is observed: (1) Roughening of the flux front with scaling exponents identical to those observed in burning paper including two distinct regimes where respectively spatial disorder and temporal disorder dominate. In the latter regime Kardar-Parisi-Zhang behavior is found. (2) Fractal penetration of flux with Hausdorff dimension depending on the critical current anisotropy. (3) Penetration as 'flux-rivers'. (4) The occurrence of commensurate and incommensurate channels in films with anti-dots as predicted in numerical simulations by Reichhardt, Olson and Nori. It is shown that most of the observed behavior is related to the non-linear diffusion of vortices by comparison with simulations of the non-linear diffusion equation appropiate for vortices.

In a recent paper [1], it has been shown from transport measurements that artificial reversible pinning can be created by the application of a magnetic tape on the surface of a thin superconducting film. We study the influence of a periodic array of magnetic stripes, generated from a signal pre-recorded on a magnetic tape, on the pinning of vortices. With our high resolution magneto-optical technique [2], we visualize in two dimensions the penetration of the flux in a thin YBa2Cu3O7-δ film partially covered by such a magnetic tape. In particular, we investigate the evolution of flux entry in the superconducting strip under and next to the magnetic tape. We present results obtained for various temperatures and various applied fields.

Kinetic roughening of flux fronts penetrating in superconducting thin films are studied by means of a high resolution magneto-optic technique. The roughening exponent (α = 0.64) and growth exponent (β = 0.65) obtained from a dynamic scaling analysis of the initial stage of flux penetration and, at small length scales, are characteristic for a static disorder dominated nonlinear diffusion such as also observed in the directed percolated depinning model. At large length scale, α = 0.46 indicates a transition towards dynamic stochastic disorder, similar to the behavior of Kardar-Parisi-Zhang systems. There is a striking similarity with the behavior of combustion fronts in burning paper.

In this study the role of anisotropy on flux penetration in c-axis epitaxial Tl2⁢Ba2⁢CuO6+𝑥 and YBa2⁢Cu3⁢O7−𝑥 films is investigated by magneto-optics. We study thin films of Tl2⁢Ba2⁢CuO6+𝑥 on substrates with vicinal angles of 0° (well-oriented), 0.5°, 2.5°, and 4° and YBa2⁢Cu3⁢O7−𝑥 films as a function of chain-conduction-induced anisotropy. A crossover from fractal to nonfractal flux penetration is observed with increasing anisotropy. Numerical simulations of anisotropic flux motion are compared with experiment.

High quality twin-free c-axis oriented YBa₂Cu₃O_7-δ films were grown by pulsed laser deposition on NdGaO₃. We find resistive anisotropies between 1 < ρ_a/ ρ_b < 1.6. The resistive anisotropy above T_c appears to be correlated to the anisotropy in the critical current below T_c. The normal state properties of the chains along b seem to have a significant influence on the superconducting properties of YBa₂Cu₃O_7-δ.

A new method has been developed to synthesize compact yttriumtrihydride by making use of a thin film technique. For electrical measurements yttrium films of typically 500 nm thickness are covered under UHV conditions by a 5 nm thick palladium overlayer which consists of electrically disconnected islands. Loading of these films with hydrogen up to the trihydride phase can then be done ex-situ in a reasonably short time (around 20-40 h) by applying gas pressures of about 60 × 10⁵ Pa. For a thicker Pd layer (above 20 nm) this time can be considerably shorter (t ∼ 125 s). The film morphology stays intact during the loading process although the film thickness increases by approximately 11% and the crystal structure changes from h.c.p. to f.c.c. and back to h.c.p. These samples are, therefore, very well suited for an investigation of the remarkable electrical and optical properties of trihydrides, as recently reported by Huiberts et al. (Nature, 380, 1996, 231). In this article we give evidence for the island structure of the palladium overlayer and make a comparison of a number of physical properties of yttrium and its related hydrides as thin films with literature values for the same material in bulk form. These properties include lattice parameters for the different hydride phases, electrical resistivity for yttrium and its dihydride and Hall coefficient for yttrium. The characteristics of the yttriumhydride thin films are very similar to those of bulk material. Furthermore, we performed concentration measurements and resistivity measurements during hydrogen loading. It is shown that the resistivity rises three orders of magnitude when yttrium is loaded up to the trihydride phase at 60 × 10⁵ Pa.

Low-electric-field E-js curves, normalized dynamical relaxation rate Q(T)=d ln(js)/d ln(E), and experimental critical current density js(E=Ec) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(js,T)=Uc(T)f[js(T)/jc(T)], with t=T/Tc, we obtained the U(j,T=0 K) relation by scaling all the E-js curves of different temperatures to one smooth curve with Uc(T) and jc(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c=ln(v0B), which was actually taken as a fitting parameter in the scaling process in previous work, can be predetermined. It is also found that the resulting U(j,T=0 K) relation based on the E-js curves for T 79 K can be well interpreted with the collective-pinning model. However, the data for T>79 K start to deviate from this interpretation, which may be attributed to the reverse hopping of vortices.