B. Dam

The origin of high critical currents in YBa2Cu3O7-d thin films

Thin films of the high-temperature superconductor YBa2Cu3O7−δ exhibit both a large critical current (the superconducting current density generally lies between 1011 and 1012u2009Au2009m−2 at 4.2u2009K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects,. But it has hitherto been unclear which types of defect—dislocations, grain boundaries, surface corrugations and anti-phase boundaries—are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped quantitatively by this technique, are the linear defects that provide the strong pinning centres responsible for the high critical currents observed in these thin films. Moreover, we find that the superconducting current density is essentially independent of the density of linear defects at low magnetic fields. These natural linear defects, in contrast to artificially generated columnar defects, exhibit self-organized short-range order, suggesting that YBa2Cu3O7−δ thin films offer an attractive system for investigating the properties of vortex matter in a superconductor with a tailored defect structure.

Destabilization of the Mg-H System through Elastic Constraints

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

Synthesis of yttriumtrihydride films for ex-situ measurements

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 20–40h) 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.

Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of Y1−yMgyHx, films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating MgH2 is formed together with cubic YH3−δ. In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.

Epitaxial switchable yttrium-hydride mirrors

By means of x-ray scattering and scanning probe microscopy it is shown that high-quality epitaxial Y films can be deposited on (111)-CaF2 substrates. The films can reversibly be switched from metallic YH2 to transparent insulating YH3−δ. Although hydrogen absorption involves an expansion of the lattice and a symmetry change from hcp to fcc, the epitaxiality of the film remains intact during the switching process. The transparency and the insulating nature of the substrate opens unique possibilities to investigate electrically and optically these switchable mirror films in the single crystalline state.

LASER ABLATION THRESHOLD OF YBA2CU3O6+X

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa2Cu3O6+x target. Below a threshold fluence of 1 J/cm2 the roughness of the irradiated target increases dramatically due to non‐stoichiometric ablation. The overall target surface composition becomes increasingly Y rich and Cu poor, while the opposite is found for the corresponding ablated thin films. Above the threshold fluence the composition of the ablated target surface is conserved. As a result of the energy homogeneity of the laser beam obtained by means of projection optics, the optimization of the deposition parameters has been improved leading to the reproducible fabrication of flat, stoichiometric YBa2Cu3O7 films with Tc0’s over 91 K.

Quasifree Mg–H thin films

The thermodynamics of hydrogen absorption in Pd-capped Mg films are strongly dependent on the magnesium thickness. In the present work, we suppress such dependency by inserting a thin Ti layer between Mg and Pd. By means of optical measurements, we show that the surface energy contribution to the destabilization of MgH2 is negligible. The inserted Ti layer prevents Mg–Pd alloy formation at the Mg/Pd interface, leading to quasifree Mg films and enhancing the kinetics of hydrogen desorption. Our observations are important for the development of thin film devices.

Hydrogen absorption kinetics and optical properties of Pd-doped Mg thin films

In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPdy with 0.023⩽y⩽0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5bar H2 within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5eV⩽ℏω⩽6.0eV corresponding to 2500nm⩾λ⩾210nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH2.

Mechanism of incongruent ablation of SrTiO3

At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3u2009J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3u2009J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.

Growth and etching phenomena on pulsed laser deposited YBa2Cu3O7-d films.

Abstract The surface morphology of pulsed-laser deposited YBa 2 Cu 3 O 7−δ films is investigated by STM AFM. Instead of spiral growth, a 2D nucleation and growth behaviour is observed. As we find these 2D nuclei also on high-oxygen pressure DC sputtered films grown at a much lower growth rate, we conclude that the supersaturation is not a decisive parameter for the predominance of either growth mode. Instead, we attribute the absence of growth spirals to the non-steady state growth conditions inherent to the pulsed nature of the laser-ablation process. Growth spirals only develop, if a non-vanishing diffusional flow of adatoms towards the step edge is maintained. The number of growth spirals observed on a films is therefore not necessarily a measure for the number of screw dislocations. After wet-etching the films in Br-ethanol, we observe that etch pits are formed consisting of concentric steps. We conclude that these pits are due to repetitive nucleation around linear defects. The etchpit density identified in this way is of the order of 1 per μm 2 .