J.H. Rector

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 1012 A m−2 at 4.2 K 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.

Hydriding kinetics of Pd capped YHx switchable mirrors

The kinetics of the metal–insulator transition in polycrystalline, Pd-capped YHx switchable mirrors upon hydrogenation is investigated. Using the accompanying optical transition, we study switching of matrix-like samples with many (∼200) combinations of Pd and Y layer thicknesses. We find that: (i) With increasing Y thickness dY, the switching time τ increases for any constant Pd thickness dPd. (ii) With increasing dPd, there are three regimes. In regime I, it is impossible to switch a device. This can mainly be related to Pd–Y compound formation consuming all Pd within the UHV system, followed by surface oxidation in air. In regimes II and III switching is possible, but only in regime III does Pd form a closed cap layer. The Pd thickness needed for a closed cap layer depends on dY. (iii) An oxide buffer layer hinders Pd–Y interdiffusion, so that a thinner Pd cap layer is needed for switching than in the case without buffer layer. This is interesting for potential applications since it yields a higher opt...

Visualization of hydrogen migration in solids using switchable mirrors

Switchable mirrors made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α-YHx <0.23 is a shiny, hexagonally close-packed metal, β-YH2±δ is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ-YHx >2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH3−δ behaves as a negative ion, in agreement with recent strong-electron-correlation theories,. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

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.

Structural and optical characterization of porous anodic aluminium oxide

Spectroscopic ellipsometry and scanning electron microscopy (SEM) experiments are employed to characterize porous aluminum oxide obtained by anodization of thin aluminum films. Rutherford backscattering spectra and x-ray diffraction experiments provide information on the composition and the structure of the samples. Results on our thin film samples with a well-defined geometry show that anodization of aluminum is reproducible and results in a porous aluminum oxide network with randomly distributed, but perfectly aligned cylindrical pores perpendicular to the substrate. The ellipsometry spectra are analyzed using an anisotropic optical model, partly based on the original work by Bruggeman. The model adequately describes the optical response of the anodized film in terms of three physically relevant parameters: the film thickness, the cylinder fraction, and the nanoporosity of the aluminum oxide matrix. Values of the first two quantities, obtained from fitting the spectra, are in perfect agreement with SEM results, when the nanoporosity of the aluminum oxide matrix is taken into account. The validity of our optical model was verified over a large range of cylinder fractions, by widening of the pores through chemical etching in phosphoric acid. While the cylinder fraction increases significantly with etch time and etchant concentration, the nanoporosity remains almost unchanged. Additionally, based on a simple model considering a linear etch rate, the concentration dependence of the etch rate was determined.

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.

Pressure dependence of the T/sub c/ of YBa/sub 2/Cu/sub 3/O/sub 7/ up to 170 kbar

The superconducting onset temperature T/sub co/ of single-phased YBa/sub 2/Cu/sub 3/O/sub 9-//sub delta/ (with deltaapprox. =2) measured resistively in a diamond anvil cell is found to increase at a rate dT/sub co//dp = 0.043 K kbar/sup -1/ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which dT/sub co//dp = 0.64 K kbar/sup -1/. The pressure dependence of the high-T/sub c/ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.

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 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/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 .