N.J. Koeman

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.

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.

Performance enhancement of metal-hydride switchable mirrors using Pd/AlOx composite cap layers

A drastic improvement of the optical properties and lifetime of switchable mirrors is obtained by placing a thin AlOx buffer layer between the Pd cap layer and the optically active, rare earth layer. The buffer layer lowers the minimum necessary Pd thickness to ∼1 nm, resulting in a ≈20% increase of the maximum transmittance. The optimal Pd and Al layer thicknesses are determined for the YHx and LaHx system using a powerful combination of optical and matrix film techniques. The AlOx buffer is shown to be superior to the native oxide layers YOx and LaOx. The buffer layer is essential for lanthanum, which is a particularly vulnerable, but fundamentally very important material. Using this composite cap layer, we have been able to switch LaHx films several times.

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 .

Roughness in Nb/Cu multilayers determined by x‐ray diffraction and atomic force microscopy

Nb/Cu multilayers grown by molecular beam epitaxy have been studied by x‐ray diffraction and atomic force microscopy. X‐ray diffraction provides the average interface roughness while atomic force microscopy shows the roughness and topology of the upper surface. Comparison of both methods shows that high‐angle diffraction averages over a lateral length which is in good agreement with the typical grain size.

Structural, optical and electronic properties of LaMgHx switchable mirrors

Structural, optical and electronic properties of lanthanum magnesium alloy thin films are studied in situ in real time during hydrogenation. X-ray data show that the as-deposited films contain the intermetallic phase LaMg with CsCl structure as well as fcc β-La and fcc LaHx. Hydrogenation initiates a transformation of β-La to β-LaHx, while the intermetallic phase disproportionates. The fcc structure stays intact during further uptake of hydrogen up to LaH3, and Mg transforms to the rutile structure of MgH2. During hydrogen absorption the alloys change from a good conducting, metallic state with high reflectance, to a transparent color neutral insulator with a band-gap of about 3 eV.

Pressure-composition isotherms and lattice dilation of YBa2Cu3Ox

Abstract The oxygen concentration x (6.2 2 Cu 3 O x is measured as a function of oxygen pressure for temperatures between 800 K and 1100 K. The data are in agreement with the predictions of a simple order-disorder model for the orthorhombic-to-tetragonal phase transition which takes into account a repulsive oxygen-oxygen interaction and a concentration dependent enthalpy of oxygen solution. The calculated pressure-composition isotherms show that at T > 800 K pressures well in excess of 1 kbar are required to reach x = 7.0. From measurements of the lattice dilation as a function of oxygen pressure it is concluded that oxygen contracts the lattice strongly in the orthorhombic structure, but (almost) not in the tetragonal phase.

Thermal expansion, compressibility and Grüneisen parameter of YBa2Cu3Ox

Abstract The thermal expansion of YBa2Cu3Ox has been measured between 4 K and 1100 K. The bulk isothermal compressibility has been determined from direct measurement of the pressure-volume relation on compacted samples by means of a simple powder model which takes into account the porosity of the sample. From these data the average Gruneisen parameter 〈γ〉 ≡ -d ln 〈ω〉 / d ln V is found to be 3.1.