Rinke J. Wijngaarden

Superconductivity at 108 K in YBa2Cu4O8 at pressures up to 12 GPa

Abstract Resistive measurements in a cryogenic diamond anvil cell show that the T c of YBa 2 Cu 4 O 8 can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p ⪷ 5 GPa, d T c /d p ≊ 5 K/GPa. At higher pressures d T c /d p decreases gradually and T c saturates at ≈ 108 K. This behaviour is reproduced by a phenomenological model where T c depends only on the number of holes in the CuO 2 planes.

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.

Halving the Casimir force with Conductive Oxides

The possibility to modify the strength of the Casimir effect by tailoring the dielectric functions of the interacting surfaces is regarded as a unique opportunity in the development of micro- and nanoelectromechanical systems. In air, however, one expects that, unless noble metals are used, the electrostatic force arising from trapped charges overcomes the Casimir attraction, leaving no room for exploitation of Casimir force engineering at ambient conditions. Here we show that, in the presence of a conductive oxide, the Casimir force can be the dominant interaction even in air, and that the use of conductive oxides allows one to reduce the Casimir force up to a factor of 2 when compared to noble metals.

Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films.

We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

Fast determination of 2D current patterns in flat conductors from measurement of their magnetic field

Abstract An extremely fast method is presented to calculate the local current–density vector in a flat conductor from the z -component of the magnetic field measured above its surface, e.g. by means of magneto-optical indicators, Hall-probe arrays or scanning SQUIDs. The method may be used for samples of arbitrary thickness provided that the current vector has only x - and y -components. The method combines the conjugate gradient (CG) method and fast Fourier transform to invert the relevant Toeplitz matrix equation. For a current map of n × n pixels, the number of operations needed is of order n 2.8 only, compared to n 4.5 or higher for earlier methods. The increase in speed for 512×512 pixels is found to be a factor 135 with respect to the fastest existing CG method.

Pressure dependence of the Tc of BiCaSrCuO up to 80 kbar

Abstract The superconducting transition temperature T c of polycrystalline BiCaSrCuO ( T c =92.4 K) was measured by monitoring the electrical resistivity in a diamond anvil cell. T c is found to decrease at a rate of d T c /d p =−0.16 K/kbar up to 80 kbar. This is the first high temperature superconductor which shows a clear decrease of T c with pressure. The pressure dependence is qualitatively explained in terms of a Hubbard-like model.

Avalanche dynamics, surface roughening, and self-organized criticality: Experiments on a three-dimensional pile of rice

We present a two-dimensional system that exhibits features of self-organized criticality. The avalanches that occur on the surface of a pile of rice are found to exhibit finite size scaling in their probability distribution. The critical exponents are tau=1.21(2) for the avalanche size distribution and D=1.99(2) for the cutoff size. Furthermore, the geometry of the avalanches is studied, leading to a fractal dimension of the active sites of d(B)=1.58(2). Using a set of scaling relations, we can calculate the roughness exponent alpha=D-d(B)=0.41(3) and the dynamic exponent z=D(2-tau)=1.56(8). This result is compared with that obtained from a power-spectrum analysis of the surface roughness, which yields alpha=0.42(3) and z=1.5(1) in excellent agreement with those obtained from the scaling relations.

Mg/Ti multilayers: Structural and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer shortrange order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming

Dendritic flux penetration in Pb films with a periodic array of antidots

We explore the flux-jump regime in type-II Pb thin films with a periodic array of antidots by means of magneto-optical measurements. A direct visualization of the magnetic flux distribution allows us to identify a rich morphology of flux penetration patterns. We determine the phase boundary

Microtorquemeter for magnetization measurements on small superconducting samples

{H}^{*}(T)