B. Wuyts

Photoinduced enhancement of superconductivity

We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak‐link devices with in situ tunable superconducting characteristics.

Controlled preparation of oxygen deficient YBa2Cu3Ox films

We describe a simple method based on the oxygen pressure‐temperature phase diagram to prepare oxygen deficient YBa2Cu3Ox thin films. Systematic critical temperature and x‐ray diffraction experiments clearly show that films with different oxygen contents (6.6≤x≤7) are obtained in a controlled, reproducible, and reversible way.

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.

Growth of particle-free YBa2Cu3O7 films by off-axis sputtering

Abstract The preparation of good YBa 2 Cu 3 O 7 thin films is often hindered by the production of non-stoichiometric particles which degrade the superconducting properties of the layer. A systematic study of films prepared by off-axis single target magnetron sputtering shows that the presence of copper-rich particles dramatically decreases the critical current density but does not affect the critical temperature. The formation of these particles can be avoided by carefully adjusting the position of the substrate with respect to the target. Films without off-stoichiometric particles have excellent superconducting properties.

Epitaxial YBa2Cu3O7 superconducting films, without twin planes on Y2O3/YSZ/Si

Abstract Epitaxial c -axis oriented YBa 2 Cu 3 O 7 superconducting thin films have been grown by off-axis sputtering on Si substrates covered with an Y 2 O 3 /YSZ buffer layer. The films show good superconducting properties and are crack-free for thickness as high as 120 nm. TEM observations reveal an epitaxial growth of YBCO on Y 2 O 3 with an angle of 45° between the [100] directions. Both layers display a microstructure which consists of slightly misoriented rectangular twin-free monocrystalline mosaic blocks.

Oxygen Bonding in High-Temperature Superconductors

Oxygen evolution experiments in low- and high-Tc ceramic superconductors are used to identify the desorption kinetics and activation energies. It is found that oxygen desorps at 600 °C from Cu-O chains (where present) with an activation energy of 1.2 eV, whereas the desorption from other oxygen-containing planes occurs at 800 °C with a higher activation energy of 2.8 eV.

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa2Cu3O6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm−1, O(2)-O(3) at 338 cm−1, and O(4) at 488 cm−1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequenciesω, which is in agreement with the enhancement of the static conductivityσ(ω→0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.

A new method of synthesizing high-Tc superconducting materials

Abstract A new sol-gel method for the preparation of the ceramic superconductors has been developed based upon the formation of complexes of the metal cations by Ethylene Diamine Tetraacetic Acid (EDTA). The materials have been characterized by transport, magnetisation, critical field, XRD and SEM measurements. Screen printed thick films made out of the powders have been fabricated and characterized.

Oxygen Ordering and Superconducting Phases in Ceramics

The superconducting transition temperature and the orthorhombic distortion in YBa2Cu3O7-δ are found to be determined mainly by the oxygen occupancy of the Cu-O chains and may explain the discrepancies between experiments with equal total oxygen stoichiometry. These results are obtained from simultaneous gas evolution, superconductivity and X-ray diffraction experiments geared towards understanding the role of the oxygen ordering in ceramic superconductors. The evolution kinetics is found to proceed by an initial large change followed by a constant chain occupancy. The superconducting transition temperature follows the change in chain occupancy and clear evidence is presented for the occasional presence of two distinct superconducting oxygen phases which are difficult to uniquely establish from scattering experiments. The presence of these phases is critically dependent on details of the preparation condition.

Photoexcitation effects in YBa2Cu3Ox

Abstract We show through photoexcitation experiments carried out on YBa 2 Cu 3 O x c-axis oriented films that photoinduced phenomena are not restricted to insulating samples. Superconducting films with critical temperatures varying between 2 K and 40 K show a clear enhancement of the superconducting transition after illumination. The increase in T c is accompanied by a corresponding decrease in the resistivity ρ, and an increase in the Hall coefficient R H , indicative of an increased carrier density. Furthermore, the carrier mobility is also affected by illumination.