F.J.G. Roesthuis

Superconducting thin films of MgB2 on Si by pulsed laser deposition

Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films.

The road to magnesium diboride thin films, Josephson junctions and SQUIDs

The remarkably high critical temperature at which magnesium diboride (MgB2) undergoes transition to the superconducting state, Tc approx 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB2 films. An overview of the deposition techniques for MgB2 thin film growth will be given, with a special focus on the in situ two-step process.

Material aspects for preparing HTS quasiparticle injection devices

Quasiparticle (QP) injection devices based on HTS could play an important role in future superconducting applications if material aspects can be better controlled. One reason why this kind of device received little attention in the past is the lack of an appropriate barrier for QP tunnelling. In a series of experiments, we used different barriers to test if they are suitable, i.e. if a current and possibly a voltage gain can be achieved. We improved the performance of planar YBCO/natural barrier/Au devices and a current gain of more than 6 at 40 K was observed. Most devices, however, showed signs of heating effects. Another barrier material was SrTiO/sub 3/ with layers of 5-6 nm thickness. Current-voltage characteristics showed that the barriers were continuous and we observed current gains of up to 3 at 60 K. PrBa/sub 2/Cu/sub 3/O/sub 7-x/ is an interesting candidate if one could overcome the problem of resonant inelastic tunnelling for QP. In a series of experiments we demonstrated that, even for 3 Mn thin PBCO barriers on a- and c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-x/, most devices showed at best a current gain of 1. However, we have indications that a current gain of 10 could be possible with unity voltage gain.

Quasiparticle injection effects in YBa2Cu3Ox-based planar structures at high operating temperatures

The modulation of the supercurrent Is of a YBCO bridge by the quasiparticle-injection current IG from the YBCO/Au or YBCO/PBCO/Au junctions at temperatures of 60?85 K is determined by two effects: (1) summation of the currents IS and IG in the YBCO bridge, and (2) nonequilibrium suppression of IS by the quasiparticle injection. At a thickness of the PBCO barrier of 40 nm the modulation of IS can be described by the current-summation effect only. For YBCO/Au structures the current gain ?IS/?IG increases linearly with decreasing temperature, reaching the value of 2 at 60 K. Numerical simulations of the current- voltage characteristics show an increase of the effective temperature T* of the YBCO bridge under injection only at small thicknesses of the PBCO barrier. Visualization of the voltage drop over the junction area by laser scanning microscopy shows a qualitative agreement with the electrical measurements with respect to the current summation and nonequilibrium effects.

Pulsed laser ablation of LaSrCoO

The ablation process of La0.5Sr0.5CoO3 by a spatial uniform 248 nm excimer laser beam has been characterized. Ablation rates, changes in the target surface morphology and composition have been studied as a function of the laser fluence. The dependence of the ablation rate on the laser fluence has been explained by a model based on plasma absorption. The threshold fluence for complete dissociation of the surface has been determined and is found equal to 0.7 J/cm2. Above this threshold, a steady-state target surface condition is obtained within 30 pulses. Below the threshold fluence, CoO nuclei form, which have a pronounced effect on the target composition and morphology. In addition, ellipsometric and reflectance spectra have been measured in the photon energy range of 1.8 to 5.0 eV.

Investigation of the microstructure of ramp-type YBa2Cu3O7-δ structures

We studied the morphology of ramps in YBa2Cu3O7-δ films and, subsequently, the barrier layer. The ramps have been fabricated by Ar ion beam milling using standard photoresist masks. SEM and AFM showed the formation of tracks along the slope of the ramp, originating from the irregular shape of the edge of the photoresist mask. A proposed modified reflowed resist and pre-annealing process show a significantly smoother ramp surface, important for the fabrication of reproducible Josephson junctions.

Observation of the AC Josephson effect up to THZ frequencies in YBCO/PBCO/YBCO ramp-type Josephson junctions

We present the response to 100 GHz irradiation of high-Tc Josephson junction devices for mixer/detector applications in the (sub-) mm wave range. These devices consist of a YBCO/PBCO/YBCO ramp-type junction combined with a planar logarithmic periodic antenna. The critical current and the first two Shapiro steps modulate with 100 GHz power according to the resistively shunted junction (RSJ) model. At 10 K clear Shapiro steps have been observed up to a voltage of about 7.5 mV. This corresponds to phase locking of 3.6 THz AC Josephson oscillations by the 36th harmonic of the 100 GHz signal. The number of observed steps is currently limited by the available power, but they are present up to voltages strongly exceeding the IcRn product of the junction at all temperatures.

Quasiparticle-injection effect in YBa2Cu3Ox-based planar structures

The supercurrent IS of a YBCO bridge can be modulated by the quasiparticle-injection current IG from YBCO/Au or YBCO/PBCO/Au junctions. The behavior of these structures is determined by two effects: 1) summation of the currents IS and IG in the YBCO bridge; 2) nonequilibrium suppression of the supercurrent IS by the quasiparticle-injection. The current gain coefficient ?IS/?IG increases linearly with decreasing temperature, reaching a value of 1.5 for YBCO/Au structures at 65 K. The nature of the nonequilibrium state and the effectiveness of the PBCO barrier layer for the formation of the quasiparticles are analyzed

Temperature and electric-field dependencies of PBCO c-axis resistivity in YBCO/PBCO/Au structures

The current-voltage characteristics of YBCO/PBCO/Au planar structures reflect the resistance behavior of PBCO in the c-axis direction. With increasing applied voltages the PBCO barrier shows a transition from thermally-activated hopping conductivity to an activationless-hopping regime. Variable-range hopping and weak-localization models are discussed to explain the experimental data. An account of the Lifshitz correlation in the hopping conductivity gives an satisfactory agreement with the junction resistivity for c-axis PBCO barrier thicknesses of 10 to 40 nm.

Fabrication aspects of microwave devices, including ramp-type high-Tc Josephson junctions and log-periodic antennas

We describe the development of high-Tc Josephson junction devices for applications at millimeter wave frequencies. These devices consist of ramp type YBCO/PBCO/YBCO Josephson junctions that are equipped with a noble metal log-periodic antenna. Growth conditions of all layers, as well as etching, cleaning and annealing procedures are being optimized, to guarantee well-defined device properties. Lowering the deposition temperature of the thick PBCO layer strongly improved its isolating properties, which is of extreme importance for good reproducability of junction fabrication. Special attention is being focused on the optimization of the contact of noble metal to YBCO as well its adhesion to the substrate. Best results are obtained using sputtered gold contacts, after a soft Ar ion sputter clean treatment of the damaged YBCO surface, followed by an anneal procedure.