Jakob Flokstra

Preparation of YBa2Cu3O7- by citrate synthesis and pyrolysis

A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-Tc superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.

(Double) relaxation oscillation SQUIDs with high flux‐to‐voltage transfer: Simulations and experiments

Relaxation oscillation superconducting quantum interference devices (SQUIDs) (ROSs) and double relaxation oscillation SQUIDs (DROSs) have been fabricated and characterized. The SQUIDs are based on hysteretic Nb/Al,AlOx/Al/Nb Josephson tunnel junctions. The relaxation oscillations were simulated by computer. By these simulations the minimum and maximum values of the shunt inductance at which the relaxation oscillations persist have been determined for different values of the shunt resistance and of the critical current and capacitance of the junctions. The transfer from flux to dc voltage in ROSs increases linearly with the shunt resistance. The operation range of the shunt inductance, however, becomes smaller with increasing shunt resistance. In ROSs with a SQUID inductance of 20 pH, a maximum voltage modulation width of 400 μV and a flux‐to‐voltage transfer δV/δφ of 4 mV/φ0 have been obtained. Two ROSs have been operated in a simple flux‐locked loop with direct voltage readout. The experimental white flu...

High Tc thin films prepared by laser ablation: material distribution and droplet problem

The lateral material distribution of laser-deposited YBa2Cu3O7?? films and the density of droplets coming from the target were studied by varying the laser pulse energy, the laser spot size and the target-to-substrate distance. Silicon wafers at ambient temperature were used as substrates to guarantee a large sticking coefficient of the particles. The deposition rate is found to depend linearly on the laser energy density E and quadratically on the spot size S at the target, whereas the droplet density is slightly dependent on E and increases linearly with 1/S, yielding a threshold energy of 0.9 J cm?2. With a laser spot size of 7.15 mm2 and a laser energy density of 1.2 J cm?2, we were able to reduce the number of droplets to one to two per 500 ?m2 for a high quality high Tc film with a typical thickness of 100 nm.

Vortex trapping and expulsion in thin-film YBa2Cu3O7−δ strips

A scanning superconducting quantum interference device microscope was used to image vortex trapping asa function of the magnetic induction during cooling in thin-film YBa2Cu3O7-δ (YBCO) strips for strip widths W from 2 to 50 m. We found that vortices were excluded from the strips when the induction Ba was below a critical induction Bc. We present a simple model for the vortex exclusion process which takes into account the vortex-antivortex pair production energy as well as the vortex Meissner and self-energies. This model predicts that the real density n of trapped vortices is given by n=BaâBK /0 with BK=1.650 /W2 and 0=h/2e the superconducting flux quantum. This prediction is in good agreement with our experiments on YBCO, as well as with previous experiments on thin-film strips of niobium. We also report on the positions of the trapped vortices. We found that at low densities the vortices were trapped in a single row near the centers of the strips, with the relative intervortex spacing distribution width decreasing as the vortex density increased, a sign of longitudinal ordering. The critical induction for two rows forming in the 35 m wide strip was 2.89+1.91 0.93Bc, consistent with a numerical prediction.

Thermogravimetry and neutron thermodiffractometry studies of the H-YBa2Cu3O7 system.

The high Tc superconducting oxide YBa2Cu3O7?x reacts with hydrogen gas. Thermogravimetric, X-ray and neutron scattering experiments allow us to propose a two-step type of hydrogen bonding. Firstly, a few hydrogen atoms fill some oxygen vacancies and may favourably modify the electron state, giving rise to a slight increase in the critical temperature. Secondly, after a prolonged heating period, the collapse of the YBa2Cu3O7?x type framework and of superconductivity were observed, and a new, highly hydrogenated material appeared.

Fetal magnetocardiography: clinical relevance and feasibility

We investigated the feasibility of a high-Tc SQUID system for fetal magnetocardiography (fetal MCG) aiming at a system without a magnetically shielded room and cooled by a cryocooler. The targeted SQUID resolution was 50 fT/√Hz (1–100 Hz). The research was performed along three lines: environmental noise suppression, cooling and low-Tc experiments. Environmental noise can be suppressed by forming second-order gradiometers from individual magnetometers. Concerning cooling, we investigated the applicability of commercially available coolers. In the low-Tc experiments, the medical relevance of fetal MCG was clearly shown. However, they also indicated that, in order to fully exploit the medical potential, the targeted resolution has to be 10 fT/√Hz. This increased resolution, in combination with the required high reliability of the sensors, will be hard to realize in high-Tc technology. This paper describes the results of the project and discusses the feasibility of a clinical system.

Four layer monolithic integrated high Tc dc SQUID magnetometer

YBa2Cu3O7−x based monolithic integrated dc SQUID magnetometers, consisting of a dc SQUID integrated with a flux transformer on a single bicrystalline substrate, have been fabricated and characterized. The devices consist of four layers, including two superconducting layers, and first realizations operate up to 73 K. A maximum voltage modulation of 32 μV is observed at 40 K. A field sensitivity of 0.17 pT/√Hz is obtained above 200 Hz at 45 K and 0.49 pT/√Hz at 1 Hz and 65 K.

Fabrication and properties of Nb/Al,AlOx/Nb Josephson tunnel junctions with a double oxide barrier.

High‐quality Nb/Al, Alox/Nb Josephson tunnel junctions using double‐oxide layers as barriers have been fabricated. The critical current density is controlled by the thickness of the second Al layer. This layer has to be oxidized completely through in order to obtain high‐quality junctions. Typically, gap voltages of 2.8–3.0 mV and Vm up to 70 mV at 4.2 K were obtained.

Sensitivity of the spherical gravitational wave detector MiniGRAIL operating at 5 K

We present the performances and the strain sensitivity of the first spherical gravitational wave detector equipped with a capacitive transducer and readout by a low noise two-stage SQUID amplifier and operated at a temperature of 5 K. We characterized the detector performance in terms of thermal and electrical noise in the system output signal. We measured a peak strain sensitivity of 1.5x10{sup -20} Hz{sup -1/2} at 2942.9 Hz. A strain sensitivity of better than 5x10{sup -20} Hz{sup -1/2} has been obtained over a bandwidth of 30 Hz. We expect an improvement of more than 1 order of magnitude when the detector will operate at 50 mK. Our results represent the first step towards the development of an ultracryogenic omnidirectional detector sensitive to gravitational radiation in the 3 kHz range.

Radio frequency power sensor based on MEMS technology with ultra low loss

A wideband 100 kHz-4 GHz power sensor, of which the basic design was presented recently by L. J. Fernandez et al. (2004), was successfully realized. The sensor is based on sensing the electrical force between the RF signal line and a suspended membrane. Optimization of the design with SONNET has resulted in measured reflection losses (S11) less than -30 dB, transmission losses (SI2) better than -0.2 dB, and a sensitivity of 80 aF/mW.