F. Schmidl

Thin‐film dc SQUID gradiometer using a single YBa2Cu3O7−x layer

A dc SQUID gradiometer with a large effective pickup area using only a single YBa2Cu3O7−x layer and step‐edge Josephson junctions on SrTiO3 substrate has been prepared. The planar gradiometer consists of two loops coupled directly to a small area SQUID in the center of the structure. The main advantage in comparison to magnetometers is the possibility of direct measurements in a magnetically unshielded environment. At 77 K without bias reversal technique we obtain a white noise level of about 4.5×10−5 Φ0 Hz−1/2 or 8×10−13 T cm−1 Hz−1/2 down to 10 Hz. At 1 Hz noise increases up to 6×10−12 T cm−1 Hz−1/2.

BaFe1.8Co0.2As2 thin film hybrid Josephson junctions

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe1.8Co0.2As2 thin film electrode, an Au barrier, and a PbIn counter electrode. The junction shows resistively shunted junction-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2 K. The temperature dependence of the critical current shows nearly linear behavior near TC. Well-pronounced Shapiro steps are observed at microwave frequencies of 10–18 GHz. Assuming an excess current of 200 μA at 4.2 K the effective ICRN product calculates to 7.9 μV.

Preparation and first measurements of TBCCO thin film intrinsic stacked Josephson junctions

We describe the preparation of high-Tc thin film intrinsic stack junctions (mesas) and first realizations with TBCCO films. For contact areas ranging from 3*3 to 7*7 mu m2 and stack heights between 50 and 100 nm we find intrinsic Josephson behaviour as it is well known from experiments with single crystals.

Dry-etching processes for high-temperature superconductors

Abstract Patterning of thin films and multilevel structures of high-temperature superconductors is a key technology for microelectronic applications. We performed a comparative study of Ion Beam Etching (IBE) and Reactive Ion Etching (RIE) processes for YBa 2 Cu 3 O 7−δ thin films. The RIE process with a pure chlorine plasma yielded small etching rates, caused by chemical modifications of the sample surface which result in a passivation layer reducing the chemical etching rate. Using IBE, microstructures down to the 1 μm regime could be fabricated without reducing the critical temperature T c and the critical current density J c of the material. Etching rates up to 40 nm/min could be achieved without deteriorating the properties of the superconducting film by cooling the sample effectively during the etching process. The influence of the etching process on J c was investigated by imaging the spatial distribution of the critical current along the patterned microstructures using Low-Temperature Scanning Electron Microscopy (LTSEM).

High-Tc directly coupled direct current SQUID gradiometer with flip-chip flux transformer

We have designed and fabricated a flip-chip-type gradiometer as a sensor for measurement of weak magnetic signals. The gradiometer consists of a directly coupled direct current superconducting quantum interference device (SQUID) gradiometer coupled to a flux transformer made from a YBa2Cu3O7*/YBa2Cu3O7/CeO2/YSZ multilayer on 2 in. Si wafer, where YBa2Cu3O7*(YBCO*) is a nonsuperconducting crystalline film. The coupling coefficient of 0.18 is obtained for the flip-chip gradiometer. The transformer increases effectively the resolutions of the gradiometer. We obtain a magnetic field gradient resolution of 73 fT cm−1 Hz−1/2 in the white region, and 596 fT cm−1 Hz−1/2 at 1 Hz. The magnetocardiograms were successfully measured at a distance of nearly 100 mm from the gradiometer center to the human heart in a shielded room.

On ageing and critical thickness of YBa2Cu3O7 films on Si with CeO2/YSZ buffer layers

Abstract The ageing behaviour of epitaxial YBa 2 Cu 3 O 7 (YBCO) films on Si (100) with CeO 2 /YSZ buffer layers is studied. For the YBCO/CeO 2 /YSZ/Si system, its ageing is weaker than the YBCO/YSZ/Si system. The experimental critical thickness of YBCO films for YBCO/CeO 2 /YSZ/Si systems is only 40 nm, otherwise cracks appear in the film. The formation of cracks is analyzed by means of fracture mechanics theory. The estimated thermal stress and elastic strain in crack-free YBCO layers reach 2.41 GPa and 1.5% at 77 K, respectively. Depositing another non-superconducting YBCO (YBCO*) film on top of YBCO as a passivation layer makes the critical thickness increase up to 70 nm. These films cannot bear surface etching and tempering temperatures higher than 110°C in lithography.

Planar gradiometers with high- DC SQUIDs for non-destructive testing

We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.

Large-area YBCO films for device fabrication

We have prepared YBCO thin films by a pulsed laser deposition (PLD) technique on substrates and on the naturally oxidized 2 in diameter silicon wafers with buffer layers. This large-area film PLD technique allows the preparation of homogeneous and reproducible YBCO films with high quality on classical and technical substrates. For 2 in silicon wafer, we obtained the critical temperature in the range from 87.5 K to 88.2 K and a critical current density at 77 K of from 0.53 to over the whole wafer.

One-dimensional intrinsic Josephson arrays based on HTS thin films

Abstract Mesa structures were produced from different highly anisotropic HTS thin films showing the intrinsic Josephson effect in c -direction. We observed clear dependencies of the critical current on both external magnetic field and microwave field. For the first time we were able to detect the modulation of the critical current of BSCCO thin film mesas in external magnetic field. For BSCCO as well as for TBCCO we found modulation periods in the Tesla range which is in good agreement with the lateral dimensions of the junctions. In the current-voltage characteristics of TBCCO mesas we found subgap structures and measured their dependence on temperature, external magnetic field and microwave irradiation.

Recovery of superconductivity and recrystallization of ion-damaged YBa2Cu3O7−x films after thermal annealing treatment

Abstract We have systematically studied the annealing effects of high-quality epitaxial YBa 2 Cu 3 O 7− x (YBCO) films on (100) SrTiO 3 substrates implanted with 100 keV Ar ions at different fluences. The mechanisms for T c degradation induced by ion implantation have been discussed. After annealing in oxygen atmosphere in a temperature range from 450 to 950°C, R ( T ) curve measurements show a partial or complete recovery of superconductivity. The recrystallization of ion-implanted YBCO films have been investigated using X-ray diffraction (XRD) and Rutherford backscattering spectrometry with channeling. It is found that the annealing at 450°C indeed results in the movement of oxygen defects and ordering of the oxygen sublattice in the sample implanted with low to moderate fluences, however, only a small partial recovery of superconductivity is observed. The results suggest that ion implantation induced T c degradation is mainly caused by disordering of the cation sublattice not the oxygen sublattice. After annealing at 850°C, the recovery of superconductivity and recrystallization of implanted YBCO films are related to the fluence. For YBCO films implanted with low to moderate fluences (1 × 10 11 −1 × 10 14 /cm 2 ), the superconductivity can be nearly completely recovered, and the pure c -axis orientation of the samples is observed in the XRD spectra. However, recrystallization of amorphous YBCO films implanted with high fluences (1 × 10 15 −1 × 10 16 /cm 2 ) is quite different. The orientation of the samples depends on the annealing temperature. With the increasing of the annealing temperature from 750 to 950°C, the regrowth orientation gradually changes from a - to c -axis orientation. After annealing at temperatures of 750 and 800°C, the samples have the nearly pure a -axis orientation. At 850°C, the sample has a mixed a - and c -axis oriented structure but the a -axis orientation is still dominant. Above 900°C the structure of the annealed sample has the pure c -axis orientation. In the case of amorphous YBCO films caused by high-fluence Ar ion implantation, the T c0 is lower than 77 K in the whole annealing temperature range due to the polycrystalline structure. The experimental results indicate that the amorphous YBCO films formed by ion implantation regrow homogeneously rather than epitaxially and thus have the polycrystalline structure. This effect can be used to make a -axis oriented YBCO films and Josephson junctions or weak links by ion implantation with a narrow bridge mask or focused ion beam technique.