H. Schneidewind

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 Φ0u2009Hz−1/2 or 8×10−13 Tu2009cm−1u2009Hz−1/2 down to 10 Hz. At 1 Hz noise increases up to 6×10−12 Tu2009cm−1u2009Hz−1/2.

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.

Resonant terahertz radiation from Tl2Ba2CaCu2O8+δ thin films by ultrafast optical pulse excitation

We have observed the free-space radiation of the resonant terahertz wave from c-axis oriented Tl2Ba2CaCu2O8+δ thin films by femtosecond optical pulse excitation under a radial magnetic field of about 100 Oe nearly parallel to the c axis of the thin film. The observed wave form showed clear oscillations up to 80 K. The frequency was shifted from ∼620u2009GHz at 24 K to ∼300u2009GHz at 80 K, corresponding to an increase in c-axis penetration depth, λc, from ∼26u2009μm to ∼53u2009μm, and disappeared above TC as expected for the Josephson plasma resonance.

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.

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.

High-T/sub c/ Josephson junctions and DC SQUIDs

Step edge Josephson junctions and DC superconducting quantum interference devices (SQUIDs) made of laser ablated and sputtered YBaCuO (YBCO) films and grain boundary junctions in polycrystalline TlBaCaCuO (TBCCO) films have been investigated. Bridges on the micrometer scale have been structured by laser or ion beam etching as well as by the inhibit-layer technique. The Josephson junctions are characterized comparing I-V measurements under microwave irradiation to calculations within the resistively shunted junction (RSJ) model with finite capacitance and white noise. The dependence of the I/sub C/R/sub N/ product on temperature can be fitted to the microscopic theory of superconductor-normal insulator-normal superconductor (SNINS) or superconductor-normal conductor-superconductor (SNS) junctions with additional temperature-independent pair breaking. Flux modulation and noise properties of the DC SQUIDs at 77 K are shown.<<ETX>>

Investigations on high- thin film intrinsic stacked Josephson junctions

We have prepared thin film mesa structures from highly anisotropic 2212 superconductors which act as stacks of intrinsic Josephson junctions corresponding to the planes coupled by non-superconducting parts of the unit cell. Results are shown on the current - voltage characteristics and temperature and magnetic field dependence of the critical current. The response of the junction to external microwave irradiation shows a reduction of critical current and step-like structures which should be studied in detail.

Superconducting properties of ion beam modified YBCO microbridges

We prepared ion beam modified microbridges based on thin sputtered or laser ablated YBCO films on SrTiO3 substrates. The microbridges with a width of 10 μm were irradiated through slits in a 700 nm thick double layer resist mask. In our experiments we used O+ ions with an ion energy of 30 keV or 100 keV varying the dose between 1013 ions·cm−2 and 1014 ions·cm−2. We investigated the influence of film thickness and slit width on the superconducting properties of these junctions. We show the temperature dependence of the junctions properties on microwave radiation or external magnetic fields.

First experimental investigations on a thin film Hall magnetometer with a high temperature superconducting pick-up antenna

Abstract First experimental investigations were performed on an integrated hybrid magnetometer, consisting of a thin film high-T C superconducting magnetic pick-up system and a Hall detector made of bismuth. Sensitivity measurements in the temperature range between 70 and 300xa0K were performed. We found a steep increase of the sensitivity by a factor of about 100 below the transition temperature of the pick-up system as expected from calculation of its field amplification. At a temperature of 77xa0K we measured a resolution of 8xa0pT/√Hz in the white noise range.