P. Seidel

Josephson effects in iron based superconductors

Just after the discovery of the iron based superconductors (iron pnictides) important differences from the conventional as well as to the high temperature superconducting cuprates were found. The complex band structure (multiple energy gaps, the unconventional symmetry of the superconducting order parameter and the unknown nature of the Cooper pair coupling mechanism) results in interesting new properties of these materials. This review summarizes the status of theoretical and experimental results on Josephson effects in the iron pnictides connected to these unusual properties. On one hand the investigation of Josephson effects can be a helpful tool to determine these properties and to test different theoretical models, but on the other hand these properties may lead to new kinds of Josephson junctions and future applications.

A morphological growth model for laser ablated Y1Ba2Cu3O7-x thin films

Abstract The aim of this work was to investigate the growth mechanism of thin Y1Ba2Cu3O7−x films ablated by LPVD. The Y1Ba2Cu3O7−x films were prepared by KrF-excimer laser ablation (248 nm) on SrTiO3 (100) substrates. To investigate film properties like surface morphology, epitaxy, c-axis length, critical current density jc (77 K) and critical temperature Toffc we varied the ablation parameters: target substrate distance dT-S, oxygen partial pressure pO2, substrate temperature Ts and laser energy density I. To analyze the films X-ray diffraction, Rutherford backscattering spectroscopy, scanning electron microscopy and electrical measurements were used. Three morphological images (zones) of the Y1Ba2Cu3O7−x film growth in dependence on pO2 and Ts can be distinguished and the growth model of Thornton can be applied as a general rule for optimization of Y1Ba2Cu3O7−x thin film growth. In a medium zone (zone T in the Thornton model) of pO2≈70Pa and Ts≈720°C with dT-S=4 cm and I=4.3 J/cm2 we get smooth, epitaxial films with Toffc > 90 K and jc (77 K) > 3x106 A/cm2. At high pO2, high dT-S and low I we found outgrowths on the films which are of the same stoichiometry as the surrounding film material.

Effect of heat treatment on mechanical dissipation in Ta2O5 coatings

Thermal noise arising from mechanical dissipation in dielectric reflective coatings is expected to critically limit the sensitivity of precision measurement systems such as high-resolution optical spectroscopy, optical frequency standards and future generations of interferometric gravitational wave detectors. We present measurements of the effect of post-deposition heat treatment on the temperature dependence of the mechanical dissipation in ion-beam sputtered tantalum pentoxide between 11 K and 300 K. We find that the temperature dependence of the dissipation is strongly dependent on the temperature at which the heat treatment was carried out, and we have identified three dissipation peaks occurring at different heat treatment temperatures. At temperatures below 200 K, the magnitude of the loss was found to increase with higher heat treatment temperatures, indicating that heat treatment is a significant factor in determining the level of coating thermal noise.

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.

Measurements of a low-temperature mechanical dissipation peak in a single layer of Ta2O5 doped with TiO2

Thermal noise arising from mechanical dissipation in oxide coatings is a major limitation to many precision measurement systems, including optical frequency standards, high-resolution optical spectroscopy and interferometric gravity wave detectors. Presented here are measurements of dissipation as a function of temperature between 7 K and 290 K in ion-beam-sputtered Ta2O5 doped with TiO2, showing a loss peak at 20 K. Analysis of the peak provides the first evidence of the source of dissipation in doped Ta2O5 coatings, leading to possibilities for the reduction of thermal noise effects.

Comparison of the temperature dependence of the mechanical dissipation in thin films of Ta2O5 and Ta2O5 doped with TiO2

Here we report the first results comparing the temperature dependence of the mechanical dissipation in thin films of Ta_2O_5 and Ta_2O_5 doped with TiO_2, of a type suitable for use in the multilayer optical coatings for advanced gravitational wave detectors. The results indicate that doping Ta_2O_5 with TiO_2 can significantly alter the distribution of activation energies associated with the low-temperature dissipation peak.

High mechanical Q-factor measurements on silicon bulk samples

Future gravitational wave detectors will be limited by different kinds of noise. Thermal noise from the coatings and the substrate material will be a serious noise contribution within the detection band of these detectors. Cooling and the use of a high mechanical Q-factor material as a substrate material will reduce the thermal noise contribution from the substrates. Silicon is one of the most interesting materials for a third generation cryogenic detector. Due to the fact that the coefficient of thermal expansion vanishes at 18 and 125 K the thermoelastic contribution to the thermal noise will disappear. We present a systematic analysis of the mechanical Q-factor at low temperatures between 5 and 300 K on bulk silicon (100) samples which are boron doped. The thickness of the cylindrical samples is varied between 6, 12, 24, and 75mm with a constant diameter of 3 inches. For the 75mm substrate a comparison between the (100) and the (111) orientation is presented. In order to obtain the mechanical Q-factor a ring-down measurement is performed. Thus, the substrate is excited to resonant vibrations by means of an electrostatic driving plate and the subsequent ring-down is recorded using a Michelson-like interferometer. The substrate itself is suspended as a pendulum by means of a tungsten wire loop. All measurements are carried out in a special cryostat which provides a temperature stability of better than 0.1K between 5 and 300K during the experiment. The influence of the suspension on the measurements is experimentally investigated and discussed. At 5.8K a highest Q-factor of 4.5 ? 108 was achieved for the 14.9 kHz mode of a silicon (100) substrate with a diameter of 3 inches and a thickness of 12 mm.

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.

The cubic to tetragonal phase transition in SrTiO3 single crystals near its surface under internal and external strains

The displacive phase transition in SrTiO3 was investigated by means of x-ray diffraction. We used 4.5 keV photons thus probing only a very thin region near the surface. In the low temperature phase the lattice parameters evolve substantially different than in bulk material. We also investigated the phase transition under the influence of an epitaxial coating with YBaCu2O7 and found the nature of the phase transition changed. The near-surface region behaves like an epitaxial thin SrTiO3 film.