V. Grosse

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.

Long-time stable high-temperature superconducting DC-SQUID gradiometers with silicon dioxide passivation for measurements with superconducting flux transformers

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evaluation and the relaxation of magnetic nanoparticles, it is important to maintain reliable sensor performance over an extended time period. We have designed and produced DC-SQUID gradiometers based on YBa2Cu3O7−x (YBCO) thin films which are inductively coupled to a flux transformer to achieve a higher sensitivity. The gradiometers are protected against ambient atmosphere and humidity by SiO2 and amorphous YBCO layers. The noise properties of the sensor in flip–chip configuration, especially in unshielded environments, are shown. We present a comparison of Tl2Ba2CaCu2O8+x (TBCCO) thin films on buffered sapphire or LaAlO3 substrates for the flux transformer in shielded and unshielded environments. We reach a low white field gradient noise of 72 fT cm−1 Hz−1 with the TBCCO on LaAlO3 flux transformer. The electric properties of the gradiometers (critical current IC, normal state resistance RN and the transfer function VΦ) were measured over a period of one year and do not show significant signs of degradation.

Planar high-Tc superconducting quantum interference device gradiometer for simultaneous measurements of two magnetic field gradients

Here, we present a planar superconducting quantum interference device (SQUID) structure consisting of four SQUIDs connected to antenna structures for simultaneous measurements of two magnetic field components. Using YBa2Cu3O7−x thin film technology, we are able to prepare gradiometer structures on strontium titanate (SrTiO3) tetracrystals containing two crossing 30° grain boundaries. This allows the production of four SQUID loops onto one substrate which can fulfill the high demands of symmetry inside the antenna structures. In our arrangement, the SQUID loops can act as independent field sensors which allow a measurement of two independent magnetic field gradient components.

Pulsed laser deposition of niobium thin films for in-situ device fabrication and their superconducting properties

Here, we studied the crystallographic and superconducting properties of niobium thin films grown by pulsed laser deposition. Depending on laser fluence our samples showed a critical temperature of up to 8.4 K and critical current densities of 3.0 · 106 A/cm2 at 4.2 K. X-ray diffraction measurements and TEM images suggest a granular structure with a preferred orientation of the (110) lattice plane parallel to the substrate surface. The superconducting properties of our films are significantly influenced by this granularity and the oxygen content in the film. We discuss the temperature dependence of the critical current density in the framework of an crossover from Ginzburg-Landau to Ambegaokar-Baratoff type behaviour. According to this the current transport is mainly dominated by Josephson-tunnelling in a granular network.

Observation of a Coulomb blockade in strontium titanate thin films

We report on the study of the low temperature electronic transport in strontium titanate thin films having a thickness of 25–30 nm. I-V characteristics measured below 4.2 K show unique structures which are evidence for the occurrence of a Coulomb blockade. Simulations of the measured characteristics suggest that this behavior is closely related to the formation of nanoscopic metallic islands within an insulating matrix which arrange in a network connected via tunneling currents.

Iron Pnictide Thin Film Hybrid Josephson Junctions

Thin films of iron pnictides open the way for fundamental experiments on superconductivity in this material. Thus we started to develop tunneling and Josephson junctions with pnictide film electrodes. Different preparation methods for Josephson junctions were investigated and the first results are presented. Resistive measurements show a high superconductive transition temperature of about 20 K even for the La-1111 electrode after patterning and preparation of the tunneling window. The hybrid junctions were completed with a PbIn counter electrode and normal conducting gold layers as barriers.

Higher Order HTSC Gradiometer for Measurements in Unshielded Environment

We present a new higher order gradiometer layout for bicrystal Josephson junctions based on high-temperature superconducting thin films. Our single layer gradiometers are suitable for the simultaneous measurement of magnetic field gradients with different spatial orientations. The sophisticated technology for bicrystal Josephson junctions is combined with custom substrates with two crossing 30 degree grain boundaries. We present the technological challenges focusing on the low ohmic contact resistance using gold nanoparticles embedded in the thin film. The sensor performance is characterized using SQUID electronics. Its main parameters such as critical current and ICRN-product are compared with parameters of first order gradiometers on single 30 degree bicrystal substrates. Special emphasis is put on the optimization of the noise properties in unshielded environment.

Comparison of High Temperature Superconducting Gradiometers Using Flip Chip YBCO and TBCCO Antennas

To optimize our high-temperature superconducting (HTSC)-dc-SQUID gradiometric sensors with galvanically-coupled antennas, we investigated the performance of antennas on a separate substrate. These antennas are inductively coupled to a dc-SQUID-gradiometer in a flip-chip configuration. Different combinations of thin-film and substrate materials, including Tl₂Ba₂CaCu₂O₈__x (TBCCO) on Al₂O₃ or LaAlO₃ and YBa₂Cu₃O_7-x (YBCO) on SrTiO₃, were used to produce antennas with variable layouts. These were then characterized concerning their critical current densities, critical temperature, and noise properties to demonstrate their suitability for high-resolution magnetic measurements in a flip-chip configuration. Field gradient resolution and noise properties of multiple dc-SQUID gradiometer antenna combinations were determined and compared.