Parameter study of in-situ grown superconducting YBaCuO thin films prepared by laser ablation

A parameter study has been made of YBaCuO thin films prepared by laser oblation. The films were deposited on SrTiO3, MgO, and Si with ZrO2 buffer layer at substrate temperatures between 600°C – 800°C. With optimization of different parameters like substrate temperature, pulse frequency, target-substrate distance, and oxygen pressure, zero resistivity was derived for the in-situ grown films on SrTiO3 at 89.5 K, on MgO at 84.6 K, and on Si (ZrO2) at 83.5 K

Conductance studies on different types of Nb/Al, AlOx,(/A1)/Nb Josephson tunnel junctions

The conductance-voltage characteristics of different types of Josephson tunnel junctions have been measured at 4.2 K: symmetric Nb/Al, AlOx/Al/Nb, asymmetric Nb/Al, AlOx/Nb and Nb/Al/AlOx-/AlOx/Nb, containing a double oxidation layer. The symmetric junctions can be described very well by a trapezoidal potential barrier model. The asymmetric junctions show less agreement with theory. In these junctions resistance switching occurs, possibly due to charge trapping. The resistance is lower than in symmetric junctions. The conductance measurements on double oxidation layer junctions show, that this type of junction has an inhomogeneous oxide layer

High Sensitivity Magnetic Flux Sensors with Direct Voltage Readout Double Relaxation Oscillation SQUIDs

The experimental sensitivity of double relaxation oscillation SQUIDs (DROSs) has been compared with theory and with the results obtained by numerical simulations. The experimental sensitivity ranges from 60 to 13h, where h is Planck's constant, for relaxation frequencies from 0.4 up to 10 GHz. For low frequencies the DROS characteristics can be explained by thermal noise on the critical currents. For high frequencies, the voltage-flux characteristics and the sensitivity are limited by the plasma frequency. The cross-over frequency is at 2 GHz, which is about 2% of the plasma frequency of the DROS

Josephson junctions and DC SQUIDS based on Nb/Al technology

A process for fabricating high-quality Josephson junctions and DC SQUIDS on basis of Nb/Al technology has been developed. DC magnetron sputtering is used for the deposition of the metal layers and the barrier is formed by thermal oxidation of the Al-layer. The junction area of 5 µm × 5 5 µm is obtained using anodisation. Three types of Josephson tunnel junctions have been prepared: standard Nb/Al, AlOx/Nb, symmetric Nb/Al, AlOx, Al/Nb and Nb/Al, AlOx/AlOx/Nb, the latter having a double oxide layer. We performed current-voltage and conductance-voltage measurements at different temperatures and special attention was paid to the noise behaviour. Gap and sub-gap parameters as well as barrier parameters are presented. Three different DC SQUID configurations were developed on basis of the Nb/Al Josephson junctions. The measured characteristics of the standard Tesche-Clarke DC SQUID, the resistively shunted SQUID and the inductively shunted SQUID are compared with special attention being paid to the noise properties. A 19-channel DC SQUID magnetometer with standard and/or resistively-shunted DC SQUIDs is under construction

Fabrication of planarized small area Nb/Al,AlO_x/Al/Nb Josephson tunnel junctions using reactive ion etching in SF₆

High quality Nb/Al,AlOx/Al/Nb Josephson tunnel junctions have been made with the help of a fabrication process based on reactive ion etching of Nb in SF6. The Vm value of these junctions is typically 60–70 mV at 4.2 K. At 1.6 K, a Vm of 4.1 V has been measured, which is the highest value that has ever been reported for this type of junction. The area of the junctions ranges from 1 to 25 μm2. By burying the Nb/Al,AlOx/Al/Nb trilayer in the substrate, a planarized junction configuration has been obtained. Reactive ion etching of Nb in SF6 plasmas has been studied in detail. Anisotropic etch profiles can be obtained because of the formation of a resistant layer during etching, which prevents etching of Nb under the photoresist. The etching process has been monitored with a spectrometer. The fluorine emission at 703.7 nm is shown to be suitable for end point detection

Relaxation Oscillation SQUIDs With High δV/δφ

Relaxation oscillation SQUIDs (ROSs) based on Nb/Al, AlOx/Al/Nb Josephson tunnel junctions have been designed and fabricated. The hysteretic SQUIDs (superconducting quantum interference devices) have a maximum critical current of about 130 μA and an inductance of 20 pH. A voltage modulation of 40 μV and a flux to voltage transfer δV/δΦ of 4 MV/Φ0 have been measured in these SQUIDs. Double relaxation oscillation SQUIDs (DROSs), which are based on two hysteretic SQUIDs, showed transfer coefficients up to 77 mV/Φ0. The intrinsic white flux noise of the DROSs is smaller than 3-5 μΦ0/√Hz. It is noted that the results are very promising for a next generation of SQUID systems with simplified read-out