F.C. Wellstood

Sub-gap leakage in Nb/AlO/sub x//Nb and Al/AlO/sub x//Al Josephson junctions

In an effort to determine the suitability of Josephson junctions for applications in quantum computation, we measured low-noise dc current-voltage characteristics of Nb/AlO/sub x//Nb and Al/AlO/sub x//Al junctions in the temperature range from 90 mK to 1 K. Nb-based samples were obtained from several different facilities with critical currents of a few /spl mu/A and critical current densities between 100 and 3000 A/cm/sup 2/. We fabricated the Al-based samples using double-angle evaporation and obtained critical currents of a few /spl mu/A with critical current densities of about 30 A/cm/sup 2/. We found that the sub-gap leakage current in the Nb-based samples does not depend on temperature in the range 90 mK to 1 K, whereas that for the Al-based samples follows the expected BCS behavior to about 150 mK. Our Al-based samples have a lower level of dissipation than Nb-based devices; however, both Al- and Nb-based samples achieved dissipation levels sufficiently low for some quantum computing applications.

Imaging defects in Cu-clad NbTi wire using a high-T/sub c/ scanning SQUID microscope

We have used a sensitive magnetic microscope based on the dc Superconducting Quantum Interference Device (SQUID) to examine room-temperature samples of Cu-clad NbTi wire. Our SQUID microscope allows the wire (in air at room temperature) to be brought to within about 0.05 mm of the SQUID, although typically the separation is 0.3-0.4 mm. The wires have round or rectangular cross-sections and are about 2 mm across. With the wire aligned parallel to the x-direction in the x-y scanning plane we apply currents of a few mA through the wire at a frequency of a few hundred Hz, and image the x-component of the magnetic field. Defects cause a redistribution of current and corresponding features in the magnetic field image. We show images of wires, and discuss the advantages of using the parallel component of the field.

Determination of magnetic properties using a room-temperature scanning SQUID microscope

We have used a YBCO de SQUID at 77 K to image room-temperature magnetic thin film samples. Samples imaged include Fe/sub 3/O/sub 4/, rare earth magnets such as samarium cobalt, and CMR materials. We typically saturate the magnetization of the sample in fields up to 8.5 Tesla, and then image the remanent (zero applied field) state. To help quickly interpret and quantify the SQUID microscope data, we have developed several analytical techniques. These techniques yield quantifiable results of magnetic properties, including magnetization, total dipole moment, and the demagnetizing field. We will present our results and discuss applications and limits of SQUID microscopy to the characterization of bulk and thin-film magnetic materials.

Behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor in the normal and superconducting state

We have studied the behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor (SET) in the normal state over a temperature range from 85 mK to 3 K. The fluctuator caused the SETs island charge to shift by /spl Delta/Q/sub o/=0.1/spl plusmn/0.025 e with an escape rate out of each state which was periodic in the gate voltage. We compare our results to a model which assumes the fluctuator resides in one of the tunnel junctions and discuss model predictions for when the device is in the superconducting state.