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Superconductors

Superconducting Tunnelling Junctions

The Group’s research in this field is focused on developing an entirely novel architecture of the Josephson Junction (JJ) by combining our expertise in vicinal surfaces and the Atomic Terrace Low Angle Shadowing (ATLAS) deposition technique. JJs are at the core of many quantum mechanical circuits such as SQUIDS, RSFQ logic circuits and are currently the most promising technology for the implementation of quantum computing through JJ based quantum bits.
The common JJ consists of an ultra-thin (≈2nm of AlOx) insulating tunnelling barrier sandwiched between two superconducting thin films (usually Nb or Al). The inability of AlOx to provide a reliable barrier, free of pinholes and defects, at thicknesses below 2nm is the source of many limitations of current JJs.

The new architecture explored in this project is reliant upon incorporating the insulating layer through stepped substrates. The APRG has vast experience in studying and controlling the formation of highly regular arrays of steps and terraces through the annealing of miscut substrates (See the schematic below). This has provided the Group with the essential knowledge required to control the step height and separation with atomic scale precision. Research is also ongoing into etching substrates to form a single step as an additional approach to forming atomic-scale ordered steps.

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Deposition of the superconducting material involves the collimation of the evaporating flux and precise control of the angle of the substrate relative to the collimated beam. Deposition onto only a portion of the substrate can be achieved, for example the step edge or the terrace. The resulting structure will consist of superconducting layers deposited on the substrate terraces, while the step edges are uncovered i.e. enabling us to incorporate the vital insulting JJ layer in the form of a defect-free single crystal template.

Combining these perfected techniques will result in significant improvements to 1)  the fundamental properties of the JJs themselves, i.e. increasing the upper limit of the critical current and 2) devices constructed using them.

Superconducting Tunnelling Junctions

The benefits of this new approach will objectively allow us to:

  1. Construct a tunnel barrier of a virtually perfect single crystalline dielectric.
  2. Choose the thickness of the tunnel barrier in the range from 0.2 to 4 nm and explore the region of small thicknesses and the exponential increase in critical current associated with these junction widths. 
  3. Maintain ultimate uniformity of the width of the tunnel barrier across the junction down to the level of single atom precision, thus reducing the effect of decoherence due to barrier thickness variation.
  4. Apply this methodology to a range of important materials for tunnelling barriers beyond AlOx, e.g. Mgo, Si, NiO, MgAl2O4, which cannot be investigated using the typical JJ structure.
Superconducting Tunnelling Junctions