Normal-state conductance used to probe superconducting tunnel junctions for quantum computing

Abstract

Here we report normal-state conductance measurements of three different types of superconducting tunnel junctions that are being used or proposed for quantum computing applications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands for polycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibited significant deviations from the parabolic behavior predicted by the WKB approximation models. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunneling conductances at voltages matching harmonics of Al-O stretching modes. On the other hand, such Al-O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction. This suggests that absence or existence of the Al-O stretching mode might be related to the crystallinity of the AlO tunnel barrier and the interface between the electrode and the barrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for future superconducting qubits, we observed suppression of the density of states at zero bias. This implies that the interface is electronically disordered, presumably due to oxidation of the vanadium surface underneath the MgO barrier, even if the interface was structurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not be suitable for qubit applications in its present form. This also demonstrates that the normal-state conductance measurement can be effectively used to screen out low quality samples in the search for better superconducting tunnel junctions.

Material matters in superconducting qubits

“…More recent investigations and simulations point to hybridization of the metallic Nb and Al2O3 conduction bands as being responsible for generating metal-induced gap states (MIGS) and reducing the potential barrier of the insulating barrier [240] when the two layers are in contact [241]. Increased subgap current was also observed with vanadium as the top electrode material for both Al2O3 [241] and MgO insulating layers [184].…”

“…From a historical perspective, the predominant amount of information gathered on Josephson junctions has been gathered using I-V characterization [158], [184]. One can derive information on the electrical properties of the insulating potential barrier (e.g: height, thickness) by analyzing data acquired in the normal state of the metallization electrodes [274], or the superconducting energy gap, critical current and subgap leakage current at temperatures below Tc [225].…”