Topological insulator nanoribbon Josephson junctions: evidence for size effect in transport properties

TL;DR

This study demonstrates that in Bi2Se3 nanoribbon Josephson junctions, the critical current decreases with narrower widths, primarily due to transport through top surface states, highlighting their dominant role in superconducting transport.

Contribution

It provides experimental evidence that the Josephson current in topological insulator nanoribbons is mainly carried by top surface states, with a size-dependent reduction explained by geometrical considerations.

Findings

Critical current density decreases with nanoribbon width.

Transport is mainly via top surface states.

Size effect impacts superconducting properties.

Abstract

We have used Bi$_2$Se$_3$ nanoribbons, grown by catalyst-free Physical Vapor Deposition to fabricate high quality Josephson junctions with Al superconducting electrodes. In our devices we observe a pronounced reduction of the Josephson critical current density $J_c$ by reducing the width of the junction, which in our case corresponds to the width of the nanoribbon. Because the topological surface states extend over the entire circumference of the nanoribbon, the superconducting transport associated to them is carried by modes on both the top and bottom surfaces of the nanoribbon. We show that the $J_c$ reduction as a function of the nanoribbons width can be accounted for by assuming that only the modes travelling on the top surface contribute to the Josephson transport as we derive by geometrical consideration. This finding is of a great relevance for topological quantum circuitry schemes, since it indicates that the Josephson current is mainly carried by the topological surface states.