Resonant subgap current transport in Josephson field effect transistor

TL;DR

This paper theoretically investigates the current-voltage characteristics of a Josephson field effect transistor, revealing unique subgap current structures and scaling behaviors influenced by junction length and transparency.

Contribution

It introduces a detailed theoretical analysis of subgap currents in Josephson FETs, highlighting differences from traditional junctions and providing analytical understanding of resonant MAR trajectories.

Findings

Long junctions show current peaks at harmonics of Andreev level differences.

The IVC follows a power-law scaling with junction transparency.

Shorter junctions exhibit exponential IVC behavior with subharmonic structures.

Abstract

We study theoretically the current-voltage characteristics (IVCs) of the Josephson field effect transistor - a ballistic SNINS junction with superconducting (S) electrodes confining a planar normal metal region (N), which is controlled by the gate induced potential barrier (I). The calculations were performed using the computation technique developed earlier for long single-channel junctions in the coherent multiple Andreev reflections (MAR) regime. We find significant difference of the subgap current structure in these junctions compared to the subharmonic gap structure in tunnel junctions and atomic-size point contacts. For long junctions, whose length significantly exceeds the coherence length, the IVC exhibits current peaks at multiples (harmonics) of the difference $\delta_m$ between the static Andreev levels, $eV_n = n\delta_m$. Moreover, the averaged IVC follows the power rather than exponential behavior, and has a universal scaling with the junction transparency. This result is qualitatively understood using an analytical approach based on the concept of resonant MAR trajectories. In shorter junctions whose length is comparable to the coherence length, the IVC has an exponential form common for point contacts, however the current structures appear at the subharmonics of the Andreev interlevel distance, $eV_n = \delta_m/n$ rather than the gap subharmonics $2\Delta/n$.