Fractional ac Josephson effect in unconventional superconductors

TL;DR

This paper predicts a fractional ac Josephson effect in certain unconventional superconductor junctions, where the current is carried by single electrons, leading to a 4pi-periodic relation and fractional frequency in the ac Josephson effect.

Contribution

It introduces the theoretical prediction of a fractional ac Josephson effect in p_x-wave and d-wave superconductor junctions, highlighting single-electron tunneling.

Findings

Fractional frequency eV/h in ac Josephson current.

Current proportional to first power of tunneling amplitude.

Observation of effect via microwave spectrum measurement.

Abstract

For certain orientations of Josephson junctions between two p_x-wave or two d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic relation between the Josephson current I and the phase difference phi: I ~ sin(phi/2). Consequently, the ac Josephson current has the fractional frequency eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction.