Resonant tunneling in short Josephson SFS junctions

TL;DR

This paper investigates the Josephson effect in short ballistic SINIS and SIFIS junctions, revealing resonant amplification, spin effects, and temperature-driven 0-$$ transitions related to quasi-bound states and interface transparency.

Contribution

It provides a detailed analysis of resonant tunneling, spin splitting, and temperature effects in short Josephson SFS junctions, highlighting new mechanisms for 0-$$ transitions.

Findings

Resonant peaks in critical current due to quasi-bound states in SINIS junctions.

Spin-split quasi-bound states induce 0-$$ transitions and coexistence of states.

Temperature can induce transitions between 0 and  states, depending on interface transparency.

Abstract

Josephson effect in short ballistic SINIS and SIFIS double-tunnel junctions, consisting of clean superconductors (S), a normal metal (N) or metallic ferromagnet (F), and insulating interfaces (I) is studied. For SINIS double-tunnel junctions, sharp peaks in the critical Josephson current as a function of the junction width result from the resonant amplification of the Andreev process when the quasi-bound states enter the superconducting gap. For SIFIS double-tunnel junctions spin split quasi-bound states partially amplify the supercurrent and trigger the transitions between 0 and $\pi$ states of the junction. Instead of the critical current reaching a peak value (which happens when the Andreev states cross the Fermi surface in SINIS junctions), a narrow dip opens up exactly at the peak due to the compensation of partial currents flowing in opposite directions. This is related to the spin polarization of Andreev states and explains the coexistence of stable and metastable 0 and $\pi$ states in the vicinity of the transition. With increased barrier transparency, the described mechanism of $0-\pi$ transitions is modified by the broadening and overlapping of quasi-bound states (transmission resonances). Temperature-induced transitions both from 0 to $\pi$ and from $\pi$ to 0 states are studied by computing the phase diagram (temperature vs. junction width) for different interfacial transparencies varying from metallic (transparent) to the tunnel limit.