Giant oscillations of the density of states and the conductance in a ferromagnetic conductor coupled to two superconductors

TL;DR

This paper predicts giant oscillations in the density of states and conductance in superconductor-ferromagnet-superconductor structures caused by quantum interference effects, enabling direct measurement of the exchange energy.

Contribution

It introduces the prediction of giant oscillations due to energy level degeneracy in superconductor-ferromagnet-superconductor systems when the exchange energy is smaller than the superconducting gap.

Findings

Oscillations occur near phase difference multiples of π.

Effect persists in long ferromagnetic bridges.

Allows spectroscopy of Andreev levels and measurement of exchange energy.

Abstract

Giant oscillations of the density of electronic states and the differential conductance of a superconductor-ferromagnet-superconductor structure are predicted for the case when the exchange energy, $I_0$, due to the interaction between the electron spin and the spontaneous moment of the ferromagnet is smaller than the superconductor energy gap, $\Delta$. The effect is due to the extremely large degeneration of the energy level $\epsilon = I_0$ when the superconductor phase difference $\phi$ is close to odd multiple of $\pi$ ($\epsilon$ is the electron energy measured from the Fermi-energy). This quantum interference effect persists even in long ferromagnetic bridges whose length much exceeds the ''magnetic length'' ($\hbar v_{F}/I_{0}$ in the ballistic regime and $\sqrt{\hbar D/I_0}$ in the diffusive regime; $D$ is the electron diffusion constant). The predicted effect allows a direct spectroscopy of Andreev levels in the ferromagnet as well as a direct measurement of the exchange energy,$I_0$.