Quasiparticle energy spectrum in ferromagnetic Josephson weak links

Abstract

The quasiparticles energy spectrum in clean ferromagnetic weak links between conventional superconductors is calculated. Large peaks in density of states (DOS), due to a special case of the Andreev reflection at the ferromagnetic barrier, correspond to spin-split bound states. Their energies are obtained as a function of the barrier thickness, exchange field strength, and of the macroscopic phase difference φ at the link, related to the Josephson current. In the ground state, φ can be 0 or π, depending on the ferromagnetic barrier influence. Conditions for the appearance of the zero-energy bound states (ZES) and for the spin polarized ground state (SPGS) are obtained analytically. It is shown that ZES appear only outside the weak link ground state.