Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling

摘要

We investigate theoretically how the proximity effect insuperconductor/ferromagnet hybrid structures with intrinsic spin-orbit couplingmanifests in the density of states and critical temperature. To describe ageneral scenario, we allow for both Rashba and Dresselhaus type spin-orbitcoupling. Our results are obtained via the quasiclassical theory ofsuperconductivity, extended to include spin-orbit coupling in the Usadelequation and Kupriyanov--Lukichev boundary conditions. Unlike previous works,we have derived a Riccati parametrization of the Usadel equation withspin-orbit coupling which allows us to address the full proximity regime.First, we consider the density of states in both SF bilayers and SFS trilayers,where the spectroscopic features in the latter case are sensitive to the phasedifference between the two superconductors. We find that the presence ofspin-orbit coupling leaves clear spectroscopic fingerprints in the density ofstates due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFSstructures without spin-orbit coupling, the density of states in the presentcase depends strongly on the direction of magnetization. We show that thespin-orbit coupling can stabilize singlet superconductivity even in thepresence of a strong exchange field $h \gg \Delta$. This leads to thepossibility of a magnetically tunable minigap: changing the direction of theexchange field opens and closes the minigap. We also determine how the criticaltemperature $T_c$ of an SF bilayer is affected by spin-orbit coupling anddemonstrate that one can achieve a spin-valve effect with a single ferromagnet.We find that $T_c$ displays highly non-monotonic behavior both as a function ofthe magnetization direction and the type and direction of the spin-orbitcoupling, offering a new way to exert control over the superconductivity ofproximity structures.