Evidence for Suppression of Superconductivity by Spin Imbalance in Co-Al-Co single electron transistors

Spin imbalance is predicted to lead to suppression of superconductivity. We report phenomena manifesting this effect under spin-polarized quasiparticle currents in ferromagnet-superconductor-ferromagnet single electron transistors. The measured superconducting gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are in opposite orientations. The effect of suppression increases with increas-ing voltage but decreases at elevated temperatures. The possible explanations for these dependences are given. This method may render it applicable to control superconductivity at low temperatures and low fields.