Observation of Rashba-Edelstein effect using a Josephson junction

At the interface of heavy metals (with large spin-orbit coupling) and normal metal the interaction of Pauli spin matrices ?? with the momentum vector k leads to the splitting of spin contours at the Fermi level, usually known as the Rashba effect. Edelstein [2] first recognized that a charge current at such a Rashba interface would naturally lead to a non-zero spin accumulation at the interface due to the shifting of Fermi contours along the direction of current. Inversely, spin injection to a Rashba interface can generate a charge current at the interface. This Inverse Rashba-Edelstein effect has been recently demonstrated [3] experimentally. In this work we have attempted to utilize the extreme phase sensitivity of a Josephson junction to look for signatures of the Rashba-Edelstein (RE) effect at the heavy metal/normal metal (Pt/Cu) interface. Since the RE effect produces a net spin accumulation (polarization) at the Pt/Cu interface, a Josephson junction realized across such an interface would see a net magnetic moment due to the spin polarization. Therefore, the Fraunhoffer pattern (magnetic field dependence of critical current or, equivalently, the magnetic field dependence of the voltage across the junction at a fixed current bias) of the Josephson junction should show a field shift corresponding to the effective magnetic moment. In this experiment, using Nb superconducting electrodes on Pt/Cu bilayers, we have realized planar Josephson junctions to study this effect. We indeed observe a shift in the Fraunhoffer pattern of the junctions which changes direction when the magnetic field direction is reversed. We also found that the magnitude of the shift in the Fraunhoffer pattern depends on the thickness of the Pt layer in the Pt/Cu bilayer.

References: [1] S. Maekawa et al., (eds) Spin Currents (Oxford University Press, 2012). [2] V. M. Edelstein, Solid State Commun. 73, 233–235 (1990). [3] J.C. Rosas Sánchez et al.,. Nat Commun 4, 2944 (2013).