Phase biasing of a Josephson junction using Rashba-Edelstein effect

Abstract

Manifestation of orbital coupling of spin degree of freedom in condensed matter systems has opened up a new dimension for the field of spintronics. The most appealing aspect of the spin-orbit coupling is the apparent Magnus force sensed by a spin system which locks the Fermi momentum with electron spin in a fascinating manner. In the current carrying state, the resulting macroscopic spin polarization becomes directly accessible in the form of spin current or spin density. At a Rashba interface, for example, a charge current shifts the spin-locked Fermi surface, leading to a non-equilibrium spin density at the interface, commonly known as the Rashba-Edelstein effect. Since the Rashba-Edelstein effect is an intrinsically interface property, direct detection of the spin moment is harder to set-up. Here we demonstrate that a simple planar Josephson Junction geometry, realized by placing two closely spaced superconducting electrodes on such a Rashba interface, allows a direct estimation of strength of the non-equilibrium spin moment. Measurements of Fraunhofer patterns of Nb-(Pt/Cu)-Nb planar Josephson junctions in a perpendicular magnetic field showed a shift of the center of the Fraunhofer pattern to a non-zero field value. By performing extensive control measurements, we argue that the screening currents in the junction effectively lock the external field with the spin moment of the Rashba-Edelstein effect induced spin-density, leading to the observed shift in the Fraunhofer patterns. This simple experiment offers a fresh perspective on direct detection of spin polarization induced by various spin-orbit effects. Very interestingly, this device platform also offers the possibility of retaining a controllable phase at zero field in the junction without using any magnetic material, and thereby useful as phase batteries for superconducting quantum circuits.