Revealing the microscopic origin of φ0 Josephson junctions (Vol. 46 No. 5-6)

The dependence of φ0 on the length L of the N bridge (see inset) with an intrinsic SOC and spin-splitting field

A spontaneous dissipationless current (supercurrent) can flow in a superconducting ring even in the absence of a magnetic flux, if the ring is interrupted by a so-called φ0 junction. In the present work the authors present a full microscopic theory that explains the appearance of the anomalous φ0 phase in junctions with an intrinsic spin-orbit coupling (SOC) and a spin-splitting field like the exchange field in ferromagnets. The SOC generates the spin precession of moving particles, and, in addition, it causes a spin-dependent deflection of electron trajectories. The latter can be interpreted in terms of an effective spin- dependent SU(2) magnetic field that in normal systems is the origin of the intrinsic spin Hall effect and the existence of spin currents in the equilibrium state. A finite φ0 in a Josepson junction is directly related to the appearance of an equilibrium spin current with a spin projection parallel to the exchange field. These findings are the first steps towards spin-orbitronics with superconductors by making a natural connection between charge-spin conversion in dissipative and superconducting structures.

F. S. Bergeret and I. V. Tokatly, Theory of diffusive φ0 Josephson junctions in the presence of spin-orbit coupling, EPL, 110, 57005 (2015)