Hybrid helical state and superconducting diode effect in superconductor/ferromagnet/topological insulator heterostructures

Physical Review B: Condensed Matter and Materials Physics. 2022. Vol. 106. No. 22. Article 224509.

Karabassov T., Bobkova I. V., Golubov A., Vasenko A.

It is well-known that the ground state of homogeneous superconducting systems with spin-orbit
coupling (SOC) in the presence of the Zeeman field is the so-called helical state, which is characterized
by the phase modulation of the order parameter, but zero supercurrent density. In this work
we investigate the realization of the helical state in a hybrid system with spatially separated superconductivity
and exchange field by considering superconductor/ferromagnet (S/F) bilayer on top
of a 3D topological insulator. This system is characterized by strong spin-momentum locking and,
consequently, provides the most favorable conditions for the helical state generation. The analysis
is based on the microscopic theory in terms of the quasiclassical Green’s functions. We demonstrate
that in the bilayer the helical state survives if the exchange field has non-zero component perpendicular
to the S/F interface even in spite of the fact that the superconducting order parameter and
the exchange field are spatially separated. At the same time, in this spatially inhomogeneous situation
the helical state is accompanied by the spontaneous currents distributed over the bilayer in
such a way that the net current vanishes. Further, we show that this hybrid helical state gives rise
to nonreciprocity in the system. We demonstrate the realization of the nonreciprocity in the form
of the superconducting diode effect and investigate its dependence on the parameters of bilayer.

Keywords: topological insulator, mesoscopic superconductivity, Hybrid helical state, superconducting diode effect

Publication based on the results of: Experimental and theoretical studies of quantum effects in low-dimensional systems (2022)