Andreev reflection at half-metal/superconductor interfaces with nonuniform magnetization

Abstract

Andreev reflection at the interface between a half-metallic ferromagnet and a spin-singlet superconductor is possible only if it is accompanied by a spin flip. Here we calculate the Andreev reflection amplitudes for the case that the spin flip originates from a spatially non-uniform magnetization direction in the half metal. We calculate both the microscopic Andreev reflection amplitude for a single reflection event and an effective Andreev reflection amplitude describing the effect of multiple Andreev reflections in a ballistic thin film geometry. It is shown that the angle and energy dependence of the Andreev reflection amplitude strongly depends on the orientation of the gradient of the magnetization with respect to the interface. We calculate the resulting effects on the subgap conductance as well as Josephson current for a few exemplary cases. Establishing a connection between the scattering approach employed here and earlier work that employs the quasiclassical formalism, we connect the symmetry properties of the Andreev reflection amplitudes to the symmetry properties of the anomalous Green function in the half metal.

Majorana fermions in superconducting helical magnets

“…Otherwise, s-wave superconductivity projected onto either a spin-up or spin-down Fermi surface would give zero, as is the case for a uniform ferromagnet, h = (J,0,0). 22 For HMS h z (p) vanishes when p z = 0, and a line of nodes in the superconducting gap appears at the Fermi surface. 16,17 …”

Majorana fermions emerging from magnetic nanoparticles on a superconductor without spin-orbit coupling

“…As we will show, the transition into the topologically nontrivial phase is governed by the competition of two types of disorder: On the one hand, a variation in the orientation of the magnetic moments on nearby nanoparticles is needed to open a gap in the excitation spectrum 21 -which is a prerequisite for a topological phase. On the other hand, disorder in the hopping energies localizes the states on short segments of the chain-suppressing the superconducting order needed for the topological phase.…”

Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)

“…Its origin depends on microscopic magnetic state at the SC/F interface, character of local magnetic moments coupling with itinerant electrons, etc., and even varies from sample to sample. But, the exact microscopic origin of spin-flip processes at the interface is important only for the effective interface scattering matrix [92,93,96,97] and not for superconducting phenomena, since Cooper pairs are of the size of the coherence length ξ S which is much larger than the atomic scale.…”

“…) Naturally, the physical mechanisms applicable for sSC/HMF heterostructures remain in force for the pointcontact geometry, as well. The impact of spin-mixing and spin-flip AR processes at sSC/HMF interface on PC spectra was theoretically studied just recently in [93,[96][97][98][99]. It was found that spin-active interface and spin-flip AR can be responsible for the long-range triplet proximity effect.…”