Topological superconductivity with deformable magnetic skyrmions

Abstract

Magnetic skyrmions are nanoscale spin configurations that are efficiently created and manipulated. They hold great promises for next-generation spintronics applications. In parallel, the interplay of magnetism, superconductivity and spin-orbit coupling has proved to be a versatile platform for engineering topological superconductivity predicted to host non-abelian excitations, Majorana zero modes. We show that topological superconductivity can be induced by proximitizing skyrmions and conventional superconductors, without need for additional ingredients. Apart from a previously reported Majorana zero mode in the core of the skyrmion, we find a more universal chiral band of Majorana modes on the edge of the skyrmion. We show that the chiral Majorana band is effectively flat in the physically relevant parameter regime, leading to interesting robustness and scaling properties. In particular, the number of Majorana modes in the (nearly-)flat band scales with the perimeter length of the system, while being robust to local disorder. 1 arXiv:1904.03005v3 [cond-mat.supr-con] 23 Oct 2019 8 where all energies are in units of the bandwidth t, all distances are in units of the lattice spacing a (see Supplementary Note 1 and Supplementary Figure 1 for details). For precisely |m J | = |m * J | the wire is at the topological transition and has a gapless spectrum, giving our model a bulk-gap-closing point as shown in Fig. 2c.The MFB found here has a protection by a chiral symmetry, as MFB's were found to have in models with translational symmetries 39,40,51 . Note that the wire Hamiltonian and its MFB become a correct model for our texture Eq. (1) if we choose q = 0 and thereby nullify the H slope m J (r) term. Physically, this is a special case where instead of the skyrmion shape the texture becomes coplanar (in the xz-plane, see Eq. (1)), and the orthogonal direction provides a chiral operator Ξ = τ y σ ythat anticommutes with the Hamiltonian (see Eq. (2)). Since all the MFB states have the same chirality, they cannot hybridize among themselves. It is difficult to remove the MFB states 51 , namely, a perturbation must have energy larger than the effective gap; or, it should hybridize the MFB with low energy bulk states at |m J | ≈ |m * J |, which are few; or, chirality symmetry must be broken (out-of-xz-plane exchange field). We note that the proof of existence of the MFB rests on the rotational symmetry of the q = 0 coplanar texture, since this symmetry provides the m J quantum number. Consider now deformations of the shape of the edge imposed on our q = 0 coplanar texture. These geometric deformations would generally mix the m J sectors, yet the described stability of the MFB implies that the deformations would be inefficient in removing the MFB states.We can now proceed to the relevant model for a skyrmion with arbitrary q = 0:The single term H slope m J (r) breaks the chiral symmetry Ξ, and there are no other chiral operators. The term H slope m J (r) exactly contributes an energy ε edgestate (m J ) ∼ m J to an MFB state ...

Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45

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Majorana bound states in magnetic skyrmions imposed onto a superconductor

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Topological superconductivity in skyrmion lattices

“…Note that the creation of Majorana modes in single skyrmions has previously been discussed in refs. 18,19 . As Kondo screening is suppressed by the full superconducting gap, the spins S r are taken to be classical vectors of length S. We assume that the triangular lattice of skyrmions is commensurate with the underlying triangular surface lattice, thus allowing the skyrmion radius R to take integer and half-integer values of a 0 .…”