SFB Colloquium

Two-dimensional materials subject to strong spin-orbit interactions have open the pathway to fine tune the magnetic and electronic behavior among a wide range of physical ground states of great fundamental and technological interest. Examples of this are spin-momentum locking of Dirac fermions, giant Rashba-splitting of conduction electrons, coexisting superconductivity and magnetic order and non-collinear magnetic arrangements.A key factor for the current understanding has been the possibility to characterize surfaces in great detail using powerful techniques such as photoemission spectroscopies, scanning tunneling spectroscopy maps in momentum space and spin polarized STM. In the frame of this experimental tool box, I will address our recent advances in the fields of: (i) Interaction of magnetic moments with helical spin-textures in strong topological insulators and; (ii) Artificial spin spirals fabricated on non-collinear magnetic substrates.

In the first part of the talk it will be shown that the spin-momentum locking prohibits backscattering in the surface states of strong topological insulators, which allows the flow of 100% spin polarized electrical currents. Here, the deposition of just a few individual magnetic moments on the surface enables backscattering events that destroy the topological spin texture. However, the intrinsic Se-Te chemical disorder in Bi based ternary compounds such as Bi2Se2Te preserves time reversal symmetry even in the presence of magnetic doping [1]. This implies that, under these premises, topological surface states can coexist together with a net surface magnetization.

Second, I will discuss Co atomic wires built by atomic manipulation over the non-collinear magnetic state of Mn/W(110). It consists in a 2D antiferromagnetic spin-spiral stabilized by the Dzyaloshinskii-Moriya interaction. This provides a periodic template with multiple spin directions. Thanks to this, the Co spin alignment imposed by the substrate [2] can be chosen by the construction direction of the wire. This coupling competes with the direct Co-Co exchange interaction, which allows us to design 1D spin spirals with unique rotational sense, theoretical candidates to host Majorana fermions when coupled to a superconductor. It is noteworthy that, for chiral spin densities, all components of the atom's spin moment can be retrieved from a single spin polarized STM image [3].

References
1. M.C. Martínez-Velarte, B. Kretz, M. Moro-Lagares et al., Nano Letters 17, 4047 (2017)
2. D. Serrate, P. Ferriani, Y .Yoshida, S.W. Hla et al., Nature Nanotechnlogy 5, 350 (2010)
3. D. Serrate, Y. Yoshida, M. Moro-Lagares et al., Phys. Rev. B 93, 125424 (2016)