Emergent Majorana States in Defect-Free Bottom-Up Constructed Atomic-Scale Magnet-Superconductor Hybrid Systems

Majorana states in atomic-scale magnet-superconductor hybrid systems have recently become of great interest because they can encode topological qubits and ultimately provide the basis for topological quantum computation. However, an unambiguous identification of Majorana states requires well defined model-type platforms and appropriate experimental tools for their investigation. Our experimental approach is based on the use of STM-based single atom manipulation techniques in order to fabricate well-defined defect-free 1D atomic chains as well as 2D arrays of magnetic adatoms on s-wave superconductor substrates with high spin-orbit coupling [1-3]. Spin-polarized STM measurements [4] allow us to reveal the presence of collinear [5] or non-collinear [1] spin textures, i.e. spin spiral ground states, of the 1D chains. Simultaneously performed scanning tunneling spectroscopy on the magnetic atom chains proximity-coupled to the superconducting substrates reveal the evolution of the spatially and energetically resolved local density of states as well as the emergence of zero-energy bound states at both chain ends above a critical chain length for Fe chains on Re(0001) [1] as well as Mn chains on Nb(110) [6]. Based on the exact knowledge of the geometrical, electronic, and spin structure of the magnetic chain – superconductor hybrid system, the experimental results can be compared rigorously with ab-initio and model-type tight-binding calculations supporting the interpretation of the spectroscopic signatures at the ends of the chains as Majorana states [1,6-8].

[1] H. Kim et al., Science Advances 4, eaar5251 (2018).

[2] L. Schneider et al., Nature Commun. 11, 4707 (2020).

[3] A. Kamlapure et al., Nature Commun. 9, 3253 (2018).

[4] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).

[5] L. Schneider et al., Science Advances 7, eabd7302 (2021).

[6] L. Schneider et al., arXiv:2104.11503.

[7] D. Crawford et al., arXiv:2109.06894.

[8] L. Schneider et al., Nature Physics 17, 943 (2021).