Periodic Reporting for period 3 - ADMIRE (Atomic-scale Design of Majorana states and their Innovative Real-space Exploration)
Berichtszeitraum: 2022-01-01 bis 2023-06-30
The unambiguous detection and identification of Majorana states in systems being controlled at the atomic level will push novel concepts of fault-tolerant topological quantum computation - one of the biggest goals for advancing computation for future needs of our society.
I) The unambiguous detection and identification of emerging Majorana bound states at both ends of a perfect atomic magnetic chain on a conventional superconducting substrate.
We have applied scanning tunneling microscopy (STM) based single-atom manipulation techniques to create perfect, disorder-free atomic spin chains on atomically clean elemental superconducting substrates and demonstrated the non-local nature of the Majorana bound states at both ends of such chains. Moreover, we have successfully applied Bogoliubov quasiparticle interference analysis of the 1D atomic chains, thereby revealing their low-energy band structure from which the topological nature of the hybrid magnet-superconductor system can be derived.
II) The detailed investigations of the existence of Majorana bound states in dependence of the atomic chain structure (atomic spacing, elemental species, etc.) and choice of superconducting substrate.
We investigated the emergence or absence of Majorana bound states in artificially built atomic chains of transition metals (Mn, Fe, and Co) on various elemental superconducting substrates (Re, Ta, La, and Nb). By employing STM-based single-atom manipulation techniques, we were able to tune the structure of the chains (linear vs. zig-zag), the interatomic spacings within the chains, as well as the chemical composition of the chains (pure vs. hybrid chains). Moreover, we have varied the crystallographic direction of the atomic chains with respect to the crystalline lattices of the superconducting substrates having different crystal structures and surface orientations.
III) In-depth studies of the role of magnetic order (collinear ferromagnetic vs. non-collinear spin spiral) for the formation of Majorana bound states.
By tuning the structure and chemical composition of the atomic chains as well as the chemical nature of the superconducting substrate, we were able to tailor the magnetic ground state of the atomic chains, being either collinear (ferromagnetic or antiferromagnetic) or non-collinear (spin spiral states). In the latter case, it was possible to tune the magnetic period of the spin spirals by an appropriate choice of magnetic adatoms and superconducting substrates.
Within the reporting period, we could demonstrate the emergence of Majorana bound states in artificially fabricated atomic Fe and Mn chains on elemental superconducting substrates (Re, Nb) as well as the emergence of chiral Majorana edge modes in 2D magnet - superconductor hybrid systems with atomically engineered interfacial layers.
Moreover, we have developed a novel atomic-resolution spin-sensitive imaging techniques based on the 100% spin-polarized Shiba states of magnetic impurities at the front end of sharp superconducting probe tips, allowing high-contrast mapping of artificially built atomic-scale magnets as well as 1D and 2D arrays of magnetic adatoms on surfaces.
Based on the atomic-resolution spin mapping capabilities, we discovered a novel type of complex non-collinear spin texture, namely a magnetic triple-q structure, in the framework of the project. If this triple-q structure is brought in contact with an elemental superconductor, topological superconductivity as well as Majorana modes can emerge, as we have predicted recently in collaboration with our international partners in Chicago and Melbourne.
Another major achievement within the reporting period was the demonstration of magnetic skyrmions in Pd/Fe/Ir triple-layers grown on superconducting Re(0001) as well as skyrmion lattice states in monolayer Fe islands on superconducting Ir(111)/Nb(110). Magnetic skyrmions interacting with superconductors are theoretically predicted to host Majorana states as well. We will test these predictions experimentally within the second half of the project.
Our ultimate goal by the end of the project is the demonstration of the manipulation of Majorana bound states, i.e. their creation, annihilation, adiabatic motion and braiding in artificially built model-type structures. We have now numerous types of 1D and 2D platforms available after the first half of the project so that we can go for the next steps toward the realization of Majorana state manipulations.