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NanoSpin Report Summary

Project ID: 616623
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - NANOSPIN (Nanoscale spin interactions and dynamics on superconducting surfaces)

The latest concepts for quantum computing and data storage envision the use of single spins, which can be addressed and manipulated reliably. One of the main limitations towards this challenging goal is the ultra-short lifetime of excited spin states due to the interaction with the contacting leads. Typically, spin-lattice relaxation times in single atoms or molecules in contact to a metal surface are in the range of ps due to the exchange of energy and angular momentum with the itinerant electrons of the substrate.
Superconductors are interesting materials to support magnetic adsorbates. First, they provide an energy gap, which serves to protect excited spin states from fast relaxation into the bulk. Second, magnetic interactions between adsorbate spins and the superconductor can lead to interesting many-body states with so-called Yu-Shiba-Rusinov (short: Shiba) states inside the superconducting energy gap, while at the same time a Kondo resonance outside the gap is formed. Third, the superconductor may provide intriguing pathways of coupling magnetic moments of individual atoms/molecules due to the presence of the Cooper pairs in the superconducting condensate. Our project aims at using these features for creating nanostructures with coupled spin states. We intend to manipulate the spin states individually and resolve a response in the neighboring atom/molecule.
On the way to this goal, we have explored a variety of transition metal atoms and metal-organic complexes on superconducting Pb surfaces. We resolve the Shiba states inside the superconducting energy gap by scanning tunneling spectroscopy at a temperature of 1.1 K. We have identified different transport mechanisms through Shiba states and deduced the lifetime of electrons in these states (Phys. Rev. Lett. 115, 087001 (2015)). We observed that single atoms and molecules exhibit a characteristic number of Shiba states. We showed that these are a result of the atomic-scale surrounding. The substrate effectively acts as a crystal field, which removes the degeneracy of the d-levels. The Shiba states inherit the symmetry from the corresponding d-states (Phys. Rev. Lett. 117, 186801 (2016)). Additionally, the crystal field can impose magnetic anisotropy on the spin states. As a result, the Shiba states are split into characteristic multiplets. A detailed lineshape analysis of these states allowed us to unambiguously identify the many-body ground state of the system (Nature Comm. 6, 8988 (2015)).
We could also show that Shiba states of adatoms at sufficiently close distance hybridize and give rise to states with bonding and anti-bonding character. In densely packed chains, the Shiba states of the adatoms yield extended bands. The chains have been shown to host localized endstates at zero-energy, which have been discussed in the framework of Majorana modes by the Yazdani group (Nadj-Perge, et al., Science 346, 602 (2014)). Since these states are promising for quantum computation, we revisited these chains within our project. We confirmed the zero-energy modes found by the Yazdani group and provided further evidence for the topological gap size as well as for a correlation between d-states and Shiba bands (Phys. Rev. Lett. 115, 197204 (2015)).
Our achievements within the first stage of this projects provide a promising basis for the creation of magnetic nanostructures on superconducting surfaces, which qualify for the read-out, manipulation and spin coupling. We will further aim at higher precision of these steps and enlarge our tool box and experimental systems.

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