Magnetic excitations of single atoms and molecules on superconductors probed by scanning tunneling spectroscoppy
Final Report Abstract
The goal of the project was to gain a profound understanding of magnetic excitations of single atoms, molecules and small nanostructures on superconducting surfaces. Using low-temperature scanning tunneling microscopy and spectroscopy, we resolved Shiba states inside the superconducting gap of a variety of magnetic adsorbates on superconducting Pb surfaces of different surface orientation. Our choice of adsorbates was motivated by covering a large range of magnetic interaction strengths and structural flexibility. Atoms are very stable on the surface and withstand several orders of magnitude of junction conductance when approaching with the STM tip. This allowed us to disentangle different transport regimes: At low junction conductance, the tunneling current is mainly carried by single-electron processes, while at strong tunnel coupling, resonant Andreev reflections through the Shiba states dominate the transport. We observed multiplets of Shiba states in atomic and molecular adsorbates. These have different physical origins. In the atomic adsorbates, all d states are rather strongly coupled to the substrate, thus creating a scattering potential individually. In molecular adsorbates, the overlap of the d levels with the substrate states may be very different. However, Hund´s coupling and magnetocrystalline anisotropy within the molecule may results in Shiba states, which are split by this anisotropy. The intensity distribution of these anisotropy-split states further provided an unambiguous fingerprint of the many-body ground state of the molecular system. The coupling strength and magnetic anisotropy can be tuned by additional adsorbates and interaction with the STM tip. Magnetic coupling between neighboring adsorbates became particularly important when it was suggested that ferromagnetic transition metal chains on a superconductor with strong spin-orbit coupling may host Majorana bound states. Whereas Fe chains on Pb(110) show localized zeroenergy modes, Co chains do not exhibit this property despite of ferromagnetic d bands and Shiba bands extending along the chains. We ascribe this fundamental difference between Fe and Co to a different band occupation and a different number of bands crossing the Fermi level (odd in Fe, even in Co).
Publications
- End states and subgap structure in proximity-coupled chains of magnetic adatoms, Physical Review Letters, 115, 197204 (2015)
Michael Ruby, Falko Pientka, Yang Peng, Felix von Oppen, Benjamin W. Heinrich, Katharina J. Franke
(See online at https://doi.org/10.1103/PhysRevLett.115.197204) - Magnetic anisotropy in Shiba bound states across a quantum phase transition, Nature Communications, 6, 8988 (2015)
Nino Hatter, Benjamin W. Heinrich, Michael Ruby, Jose I. Pascual, Katharina J. Franke
(See online at https://doi.org/10.1038/ncomms9988) - Tuning of the magnetic anisotropy of single molecules. NanoLetters, 15, 4024 (2015)
Benjamin W. Heinrich, Lukas Braun, Jose I. Pascual, Katharina J. Franke
(See online at https://doi.org/10.1021/acs.nanolett.5b00987) - Tunneling processes into localized subgap states in superconductors, Physical Review Letters, 115, 087001 (2015)
Michael Ruby, Falko Pientka, Yang Peng, Felix von Oppen, Benjamin W. Heinrich, Katharina J. Franke
(See online at https://doi.org/10.1103/PhysRevLett.115.087001) - Orbital Picture of Yu-Shiba-Rusinov Multiplets, Physical Review Letters 117, 186801 (2016)
Michael Ruby, Yang Peng, Felix von Oppen, Benjamin W. Heinrich, Katharina J. Franke
(See online at https://doi.org/10.1103/PhysRevLett.117.186801) - Exploring a Proximity-Coupled Co Chain on Pb(110) as a Possible Majorana Platform, Nano Letters 17, 4473 (2017)
Michael Ruby, Benjamin W. Heinrich, Yang Peng, Felix von Oppen, Katharina J. Franke
(See online at https://doi.org/10.1021/acs.nanolett.7b01728)
