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**Magnetic interaction between Mn atoms on the superconductor -Bi 2Pd revealed
by their Yu-Shiba-Rusinov States**
Stefano Trivini
Nanoimaging Group, CIC nanoGUNE
In the last decade, the interest in studying the coupling between magnetic
atoms arranged in linear chains on superconducting surfaces increased.1,2,3
This is due to proposals suggesting emergence of Majorana bound states (MBS)
in condensed matter systems, that have potential applications in topological
quantum computing.4 A powerful way to investigate the coupling is looking at
how it modifies the Yu-Shiba-Rusinov (YSR) states inside the superconducting
gap. These states are generated by the exchange coupling between a magnetic
impurity and the Cooper pair condensate of the superconductor.5 The coupling
mechanism can be investigated by monitoring the sub gap YSR fingerprint during
the construction of atomically precise structures. We cited studies that
report evidence of Majorana bound states. However, more control on the atomic
precision and the spectral resolution is needed to understand the coupling
mechanism and to distinguish trivial magnetic states (YSR) from topological
states (MBS). By means of a low temperature scanning tunneling microscope
(STM) working at 1.3K, we perform atomic manipulation of Mn atoms on -Bi2Pd
superconductor (Tc=5.4K, =0.78mV). A major advantage is achieved doing atomic
manipulation with superconductive tips, studying the coupling with high
resolution in Mn dimers, trimers, cross like structures, chains and other more
complex structures. Depending on the distance and geometry different coupling
effects occurs, with signatures of ferromagnetic and antiferromagnetic
coupling. Growing linear chains, we have evidence of a collective behavior
with a lowering in energy of the YSR states that saturates for chains longer
than 6-7 atoms. The substrate inhomogeneities also effect the YSR states,
underlying the importance of controlling this parameter when building
structures.
**References**
1. Nadj-Perge, S., Drozdov, I. K., Bernevig, B. A. & Yazdani, A. Proposal for realizing Majorana fermions in chains of magnetic atoms on a superconductor. Phys. Rev. B **88** , 020407 (2013).
2. Nadj-Perge, S. et al. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor. Science **346** , 602–607 (2014).
3. Kim, H. et al. Toward tailoring Majorana bound states in artificially constructed magnetic atom chains on elemental superconductors. Sci. Adv. **4** , eaar5251 (2018).
4. Alicea, J., Oreg, Y., Refael, G., von Oppen, F. & Fisher, M. P. A. Non-Abelian statistics and topological quantum information processing in 1D wire networks. Nat. Phys. **7** , 412–417 (2011).
5. Heinrich, B. W., Pascual, J. I. & Franke, K. J. Single magnetic adsorbates on s -wave superconductors. Prog. Surf. Sci. **93** , 1–19 (2018).
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