One step to create topologically non-trivial excitations on superconductors

The authors present an article published in Physical Review Letters, on the first manipulation of individual atoms on the surface of a single crystalline superconducting compound. The work presented was led by Deung-Jang Choi, currently an Ikerbasque fellow researcher at the Quantum Phenomena on Surface group at CFM, and involves researchers from CFM, CIC nanoGUNE, DIPC and Universidad Autónoma of Madrid.

Influence of Magnetic Ordering between Cr Adatoms on the Yu-Shiba-Rusinov States of the β−Bi2Pd Superconductor

Magnetic moment density (a) over a Cr2 dimer on the Bi-rich surface of the superconductor. Yellow is for magnetic moment pointing in the up direction and red in the down direction, corresponding to the antiferromagnetic ordering of Figure (b). The other cases are possible magnetic moment orientations on the surface. Spin-orbit coupling is large and non-collinear arrangements are possible although they were not found in the experiment.

Until now, atomic manipulation has been restricted to surfaces of elements or relatively simple superconducting materials. Using individual Cr atoms and manipulating these with great precision in Scanning tunneling Microscope (STM), they were able to create dimers formed by Cr atoms on different sites of the square surface lattice of the Bi atoms of beta-Bi2Pd. They characterized the interaction among differently spaced Cr dimers through so-called Yu-Shiba-Rusinov states which are coherent electron-hole excitations produced in the superconductor due to the exchange interaction between the magnetic moments and the Cooper pairs. This work shows that atomic manipulation is possible in quantum materials, opening new grounds to create topologically non-trivial excitations.

A superconductor is a metal where the electron-vibration coupling is large enough to create pairs of electrons at low temperature. If all

electrons are paired, there are no single-electron conduction channels and it is impossible to inject an electron at low energies. It is well-known that magnetic impurities weaken the pairs by creating a local magnetic field that attracts one of the electrons and repells the other electron. In the process of doing so, single-electron states appear at low energies. In the present contribution, Choi et al, study these single-electron states when two magnetic impurities are at different distance between each other. They show that the induced states in the superconductor reveal a lot of information on the impurities and particularly on their mutual interaction. If the magnetic moments of the impurities are aligned, the local “magnetic field” is enhanced and hybridization of single-electron states is possible with a characteristic signature in the differential conductance. If the magnetic moments are anti-aligned, the single-electron states do not interact and something similar to the single-impurity conductance is found. These studies are a step forward in the process of creating quantum states with atomic precision and with controlled of all the quantum properties of matter.