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Stars in superconductors

Physicists have now shown that two-dimensional materials allow to significantly increase the spatial extent of quantum states generated by magnetic atoms embedded in superconductors.

Conventional superconductivity is obtained by pairing electrons of opposite spin in Cooper pairs. This particular spin state makes the system very sensitive to magnetic impurities and, in the case of individual magnetic atom, gives rise to spin-polarized bound states called Yu-Shiba-Rusinov states. Despite two decades of research, the spatial structure of these states could never be measured because of their extent being limited to a few Angstroms. Physicists at the Institute of Nanosciences in Paris (INSP) in collaboration with a theoretical team of the Laboratoire de Physique des Solides (LPS) in Orsay and the Institute Jean Rouxel in Nantes, highlighted the intimate connection between dimensionality and spatial extent. By probing the spatial structure of the wave function associated with the destruction of Cooper pairs by a magnetic impurity, they were able to reveal its star shape governed by the electronic structure of the system. The results obtained by scanning tunneling microscopy are in perfect agreement with the theoretical models in terms of the spatial extension of the wave function, of its oscillating nature and of its star shape structure. This work has just been published in the journal Nature Physics.

Figure: a. Spectroscopic map containing many magnetic impurities located at different depths in the material. b. Simulation of a Yu-Shiba-Rusinov bound state obtained by tight binding calculations.

To obain these results, physicists have studied superconducting crystals with a lamellar structure, giving to the electronic structure a quasi- two-dimensional behavior. The growth of these monocrystals was achieved by adding a small percentage of iron impurities leading to the inclusion of magnetic defects homogeneously distributed in the sample. By moving the microscope tip over the sample, physicists were then able to measure the spatial dependence of the tunnel current and reconstruct the distribution of electronic states and thus image the structure of the Yu-Shiba-Rusinov state. These results open the door for a new approach to couple distant magnetic impurities and to engineer Majorana fermions. These excitations of superconducting systems could be a key for quantum computing because of their topological protection.


Coherent long-range magnetic bound states in a superconductor
G. C. Ménard et al.
Nature Physics 11, 1013 (2015).


Tristan Cren
Pascal Simon