Superconducting Surfaces

When the substrate is superconducting, the impurity problem experiences a twist. The superconductor’s ground state is already a many-body state of paired particles (cooper pairs). It is impossible to inject a single particle in this ground state. The STM shows signal if the bias is larger than the pairing energy of the ground state particle. In this case, the electron energy is enough to break the Cooper pair and single-particle states are available in the conduction process. An impurity can trap states that will give a signal inside the superconducting gap. Indeed, the local magnetic moment acts as a magnetic field that splits the Cooper pair, creating single particles in the gap. We have recently shown the orbital structure of these in-gap states [1].

Entangled impurities are an extra level of complexity. On a superconductor they lead to the appearance of Majorana fermions. A Majorana fermion is its own antiparticle. Two Majorana fermions annihilate. The strategy is to create a chain of magnetic atoms that host two Majorana fermions at their edges to avoid their annihilation. These particle are compound due to the mixing of Cooper pairs in the presence of spin-orbit and magnetic interactions. Contrary to the other quasiparticles of superconductors, the Majorana fermions do not follow the Fermi-Dirac statistics, they are actually anyons, following fractionary statistics. Majorana fermions in this case is a misnomer and the more precise Majorana bound state should be used. Our present studies of magnetic atoms on a superconductor are very promising. Using the atomic manipulation capabilities of the STM, we can assemble chains of magnetic atoms on the superconductor and study the in-gap states [2]. Our calculations predict that twenty chromium atoms are in the topological phase leading to Majorana bound states.

[1] Mapping the orbital structure of impurity bound states in a superconductores. Deung-Jang Choi, Carmen Rubio-Verdú, Joeri de Bruijckere, Miguel M. Ugeda, Nicolás Lorente and José Ignacio Pascual. Nature Communications 8, 1575 (2017)

[2] Building complex Kondo impurities by manipulation entangled spin chains. Deung-Jang Choi, Roberto Robles, Sichao Yan, Jacob A. J. Burgess, Steffen Rolf-Pissarczyk, Jean-Pierre Gauyacq, Nicolás Lorente, Markus Ternes and Sebastian Loth. Nano Letters 17, 6203 (2017)