Quantum phenomena on surfaces
We are interested in fundamental physics problem concerning atoms, molecules and more generally solids. The surface of solids is of particularly high interest because many phenomena take place at the surface. Additionally, the surface breaks the translational symmetry of the solid and leads to spatial confinement, creating unique physical conditions. Extraordinary examples are surface states, which mediate interactions and can be applied to e.g. quantum corrals and other interesting phenomena.
Studying atoms and molecules on surfaces gives new insights on many effects. Thanks to the scanning tunneling microscope (STM) we can image, probe and manipulate single atoms on a surface. New exciting opportunities appear when atoms can be controlled one by one. Our group is devoted to studying experimentally and theoretically all aspects of atom, molecule and nanostructures on solid surfaces. Besides the many opportunities granted by all different types of atoms and molecules (magnetic, non-magnetic, reactive, non-reactive, vibrational, rigid etc) the substrate is also extremely important. We study metals, semiconductors, insulators and superconductors, that create completey different environments with extraordinary properties.
A case of great interest is the impurity problem. The impurity problem refers to the physics of a magnetic atom in a non-magnetic host. If the host is metallic, the flow of electrons hybrizing with the atomic states lead to an effective flip of the magnetic moment of the atom. If the temperature is very low, the flips become coherent. The phase of the wavefunction is well defined and all electrons become correlated. The ground state of the system is a humangously unique wave function. This is the Kondo effect. The STM gives unprecedented insight into the Kondo effect by studying the conductance between a metallic tip position a few Ångströms away from the impurity and the holding substrate. Recently , we have shown that a group of impurities that individually cannot correlate the substrate’s electrons, when they entangle quantally, they create the Kondo many-body ground state.
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 .
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 misnommer 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 esamble chains of magnetic atoms on the superconductor and study the in-gap states . Our calculations predict that twenty chromium atoms are in the topological phase leading to Majorana bound states.
Our work takes place with several international groups, leading to many publications in high-impact journals.
Dr. Deung-Jang Choi
Dr. Carlos García
Dr. Nicolás Lorente
Ms. Cristian Mier
Mr. José Reina
Dr. Vladimir Zobac
 Influence of magnetic ordering between Cr adatoms on the Yu-Shiba-Rusinov states of the β-Bi2Pd superconductor. Deung-Jang Choi, Carlos García Fernández, Edwin Herrera, Carmen Rubio-Verdú, Miguel M. Ugeda, Isabel Guillamón, Hermann Suderow, José Ignacio Pascual, and Nicolás Lorente. Phys. Rev. Lett. 120, 167001 (2018)
 Book chapter - Magnetic Impurities on Surfaces: Kondo and Inelastic Scattering. In: Andreoni W., Yip S. (eds) Handbook of Materials Modeling. Springer, Cham. D.-J. Choi*, N. Lorente (2018).
 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)
 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)
 From tunneling to contact in a magnetic atom: The non-equilibrium Kondo effect. Deung-Jang Choi*, Paula Abufager, Laurent Limot, Nicolás Lorente. J. Chem. Phys. 146, 092309 (2017).
 Kondo resonance of a Co atom exchange coupled to a ferromagnetic tip. D. -J. Choi, S. Guisssart, M. Ormaza, N. Bachellier, O. Bengone, P. Simon, L. Limot. Nano Letters 16, 6298 (2016).