Atomic manipulation of in-gap states in the β − Bi₂ Pd superconductor

JULY 6, 2021

Cristina Mier, Jiyoon Hwang, Jinkyung Kim, Yujeong Bae, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Nicolás Lorente, Andreas Heinrich, and Deung-Jang Choi
PHYSICAL REVIEW B 104, 045406 (2021)


Atomic spin chains on a superconductor provide one of the most feasible routes for the realization of Majorana bound states, which can be used to store robust quantum information. The in-gap states created by spins on the superconductor, known as Yu-Shiba-Rusinov states, have attracted much interest since they possibly give rise to Majorana modes. In this paper, published in the journal Physical Review B, we used our home-built scanning tunneling microscope (STM) at very low temperature (several milli-Kelvin) to build chains of Cr atoms on the Bi2Pd superconductor. We built those chains along two different lattice directions, one with two-lattice-parameter spacing and one with square-root-of-two lattice parameters. The spectroscopy measured using STM revealed the evolution of these in-gap states as a function of the number of atoms in the chains. We found that only the spin chains with the square-root-of-two lattice parameters undergo a topological transition, showing that in-gap states at the edge of the chains become closer to the zero energy state in longer chains. Our results suggest that this system might provide a handy platform for Majorana-based quantum computation. This work was performed in collaboration with researchers at the DIPC research center in Spain.


Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the β − Bi2 Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for nontrivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two lattice parameters apart and one with √ 2 lattice parameters. Of these, only the second one is in a topologically nontrivial phase, in agreement with the spin interactions for this geometry.