Long-range focusing of magnetic bound states in superconducting lanthanum

TL;DR

This paper demonstrates that magnetic impurities in superconducting lanthanum can interact over very long distances through quasiparticles, enabling the design of magnetic structures with potential applications in quantum computing.

Contribution

It reveals the long-range, anisotropic interactions of magnetic impurities in superconducting lanthanum mediated by quasiparticles, supported by experimental and theoretical analysis.

Findings

Magnetic impurities interact over tens of nanometers.

Quasiparticle focusing causes giant oscillations in local density of states.

Surface states with anisotropic Fermi surface are crucial for long-range interactions.

Abstract

Quantum mechanical systems with long-range interactions between quasiparticles provide a promising platform for coherent quantum information technology. Superconductors are a natural choice for solid-state based quantum devices, while magnetic impurities inside superconductors give rise to quasiparticle excitations of broken Cooper pairs that provide characteristic information about the host superconductor. Here, we reveal that magnetic impurities embedded below a superconducting La(0001) surface interact via quasiparticles extending to very large distances, up to several tens of nanometers. Using low-temperature scanning probe techniques, we observe the corresponding anisotropic and giant oscillations in the local density of states. Theoretical calculations indicate that the quasi-two-dimensional surface states with their strongly anisotropic Fermi surface play a crucial role for the focusing and long-range extension of the magnetic bound states. The quasiparticle focusing mechanism should facilitate the design of versatile magnetic structures with tunable and directed magnetic interactions over large distances, thereby paving the way toward the design of low-dimensional magnet-superconductor hybrid systems exhibiting topologically non-trivial quantum states as possible elements of quantum computation schemes based on Majorana quasiparticles.