Quantum spins and hybridization in artificially-constructed chains of magnetic adatoms on a superconductor

TL;DR

This study constructs and investigates magnetic adatom chains on a superconductor, revealing quantum spin interactions, YSR state hybridization, and the absence of Majorana modes, advancing understanding of quantum spin systems and topological superconductivity.

Contribution

It demonstrates the construction of Fe adatom chains on NbSe₂ and explores their quantum spin interactions, YSR states, and the impact on topological superconductivity.

Findings

YSR states undergo quantum phase transitions in dimers

Chains show coherent hybridization and band formation of YSR excitations

No signatures of Majoranas observed despite spin-orbit coupling

Abstract

Magnetic adatom chains on surfaces constitute fascinating quantum spin systems. Superconducting substrates suppress interactions with bulk electronic excitations but couple the adatom spins to a chain of subgap Yu-Shiba-Rusinov (YSR) quasiparticles. Using a scanning tunneling microscope, we investigate such correlated spin-fermion systems by constructing Fe chains adatom by adatom on superconducting NbSe$_2$. The adatoms couple entirely via the substrate, retaining their quantum spin nature. In dimers, we observe that the deepest YSR state undergoes a quantum phase transition due to Ruderman-Kittel-Kasuya-Yosida interactions, a distinct signature of quantum spins. Chains exhibit coherent hybridization and band formation of the YSR excitations, indicating ferromagnetic coupling. Longer chains develop separate domains due to coexisting charge-density-wave order of NbSe$_2$. Despite the spin-orbit-coupled substrate, we find no signatures of Majoranas, possibly because quantum spins reduce the parameter range for topological superconductivity. We suggest that adatom chains are versatile systems for investigating correlated-electron physics and its interplay with topological superconductivity.