Diluted Yu-Shiba-Rusinov arrays on the $\beta$-Bi$_2$Pd anisotropic superconductor

TL;DR

This study investigates how the anisotropic Fermi surface of $eta$-Bi$_2$Pd influences the coupling and spectral features of diluted Mn adatom arrays, revealing directional modulation and multiple YSR excitations.

Contribution

It demonstrates the impact of Fermi surface anisotropy on YSR state coupling in diluted magnetic adatom arrays on $eta$-Bi$_2$Pd, supported by experimental measurements and a Green function model.

Findings

YSR peak splitting increases with atom number

Orientation modulates YSR spectral asymmetry

Extended 2D arrays show multiple YSR excitations

Abstract

Magnetic adatoms on s-wave superconductors induce bound states inside the superconducting gap, called Yu-Shiba-Rusinov states (YSR). The anisotropy of the Fermi surface determines the spatial extension of bound states in a quasi-two-dimensional superconductor. This is especially important in the diluted impurity limit since the orbital overlap determines the coupling of YSR states of neighboring atoms and the formation of the collective YSR system. Here, we build diluted arrays of Mn atoms with different dimensionalities on the surface of $\beta$-Bi$_2$Pd, and we measure the evolution of their YSR spectra with the structure. We detect the coupling as a split of YSR peaks in subgap spectra and find that the split size increases with the number of atoms. The orientation of the structures along different directions of the \bipd substrate modulates the split and particle-hole asymmetry of the YSR states due to the anisotropic character of the Fermi surface, captured by the Green function model. With the aid of the model, we found multiple YSR excitations in an extended 2D array of 25 Mn atoms, and we identified that their spatial distribution reflects a chiral LDOS.