Correlating Josephson supercurrents and Shiba states in quantum spins unconventionally coupled to superconductors

Abstract

Local spins coupled to superconductors give rise to several emerging phenomena directly linked to the competition between Cooper pair formation and magnetic exchange. These effects are generally scrutinized using a spectroscopic approach which relies on detecting the in-gap bound modes arising from Cooper pair breaking, the so-called Yu-Shiba-Rusinov (YSR) states. However, the impact of local magnetic impurities on the superconducting order parameter remains largely unexplored. Here, we use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, we reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, we detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy. Our findings have far reaching implications for phenomena that rely on the interplay between quantum spins and superconductivity.

Anomalous excitations of atomically crafted quantum magnets

“…We assume an angular momentum cutoff at 𝓁 max = 3 for the orbital expansion of the Green function, and when extracting the local density of states, a k -mesh of 450 × 450 is considered. The Cr atoms sit on the hollow stacking site relaxed toward the surface by 20% of the interlayer distance of the underlying Nb(110) surface, as found in previous studies ( 47 ).…”

“…Single Cr adatoms were deposited directly onto a cold substrate, as described in Methods. All atoms are found to be adsorbed in hollow stacking sites, which are energetically most favorable ( 47 ). Figure 1A shows a region of the Nb surface where blue dots correspond to Cr adatoms.…”

High-temperature Majorana fermions in magnet-superconductor hybrid systems

“…The side features of the Mn chains were previously ascribed to bands formed by hybridization of Yu-Shiba-Rusinov (YSR) states of particlular orbital character of the individual Mn impurities [20,36]. Individual Fe atoms do not exhibit clear signatures of multi-orbital YSR states [37,38] (see also Supplement), due to their close proximity to the coherence peaks of the substrate. Nevertheless, the linear Fe chains along the [001] direction, mentioned above, show an enhanced zero-energy LDOS at the sides of both ends (Fig.…”

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

“…In contrast, the d xy orbitals show the largest contribution out of plane as there the nearest neighbors are in the direction of the orbital lobes. At a surface the degeneracy of these orbitals would be lifted, and the orbital-induced angular dependence of the density can be resolved with scanning tunneling microscopy (STM) experiments [8,9,27,28]. Furthermore, we show in Fig.…”