Magnetism and in-gap states of 3d transition metal atoms on superconducting Re

Lucas Schneider,Manuel Steinbrecher,Manuel dos Santos Dias,Jens Wiebe,Roland Wiesendanger,Levente Rózsa,Juba Bouaziz,Krisztián Palotás,Samir Lounis

doi:10.1038/s41535-019-0179-7

Abstract

Magnetic atoms on heavy-element superconducting substrates are potential building blocks for realizing topological superconductivity in one- and two-dimensional atomic arrays. Their localized magnetic moments induce so-called Yu-Shiba-Rusinov (YSR) states inside the energy gap of the substrate. In the dilute limit, where the electronic states of the array atoms are only weakly coupled, proximity of the YSR states to the Fermi energy is essential for the formation of topological superconductivity in the band of YSR states. Here, we reveal via scanning tunnel spectroscopy and ab initio calculations of a series of 3d transition metal atoms (Mn, Fe, Co) adsorbed on the heavy-element superconductor Re that the increase of the Kondo coupling and sign change in magnetic anisotropy with d-state filling is accompanied by a shift of the YSR states through the energy gap of the substrate and a crossing of the Fermi level. The uncovered systematic trends enable the identification of the most promising candidates for the realization of topological superconductivity in arrays of similar systems.

Highlights

IntroductionMagnetic atoms locally induce pairs of bound state resonances inside the gap of their host superconductor, known as Yu-ShibaRusinov (YSR) states.[31,32,33,34,35] One route toward topological superconductivity is to weakly couple these states in so-called YSR bands, which can be realized by a dilute array of magnetic atoms on the surface of a superconductor.[17,36] In order to enter the topologically nontrivial phase, the YSR band has to cross the Fermi level EF
We show that the YSR state energy is shifting through the gap by increasing the 3d-orbital occupation going from the middle towards the right end of the 3d transition metal series, which correlates with a simultaneous increase in the Kondo coupling, and a transition from easy-axis to easy-plane magnetic anisotropy as revealed by spin-excitation spectroscopy
We see that going from the center of the 3d transition metal series (Mn) toward the end (Co), and from fcc to hcp adsorption site, the decrease in μ and increase in kBTK correlates with a systematic shift of the YSR state from the coherence peak at one gap edge across EF towards the coherence peak on the other side of the gap

Summary

Introduction

Magnetic atoms locally induce pairs of bound state resonances inside the gap of their host superconductor, known as Yu-ShibaRusinov (YSR) states.[31,32,33,34,35] One route toward topological superconductivity is to weakly couple these states in so-called YSR bands, which can be realized by a dilute array of magnetic atoms on the surface of a superconductor.[17,36] In order to enter the topologically nontrivial phase, the YSR band has to cross the Fermi level EF This requires the width in energy and the separation from EF of the YSR states of the atomic building blocks to be sufficiently small. The YSR state is accompanied by the formation of the Kondo state.[23,30] Orbital effects[22,27,29] and magnetic anisotropy[25,38] can lead to multiplicity and shifts, and exchange interactions between the atoms may split or shift the YSR states.[22,39,40,41,42] It is known since the 1960s that there is a systematic trend in the evolution of the

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