Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

T R F Peixoto ,Philipp Rüßmann,Stefan Blügel,Robert J Green,Henriette Maaß,Sonja Schatz,Raphael C Vidal ,Kai Roßnagel ,Celso I Fornari,A Barla ,Florian Diekmann,M Kalläne ,Pierluigi Gargiani,Manuel Valvidares,Laurens W Molenkamp,Hari Babu Vasili,Martin Winnerlein,S Schreyeck ,Hendrik Bentmann,Jens Buck,V B Zabolotnyy ,Moritz Hoesch,C Gould ,Fabian Stier,Chul Hee Min,V Hinkov ,Karl Brünner ,A Tcakaev ,Sebastian Rohlf,F Reinert

doi:10.1038/s41535-020-00288-0

Abstract

Since the discovery of the quantum anomalous Hall (QAH) effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)2Te3 and V:(Bi,Sb)2Te3, the search for the magnetic coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic circular dichroism and density functional theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)2Te3. State-of-the-art first-principles calculations find pronounced differences in their 3d densities of states, and show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results explain, in an unified picture, the origins of the observed magnetic properties, and establish the essential role of impurity-state-mediated exchange interactions in the magnetism of MTI.

Highlights

Doped topological insulators (MTI) form a cornerstone in the field of topological quantum materials
In a pioneering work, predicting the quantum anomalous Hall (QAH) effect in transition metal (TM)-based Magnetically doped topological insulators (MTI), the FM state was proposed to arise from a van Vleck mechanism, as a result of strong spin–orbit coupling (SOC) and the topologically non-trivial band ordering in these materials[10]
Some experimental support for this scenario has been reported[2,11,12], more recent first-principles calculations find that the strength of the exchange interactions in Cr:(Bi,Sb)2Te3 and V:(Bi,Sb)2Te3 is, largely independent of SOC, suggesting that the van Vleck mechanism, in the form proposed in ref. 10, plays no decisive role[13,14]

Summary

INTRODUCTION

Doped topological insulators (MTI) form a cornerstone in the field of topological quantum materials. In Cr-doped and V-doped (Bi,Sb)2Te3, the combination of ferromagnetism and a topologically non-trivial electronic band structure led to the discovery of the quantum anomalous Hall (QAH) effect[1,2,3,4,5], i.e. a dissipationless quantised edge-state transport in the absence of external magnetic fields. These materials are being widely utilised for possible realisations of topological superconductor[6] and axion insulator states[7], as well as in the context of metrology[8] and spintronic functionalities[9].

RESULTS

DISCUSSION

CODE AVAILABILITY