Unexpected orbital magnetism in Bi-rich Bi2Se3 nanoplatelets

Hae Jin Kim,Irene Zafiropoulou,Yannis Sanakis,Nikolaos Panopoulos,Vasileios Tzitzios,Dimitrios Gournis,Sang-Gil Lee,Jin-Gyu Kim,Seung Jo Yoo,M Pissas ,Saeed M Alhassan ,Marios S Katsiotis ,George Mitrikas ,Young‐Min Kim ,Ji–Hyun Lee ,M Fardis ,N Boukos ,Αntonios Kouloumpis ,Michael A Karakassides ,G Papavassiliou

doi:10.1038/am.2016.56

Abstract

The discovery of two-dimensional electron gas states with giant Rashba spin splitting (RSS) in electron-doped three-dimensional topological insulators (TIs) uncovered new fascinating physics and raised hopes for novel spintronic devices. Significant challenges, including synthetic constraints and control of the magnetic properties, must be addressed before any breakthroughs are possible. Here, we show how RSS in Bi-rich Bi2Se3 nanoplatelets is responsible for the appearance of remarkable orbital magnetic properties, as observed using magnetization and conduction electron spin resonance experiments and confirmed by theoretical simulations. In view of the strong spin-orbit coupling (SOC) and the proximity to the TI surface states, this discovery enlightens fundamental aspects of SOC-based functionalities of TI materials with aims for future applications. Plate-like bismuth nanocrystals can give physicists enriched mechanisms to control spin through unexpected, surface-localized magnetism. Topological insulators have crystal structures that make them non-conductive everywhere except for the quantized states on their surfaces. Hae Jin Kim's group at KBSI and co-workers (PI, “Demokritos”, and UoI) has now used a liquid-phase, solvothermal synthesis to turn bismuth selenide (Bi2Se3) topological insulators into nanometre-thin platelets with unique hexagonal shapes. The team discovered that sandwiching bismuth atoms between the nanohexagons disrupts the normal coupling interactions between electron spin and orbital motion at topological insulators surface states. This splitting generates so-called Rashba states that produce a special orbital-based type of magnetism, where spin states can be flipped with help from a weak magnetic field. Further spin engineering of this system is possible by altering the degree of bismuth intercalation. Bi-layer intercalation in Bi2Se3 nanoplatelets gives rise to an intriguing crystal structure comprised of randomly stacked Bi2Se3 and Bi2Se2. Detailed conduction electron spin resonance (CESR) and AC/DC magnetization studies prove that controlling Bi intercalation results in fine tuning the two-dimensional electron gas parabolic Rashba states, which enables the appearance of extraordinary orbital magnetism, through the coupling of the spin and orbital degrees of freedom. The methodology presented herein provides a unique and simple way for efficient spin engineering, with important potential applications.

Highlights

Topological insulator (TI) systems represent an unconventional quantum phase of matter controlled by spin-orbit coupling (SOC) with insulating bulk states and gapless metallic surface states.[1,2,3]Exemplary three-dimensional (3D) topological insulators (TIs) systems are the group V Bi1 − xSbx insulating alloys[4] and the group V–VI chalcogenide materials Bi2Se3, Bi2Te3 and Sb2Te3.1,2,5 The distinct properties of TI surface electron states appear attractive for fundamental research as well as for spintronic,[6,7] quantum information[8] and low-energy dissipation electronic applications.[9]
T1 vs temperature measurements are shown to follow the Korringa relation, which is a direct evidence of metallic behavior, that is, strong electron doping, most probably produced by Se vacancies and BiSe antisite defects.[12]
We have shown that sweeping a weak magnetic field from negative to positive values induces a stepwise reversal in the magnetization of bismuth-rich Bi2Se3 nanoplatelets, which is attributed to spin flips between Rashba states with opposite spin helicity

Summary

Introduction

Topological insulator (TI) systems represent an unconventional quantum phase of matter controlled by spin-orbit coupling (SOC) with insulating bulk states and gapless metallic surface states.[1,2,3]Exemplary three-dimensional (3D) TI systems are the group V Bi1 − xSbx insulating alloys[4] and the group V–VI chalcogenide materials Bi2Se3, Bi2Te3 and Sb2Te3.1,2,5 The distinct properties of TI surface electron states appear attractive for fundamental research as well as for spintronic,[6,7] quantum information[8] and low-energy dissipation electronic applications.[9]. Topological insulator (TI) systems represent an unconventional quantum phase of matter controlled by spin-orbit coupling (SOC) with insulating bulk states and gapless metallic surface states.[1,2,3]. Adsorbents, defects and stoichiometric deficiencies cause electron doping and strong bending of the electron bands.[11] In the case of Bi2Se3, experiments have shown that strong electron doping induced by Bi intercalation, Se vacancies and BiSe antisite defects is unavoidable[12] and leads to the surface transport properties being obscured by the bulk conductivity.[13]. Angle-resolved photoemission spectroscopy studies of Bi2Se3 have shown that band bending gives rise to two-dimensional electron gas (2DEG) states with a band bottom at the Γ point of the Brillouin zone, which coexist with the TI surface states.[14] By increasing bending, the in systems with topologically nontrivial band structures, a large OM is expected on electron states near the Dirac point.[23]

Methods

Results

Conclusion