Unexpected orbital magnetism in Bi-rich Bi2Se3 nanoplatelets

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.