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Breakthrough in Spintronics: New Class of Magnetic Topological Insulators Identified

A research team from Monash University, affiliated with the FLEET Center, has unveiled a novel methodology for understanding intrinsic magnetic second-order topological insulators. These materials offer significant potential for advancing spintronics, a field dedicated to utilizing electron spin for data transfer. The findings of this study have been shared in the journal Nano Letters.

In recent years, two-dimensional ferromagnetic semiconductors such as CrI3, Cr2Ge2Te6, and VI3 have garnered extensive research attention due to their essential characteristics in spintronic applications. Topological insulators, known for their unique properties, maintain an insulating behavior in their bulk while allowing conductivity along their surfaces.

Three-dimensional topological insulators like Bi2Se3 facilitate the observation of two-dimensional Dirac fermions on their surfaces. A more recent concept, second-order topological insulators, advances this idea by displaying (m-2)-dimensional boundary states in m-dimensional frameworks. For instance, one-dimensional hinge states arise in three-dimensional materials, while two-dimensional materials can showcase zero-dimensional corner states.

Typically, intrinsic ferromagnetic semiconductors exhibit pronounced electron-electron correlations, which can severely restrict electron interaction among adjacent atoms. This results in an atomic insulator-like system that lacks topological characteristics, complicating the pursuit of bridging these two states.

The study, spearheaded by Dr. Zhao Liu and Professor Nikhil Medhekar under FLEET THEME 1, proposes an innovative solution to this dilemma. They determined that in specific intrinsic ferromagnetic semiconductors, p orbitals from ligand anions and d orbitals from metal cations can obtain an inverted orbital arrangement.

In conventional ferromagnetic semiconductors, p-d orbitals are typically ordered so that the p orbitals are at a lower energy level, facilitating superexchange interactions between metal cations with open d shells. However, the research revealed scenarios where certain p orbitals possess higher energy than d orbitals, leading to an inverted p-d orbital system. Given that p and d orbitals have opposing parity, it is anticipated that inverted p-d orbitals result in nontrivial topological phases, contrasting with the trivial phases associated with normally ordered configurations.

Utilizing advanced density-functional theory calculations and wave function symmetry analysis, the team identified 1T-VS2 and CrAs monolayer as promising candidates for intrinsic magnetic second-order topological insulators. 1T-VS2 exhibits a hexagonal lattice structure, while CrAs has a square lattice. In both materials, the spin-up channel reveals inverted p-d orbitals, suggesting nontrivial topology, whereas the spin-down channel displays normally ordered p-d orbitals that correspond to trivial topology.

The unique growth patterns of 1T-VS2 nanoflakes into hexagonal or triangular shapes and CrAs monolayers into squares enable the use of spin-polarized scanning tunneling microscopy to detect the localized states, which are present solely at the corners of these structures.

Professor Medhekar emphasized the broader implications of their research: “Our findings can extend to Kondo insulators, where d and f orbitals serve similar functions to the p and d orbitals discussed. Exploring the possibility of second-order topological Kondo insulators could be significantly impactful, especially given the recent recognition of topological Kondo insulators in this domain.”

More information: Zhao Liu et al, Generic Approach to Intrinsic Magnetic Second-Order Topological Insulators via Inverted p–d Orbitals, Nano Letters (2024). DOI: 10.1021/acs.nanolett.4c03109

Citation: Fundamental spintronics research reveals generic approach to magnetic second-order topological insulators (2024, September 12) retrieved from https://phys.org/news/2024-09-fundamental-spintronics-reveals-generic-approach.html

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