[Paper Review] Long range dynamical coupling between magnetic adatoms mediated by a 2D topological insulator

Marcio Costa, Marco Buongiorno Nardelli+2 more|arXiv (Cornell University)|Aug 1, 2018

Topological Materials and Phenomena3 citations

TL;DR

This study demonstrates that magnetic iron adatoms on a Bi bilayer nanoribbon exhibit long-range dynamical spin coupling mediated by topologically protected edge states (TPES), enabled by the 2D topological insulator's robust spin current transport. Despite strong spin-orbit coupling, TPES preserve spin coherence over distances exceeding 8 nm, enabling terahertz-frequency spin excitations and non-exponential decay of coupling, which is absent in non-topological systems.

ABSTRACT

We study the spin excitation spectra and the dynamical exchange coupling between iron adatoms on a Bi bilayer nanoribbon. We show that the topological character of the edge states is preserved in the presence of the magnetic adatoms. Nevertheless, they couple significantly to the edge spin currents, as witnessed by the large and long-ranged dynamical coupling we obtain in our calcula- tions. The large effective magnetocrystalline anisotropy of the magnetic adatoms combined with the transport properties of the topologically protected edge states make this system a strong candidate for implementation of spintronics devices and quantum information and/or computation protocols.

Motivation & Objective

• To investigate how magnetic adatoms interact with topological edge states in a 2D topological insulator (Bi bilayer nanoribbon).
• To determine whether topological protection preserves spin coherence over long distances despite strong spin-orbit coupling.
• To explore the role of topologically protected edge states in mediating long-range dynamical exchange coupling between magnetic adatoms.
• To assess the feasibility of using such hybrid systems for spintronic and quantum information applications.
• To compare coupling behavior in topological vs. non-topological Bi (with reduced SOC) to isolate the role of TPES.

Proposed method

• Employed ab initio density functional theory (DFT) to obtain electronic structure and hopping parameters for the Bi bilayer nanoribbon.
• Constructed a multi-orbital tight-binding Hamiltonian using pseudo-atomic orbital (PAO) projection from DFT results.
• Incorporated effective spin-orbit coupling (SOC) via a local atomic approximation to model strong spin-orbit effects in Bi.
• Used many-body techniques to calculate spin excitation spectra and dynamical exchange coupling between Fe adatoms.
• Simulated both isolated adatoms and pairs at varying inter-atomic distances, comparing edge vs. center positions on the ribbon.
• Artificially reduced Bi’s SOC strength to 0.8 eV to create a non-topological reference system and isolate the role of TPES.

Experimental results

Research questions

• RQ1 How does the presence of magnetic adatoms affect the topological protection of edge states in a 2D topological insulator?
• RQ2 What is the nature and range of dynamical coupling between Fe adatoms mediated by topological edge states?
• RQ3 How does the coupling strength depend on the relative position of adatoms (edge vs. center) on the nanoribbon?
• RQ4 What role does topological protection play in enabling long-range spin current transport between distant magnetic impurities?
• RQ5 How does the coupling behavior differ in a topological vs. a non-topological Bi system with identical geometry and adatom placement?

Key findings

• The topological character of the edge states is preserved even in the presence of Fe adatoms, confirming robustness of topological protection.
• Dynamical coupling between Fe adatoms exhibits a long-range, near-plateau-like decay for distances up to 8 nm when adatoms are near the edge, indicating minimal loss over long distances.
• For adatoms at the ribbon center, coupling decays slowly but non-exponentially, with a plateau observed beyond 8 nm, contrasting sharply with exponential decay in non-topological systems.
• When SOC in Bi is reduced to 0.8 eV (non-topological), coupling decays exponentially regardless of adatom position, proving that TPES are essential for long-range coupling.
• The large effective magnetocrystalline anisotropy in Fe adatoms results in spin excitation frequencies in the terahertz range, suitable for high-speed spintronic applications.
• Topological edge states act as near-lossless spin current conduits, enabling coherent spin transport over distances exceeding 8 nm, despite strong spin-orbit coupling.