Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have conducting states on their edge or surface. The conducting surface is not what makes topological insulators unique, but the fact that it is protected due to the combination of spin-orbit interactions and time-reversal symmetry, the so-called topological surface state.

Given the size of the bulk bandgap of about 0.3 eV in Bi_{2}(Se,Te)_{3}, the topological surface state can be exploited even at room temperature offering high electron mobility, dissipation-less spin torque, topological magnetoelectric coupling, or tunneling magnetoresistance. Another crucial advantage of topological insulators is that the functionalities of the surface are protected against any perturbation that preserves time-reversal symmetry, such as contamination, structural defects, or phonon scattering.

However, to implement spintronic applications, materials with magnetic order have to be interfaced with the surface, breaking consequently time-reversal symmetry. Without time-reversal symmetry, the topological surface state becomes gapped. Hence, it is very interesting to explore the interaction mechanisms between topological surface state and individual magnetic moments that preserve the time-reversal symmetry of the topological insulator.

A team of researchers from the University of Zaragoza, USDOE’s Ames Laboratory and DIPC has compared ^{1} the impact of a ferromagnetic element, around 1% of monolayer of cobalt (one cobalt atom per surface unit cell), in the quasiparticle interference patterns of topological insulators Bi_{2}Te_{3} and Bi_{2}Se_{2}Te. They found that time reversal symmetry is preserved in the ternary compound, while in the binary one it is destroyed. The researchers demonstrate that the magnetization coming from individual Co atoms deposited on the surface can disrupt the spin coherence of the carriers in the archetypal topological insulator Bi_{2}Te_{3}, while in Bi_{2}Se_{2}Te the spin texture remains unperturbed.

Quasiparticle interference patterns obtained by scanning tunnelling microscopy have proven to be extremely sensitive to minute amounts of scatterers. The team reached their conclusions from the observation of elastic backscattering events in those patterns.

From these analysis, the researchers found that time-reversal symmetry is preserved in the Se−Te mixed surface termination of Bi_{2}Se_{2}Te under the same coverage and experimental conditions for which time-reversal symmetry is destroyed in the Te-terminated surface of Bi_{2}Te_{3}. It is the modified adsorption geometry in the disordered surface what precludes cobalt magnetic states from hybridizing with the topological surface state , preserving in this way time-reversal symmetry.

As this principle can be expected to apply in many other ternary topological insulators, it would enable the design of functional interfaces based on magnetic probes in close contact to topological surface states.

*Author: César Tomé López is a science writer and the editor of Mapping Ignorance.*

## References

- M. Carmen Martínez-Velarte, Bernhard Kretz, María Moro-Lagares, Myriam H. Aguirre, Trevor M. Riedemann, Thomas A. Lograsso, Luis Morellón, M. Ricardo Ibarra, Arán Garcia-Lekue, and David Serrate (2017) Chemical Disorder in Topological Insulators: A Route to Magnetism Tolerant Topological Surface States Nano Letters
*17*(7), 4047-4054 doi: 10.1021/acs.nanolett.7b00311 ↩