Digging into topological protection

Topological insulators are materials that impede the transport of electrons in their bulk but have spin polarized conduction channels on their surface. Despite promising for spintronics, electronics with little energy loss, and even quantum computing, topological insulators are not completely understood. EPFL scientists have now made a breakthrough insight into topological protection, a central aspect of topological insulators that relates to their robustness against environmental perturbation. The study is published in Physical Review B.

If the future does indeed hold topological quantum computers, it will inevitably require a deep understanding of topological protection. Quantum computers will need to put information bits (or “qubits”) into fragile quantum superposition states, which themselves are very vulnerable to environmental noise, and must be shielded from perturbations and fluctuations. Using topological protection to encode quantum information would improve computation and transmission of a message before it degenerates into nonsense.

In particular, it is important to create a material with a large "spin-splitting" – the separation of the spin-up and spin-down states of electrons – with the existence of the spin polarized states guaranteed by the topology of the sample. The problem is that, although many candidate materials have been identified both at the EPFL and other institutions, the microscopic nature of this protection is not well understood.

A team of scientists led by Hugo Dil at EPFL has gained new understanding into exactly this topological protection. The scientists purposely destroyed the surface of the prototypical topological insulator Bi₂Se₃ and then used a technique called "soft x-ray ARPES", which can “see” electronic states below the surface of solid materials, to look for the topological surface state deeper down into the bulk. By comparison of the experimental data with theoretical models they could uncover the secret behind the protection mechanism.

The researchers found that topological protection works by moving away from regions at the surface that contain many defects and down into more pure bulk regions. In fact, by moving down the topological states avoid scattering with adsorbates and actually become clearer and more pronounced.

The study explains the mechanism behind topological protection and opens new ways for optimizing topological properties for future technologies.

This work involved a collaboration between EPFL’s Institute of Physics, Max-Planck-Institut, the University of Zurich, Paul Scherrer Institut, and Aarhus University. It was funded by the Swiss National Science Foundation, the US National Science Foundation, the VILLUM foundation, the Deutsche Forschungsgemeinschaft, and the Danish National Research Foundation.

Reference

Queiroz R Landolt G, Muff S, Slomski B, Schmitt T, Strocov VN, Mi J, Iversen BB, Hofmann P, Osterwalder J, Schnyder AP. Dil JH. Sputtering induced re-emergence of the topological surface state in Bi₂Se₃. Physical Review B 93, 165409 2016. DOI: 10.1103/PhysRevB.93.165409