Posted: Apr 19, 2016 
Towards controlling the electronic surface structure of materials
(Nanowerk News) Topological insulators are materials that have an electronically insulating interior and an electronically conductive exterior. Much effort has gone into finding and researching the properties of these topological isolators, as they have many uses in potential applications such as quantum computers. An important property of these materials is the Zeeman effect, which is the coupling of spins with a magnetic field.

The magnitude of the Zeeman effect is influenced by the magnetic field that has been applied to the material, and an electron "gfactor". Therefore, to influence the topological surface state of a topological insulator through the Zeeman effect, it is important to determine its gfactor. However, studies to determine the gfactor of the topological surface states of materials have yielded inconsistent results.

A new method of measurement

A collaboration combining the expertise of researchers from Tokyo Tech (led by Prof. Takao Sasagawa), RIKEN (led by Dr. Tetsuo Hanaguri) and other institutes recently used spectroscopic imaging scanning tunnelling microscopy (SISTM) to determine the gfactors of the topological surface states of topological insulators Bi2Se3 and Sb2Te2Se, and reported their findings in Nature Communications ("Observation of Zeeman effect in topological surface state with distinct material dependence").


Spectroscopic imaging scanning tunnelling microscopy used to determine the gfactor of Bi2Se3 and Sb2Te2Se.

SISTM can probe electronic states in a wide energy range with high spatial and energy resolution, so it can be used to study the electronic configuration of topological insulators even if they contain defects. The team developed a methodology using SISTM to unambiguously observe the gfactor of the topological surface state of Bi2Se3 and Sb2Te2Se.

Calculating the gfactor

The collaboration first modelled the Zeeman effect of a topological surface state to develop a method to analyze their SISTM data accounting for external effects. They performed SISTM measurements using crystals of Bi2Se3 and Sb2Te2Se, a tungsten tip, and a magnetic field applied perpendicular to the sample crystal. By accounting for and removing the influence of external effects, the researchers were able to determine the genuine gfactors of Bi2Se3 and Sb2Te2Se from SISTM measurements.

Interestingly, the determined gfactors for Bi2Se3 and Sb2Te2Se were quite different despite their similar band dispersions and effective masses. The difference might be because of the different constituent elements of the materials. This strong dependence of gfactor on material type has massive potential implications for the development of topological insulators with designed spin states, as the gfactor of topological surface states could potentially be controlled by modifying the chemical composition of the topological insulators.

This knowledge could open the door to new, highquality material for use in future applications such as spintronic devices and quantum computing.
