Topological insulator could find use in quantum computing and future electronic components

The researchers say that ordinary insulating solids have energy bands that are fully occupied by electrons. In diamond, for example, the conducting band is so far away from the valence band that electrons do not have sufficient energy to move – the 'band gap' is large and no electric current can be carried. However, researchers have become interested in topological insulators (TIs); materials which act as insulators on the inside, but are highly conductive on their surfaces. An international team of scientists from Japan, the UK and the US, led by Takao Sasagawa at Tokyo Tech, have developed a TI comprised of bismuth, tellurium and chlorine. Their TI – called BiTeCl – is inversion asymmetric, meaning it has different electronic states, and therefore different polarities, on each crystal surface. As a result, it exhibits topological effects that have not been seen experimentally before. The team optimised laboratory growing conditions to produce single crystals of BiTeCl, then split each crystal to obtain two different surfaces – one Te and one Cl – and observed their electronic structures. The composition of the TI's top and bottom crystal surfaces are such that their charge carriers are opposite, leading to polarisation. This allows the TI to be used as a diode, allowing current flow in only one direction. But the crystal also exhibits pyroelectric capabilities, meaning it can generate a temporary voltage when heated or cooled. Interestingly, BiTeCl may have a role in the development of quantum based technologies. Sasagawa said: "We would like to discover a topological superconductor whose surface can host Majorana fermions – particles which are their own antiparticles and could be used for the development of topological quantum computing."