October 5, 2011 — Scientists at SIMES, the Stanford Institute for Materials and Energy Sciences, which is a joint institute of SLAC National Accelerator Laboratory and Stanford University, have combined two known topological insulators to create a new one, which carries only surface currents. The researchers fabricated thin, tiny plates with the new topological insulator and gated them, creating an insulator-based transistor.

To create the gated topological insulator architecture, Stanford Associate Professor Ian Fisher’s group prepared crystals of the new compound (comprising bismuth, antimony and tellurium). SLAC Chief Scientist Zhi-Xun Shen’s group tweaked the elemental combinations for the best electronic properties, aided by the Advanced Light Source instruments at Lawrence Berkeley National Laboratory (Berkeley Lab). The resultant material maintained the highest current flow on the material surface with the least leakage into the bulk material.

Yi Cui, an associate professor on the faculty of Stanford and SLAC, led a group that formed the compound into 6-sided nanoplates with properties controlled by switching a separate electrical current on and off. The current changed the nanoplates between n- and p-type materials.

Figure. Electrons are unimpeded in a topological insulator, traveling along the edges of the sample, regardless of where they enter or leave it. In this diagram, based on a sample of mercury telluride, red arrows correspond to electrons with "spin up" and blue arrows indicate "spin down." An electron injected into one leg of the "H" can end up in the other without bumping into other electrons or material defects. Image courtesy Shoucheng Zhang.

Controlling the material’s properties with gates will enable "future electronic devices" based on topological insulators, said Cui. This study demonstrates that it’s possible to toggle a whole piece of topological insulator between n- and p-states using a gate, a "very important" step, said Desheng Kong, a fourth-year graduate student in Cui