Topological insulators (TI) are an exciting new class of materials with a wide range of interesting properties- many of which are yet to be explored.

In the HiTIMe (High Frequency Topological Insulator devices for Metrology) project we study the properties of high frequency devices made from topological insulator materials. The main driver of the project is the prospect of using a TI nanoribbon –a very narrow nanoscale ribbon made from a TI material- to create a topologically protected single-electron charge pump.  The main application of charge pumps is in metrology where they are used to “pump” electrons one at a time; thereby creating a very well-defined current I=e*f, f being the pump frequency and e the charge of a single electron (1.6022*10-19 C).  In order to obtain a current large enough to be used in the practical applications the frequency must be of the order ~1 GHz, meaning these are high-frequency devices.

The technological development in this project will also lay the groundwork for other devices that exploit the unique properties of TI for high-frequency applications including sensing, precision measurement and topologically protected quantum computation.

Topological insulator nanoribbons

Topological insulators is an example of a new class of quantum materials that is on the cusp of finding applications in electronic devices. Focus so far has mostly been on improving our understanding of the many fascinating properties of TI materials, but it is now becoming clear that they possess electronic properties that make them interesting for a wide range of applications.  In this project we use so-called nanoribbons of Bi2Se3 and Bi2Te3.  The nanoribbons are first grown using vapour-solid deposition on e.g. quartz and then transferred to a target substrate (typically silicone) before being incorporated into a device.  The ribbons grown using this method are very long (40- 50 mm) and free standing (see fig. 1).

Current metrology

Figure 1 SEM image showing a very long nanoribbon

In 2019 some of the units of the international system of units –the SI– will be redefined.  The Ampere which is the unit for electrical current will then be defined via the charge of the electron, 1.602 176 6208(98) x 10-19 C.
One of the challenges in metrology is how to use this new definition in real-world measurements.  We need to design experiments that allow us to perform extremely accurate measurements of a known current that can then be used to e.g. calibrate measurement equipment for industry.

Conceptually, the most straightforward way of performing such an experiment is to start with a device that will transfer one –and only one- electron between two electrodes every time the device receives a “clock signal” (which could e.g. come from an extremely precise atomic clock). This is the basic idea behind so-called charge pumps.

So far most charge pumps have been made using conventional semiconducting materials such as GaAs or silicon.  Several measurement institutes (including NPL) have already demonstrated devices with extremely good performance.

In this project we will work towards realizing a charge pump made from a TI nanoribbon. One of the unique properties of such materials is that there is topological protection from back-scattering. Or –in other words- the electrons can only go one way.

Ultimately, a TI based pump could be even more accurate than existing semiconductor based pumps. Moreover, whereas existing pumps are operated at very low temperatures (~300 mK) and high magnetic field (>10 T) we hope to be able to operate our charge pump at temperatures and magnetic fields achievable using affordable table-top systems. This would make it possible to for industry to purchase and operate a quantum standards for the Ampere near a production line and without the need for expensive facilities or expertise in cryogenic measurements.