Topological insulator surface acoustoelectric current

Topological insulators (TI) are materials which, though insulating in the bulk, support carrier transport in surface states in which scattering is suppressed due to strong spin orbit interactions and topological protection.

Their conduction and valence bands are partially inverted due to an energy shift arising from their strong spin orbit interactions; the surface states (hereafter called as topological surface states, or TSS) display a linear energy dispersion (which results in the carriers being massless Dirac fermions) which can also exhibit spin-momentum locking (a result of the helical spin texture).

Both properties make them potentially highly useful energy-saving electronics with very fast response time.

The presence of the TSS has been imaged by various surface sensitive experiments, while spin transport has recently been demonstrated by electrical devices that can tune the chemical potential into the band gap. Ways to manipulate (transport and modulate) the carriers within TIs are now sought, to fully exploit their potential.

Surface acoustic waves (SAWs) are mechanical Rayleigh waves, like a microscopic earthquake, which can be launched and detected in piezoelectric materials using interdigitated transducers.

The mechanical waves are accompanied by a powerful wave of electric potential which can produce what is termed an acoustoelectric current by breaking local and lateral inversion symmetry of objects placed in the SAW beam path, and by modulation of the band structure of overlaid materials.

In this studentship, we will seek to directly manipulate (controllably transport) carriers within the TSS using the effect on the TI band structure of surface acoustic waves for the first time.

The acoustoelectric current will also reflect the Dirac electron-surface phonon coupling strength and will provide insights on practical ways to manipulate the TSS in practical devices operating at up to 10s of GHz, with relevance to many classes of electronic device.

Demonstration of the new types of band structure modification and excitation will not only be of great interest to physicists, where SAW induced transport in TIs will open up a new route to modification and exploration of their band structure but could also open up potential applications for TIs in high frequency electronics, including in delay lines, acoustic charge transport devices, and for use as current standards.