A topological insulator is a new class of material discovered just a few years ago. It has fascinating properties: it behaves as an insulator in its bulk but contains conducting states at its boundary. In three dimensions, they are simile to a common band insulator with metallic surfaces that are protected by non-trivial topological order of the band structure. In two dimensions, they are characterized by one dimensional edge currents. The charge carriers at the boundary states have their spin locked at a right-angle to their momentum (so called spin-momentum locking), which can find applications in sensors, spin-electronics (spin torque, spin filtering, etc), thermoelectricity, plasmonics, etc.

We study the charge, spin transport and thermoelectric properties of topological insulators. We use exfoliated single crystals and thin films that are grown by Molecular Beam Epitaxy with whci we fabricate our own devices.

Below, we describe some of the recent results on this topic.

Inelastic Transport at the Surface of a Topological Insulator

We report in PRL on electric-field and temperature-dependent transport measurements in exfoliated thin crystals of the Bi2Se3 topological insulator. At low temperatures (< 50 K) and when the chemical potential lies inside the bulk gap, the crystal resistivity is strongly temperature dependent, reflecting inelastic scattering due to the thermal activation of optical phonons. A linear increase of the current with voltage is obtained up to a threshold value at which current saturation takes place.We show that the activated behavior, the voltage threshold, and the saturation current can all be quantitatively explained by considering a single optical phonon mode with energy ℏΩ ≈ 8 meV. This phonon mode strongly interacts with the surface states of the material and represents the dominant source of scattering at the surface at high electric fields.