Skip to main content

Exploring the physics of 3-dimensional topological insulators

Mid-Term Report Summary - 3-TOP (Exploring the physics of 3-dimensional topological insulators)

This report summarizes the 3-TOP related activities of the EP3 group of Prof. L. W. Molenkamp for the period from 04/01/2012 to 09/30/2013. Our work to date in this project can be divided into 3 categories.

1: growth and materials development of V-VI based topological insulators.
2: transport studies on HgTe based TI devices.
3: Technological development towards a NV-centre based high resolution magnetic imagining system.

1.Our research into V-VI based compounds has focused on Bi based materials, which seem to be the most promising, having the simplest band structure and a band gap of about 0.3 eV. Our growth methods make use of Molecular Beam Epitaxy (MBE), and we have studied two approaches, to tune the Fermi level into the bulk band gap. First we aim to improve the structural quality of the crystal to reduce defects causing the bulk conductivity. To this aim, we have investigated various growth substrate and surface preparations in order to determine an optimum growth start. Second we use doping and alloying techniques to counter dope the layers.
To determine the carrier density and mobility, optical lithography was developed to fabricate microhallbars of Bi2Se3, together with an insulator growth step allowing us to manipulate the carrier density with a gate. Using all these methods we have achieved a significant reduction in bulk doping densities of Bi2Se3 compounds, but more work is needed to achieve a truly insulating bulk.
2. On HgTe based materials, following the proposal by L. Fu and C. L. Kane (Phys. Rev. B76, 045302 (2007)), that strained bulk HgTe should be a strong topological insulator, we experimentally confirmed this to be the case by measuring the quantum Hall effect (QHE) originating from surface states of a 70 nm thick HgTe film.

In the first funding period of 3-TOP, we were able to fabricate strained bulk HgTe samples with top-gate electrodes, which allow for a change of the charge carrier density in the topological surface states. An analysis of Rxy and Rxx in magnetic field indicates that the two surface states dominate the transport behaviour, and that they exhibit clear signatures of Dirac physics.

3. Building towards the magnetometer set-up, we have developed a process to create nanoscale diamond structures that will be used as cantilevers in the scanning probe apparatus. We have also established an optical setup with the ability to perform Hanbury-Brown and Twiss Interferometry, and have finished all designs for the magnetic and cryogenic components which are currently in fabrication. Assuming all delivery dates are met, we expect to have a full system operational and ready for first testing by the end of March 2014, in good agreement with our pre-project planned timeline.