The key result of this paper is a third 'knob' in which the amount of illumination of the surface with a super-band-gap photon beam can be used to reversibly tune the energy of the Dirac point in binary, ternary and quaternary 3D TI's.
The parameters explored and described by Emmanouil Frantzeskakis and colleagues include stoichiometric variation in the bulk TI crystal (tunes bulk conductivity) and adsorption of gases on the surface (yields downward band bending at the first 10's of nm of the topological insulator surface). Given illumination at fluxes no greater than those found in standard laboratory LEDs and simple laser sources, the VUV radiation from the synchrotron could be used to fully compensate the downward band-bending and thus uncover the true flat-band energy alignment of the Dirac point with respect to the bulk and surface Fermi energy.
All in all, the study shows we have four knobs with which to dial up the required energy position of the topological surface states - bulk stoichiometry, surface decoration, temperature, and photon exposure - and that these can be used in conjunction to fine tune the band energies near the surface and consequently influence whether the Fermi levels cuts only topological surface states or additionally topologically-trivial surface-confined bulk conduction band states. As a bonus, this approach allows us to reliably determine the low temperature, flat-band energy scheme at the surface for six popular 3D topological insulators, three being n-type and three, p-type in the absence of band bending.
Pick up the paper in the publications section of the webpage, or click here.
The experiments were carried at the SIS beamline of the Swiss Light Source, and the 1^2 and IDEEAA end-stations at BESSY-II/HZB in Berlin.
This research is part of the FOM/NWO-N Programme 134 'Topological Insulators', which combines the forces of groups from Leiden, Delft, Amsterdam & Twente, and is led by the Mark Golden in Amsterdam.
