Photoemission-induced gating of topological insulators

The recently discovered topological insulators exhibit topologically protected metallic surface states which are interesting from the fundamental point of view and could be useful for various applications if an appropriate electronic gating can be realized. Our photoemission study of Cu-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ shows that the surface-state occupancy in this material can be tuned by changing the photon energy and understood as a photoemission-induced gating effect. Our finding provides an effective tool to investigate the new physics coming from the topological surface states and suggests intercalation as a recipe for synthesis of a material suitable for electronic applications.

Figure 1

In-plane resistivity of the pure (black dashed line) and Cu intercalated (red solid line) Bi${}_2$Se${}_3$ crystals; the low-temperature region of the later is zoomed in the inset.

Figure 2

Excitation energy dependence of the surface-state occupancy. A weak decrease of the binding energy of the Dirac point in Bi${}_2$Se${}_3$ (a) is contrasted to a 150 meV steplike change in Cu${}_{0.07}$Bi${}_2$Se${}_3$ (b). (c) The step at 30 eV in the surface-state occupancy, shown in units of the Fermi surface area, corresponds to the lowest photon energy enabling photoemission from Bi 5$d$ core levels and consequent manifold increase of the photocurrent [see underlying plot of integral density of states (DOS)].

Figure 3

A classification of the photovoltage effects as seen by photoemission. (a) A so-called charging of the whole sample due to absence of a good Ohmic contact between the surface of the sample and the electron analyzer appears as a shift $\Delta E_F$ of the Fermi level $E_F$ of the sample under illumination with respect to its equilibrium position or to the $E_F$ of the analyzer. (b) In the most general case the light-induced photovoltage both affects the surface charge region and creates the charge of the sample. (c) In the case of a highly conductive surface (and poorly conductive or insulating sample volume), its Fermi level remains equal to that of the analyzer and the only observed photovoltage effect is the surface-state gating.