Ideal three-dimensional (3D) topological insulators (TIs) are known to have topologically protected (Dirac-like) surface states and an insulating bulk. Unfortunately, most TIs are doped such that electric measurements of the isolated surface states become cumbersome as the signal is blurred by the coexistance of (massive) bulk carriers.
We measure the quantum capacitance of TIs (Bi2Se3 and HgTe) at GHz frequencies. These measurements provide insight into the compressibility of such a two electron-fluid system. Furthermore, the dynamical response yields information about electron scattering properties. More specifically, in our measurements we track simultaneously the conductivity $\sigma$ and the compressiblity as a function of a DC-gate voltage. With these quantities at hand, we trace the energy dependence of the different conductivities. This procedure allows us to identify surface-surface, bulk-bulk as well as surface-to-bulk scattering rates. We demonstrate how orbital pseudo-spin polarization protects the surface carriers from surface-to-bulk scattering.