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The observation of helical surface states in Bi-based three-dimentional topological insulators has been a challenge since their theoretical prediction. The main issue raises when the Fermi level shifts deep into the bulk conduction band due to the unintentional doping. This results in a metallic conduction of the bulk which dominates the transport measurements and hinders the probing of the surface states in these experiments. In this study, we investigate various strategies to reduce the residual doping in Bi-based topological insulators. Flakes of Bi2Se3 and Bi1.5Sb0.5Te1.7Se1.3 are grown by physical vapor deposition and their structural and electronic properties are compared to mechanically exfoliated flakes. Using Raman spectroscopy, we explore the role of the substrate in this process and present the optimal conditions for the fabrication of high quality crystals. Despite of this improvement, we show that the vapor phase deposited flakes still suffer from structural disorder which leads to the residual n-type doping of the bulk. Using magneto-measurements we show that exfoliated flakes have better electrical properties and are thus more promising for the probing of surface states. arXiv:2001.04368v2 [cond-mat.mes-hall]

Dominating electron−electron scattering enables viscous electron flow exhibiting hydrodynamic current density patterns, such as Poiseuille profiles or vortices. The viscous regime has recently been observed in graphene by nonlocal transport experiments and mapping of the Poiseuille profile. Herein, we probe the current-induced surface potential maps of graphene field-effect transistors with moderate mobility using scanning probe microscopy at room temperature. We discover micrometer-sized large areas appearing close to charge neutrality that show currentinduced electric fields opposing the externally applied field. By estimating the local scattering lengths from the gate dependence of local in-plane electric fields, we find that electron−electron scattering dominates in these areas as expected for viscous flow. Moreover, we suppress the inverted fields by artificially decreasing the electron-disorder scattering length via mild ion bombardment. These results imply that viscous electron flow is omnipresent in graphene devices, even at moderate mobility.

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