The weak intrinsic spin-orbit coupling in graphene can be greatly enhanced by proximity coupling. Here we report on the proximity-induced spin-orbit coupling in graphene transferred by hexagonal boron nitride (hBN) onto the topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) which was grown on a hBN substrate by vapor solid synthesis. Phase coherent transport measurements, revealing weak localization, allow us to extract the carrier density-dependent phase coherence length l φ . While l φ increases with increasing carrier density in the hBN/graphene/hBN reference sample, it decreases in graphene/BSTS due to the proximity-coupling of BSTS to graphene. The latter behavior results from D'yakonov-Perel-type spin scattering in graphene with a large proximity-induced spin-orbit coupling strength of at least 2.5 meV.arXiv:1809.01113v2 [cond-mat.mes-hall]

Free-standing Bi2Se3 single crystal flakes of variable thickness are grown using a catalyst-free vapor-solid synthesis and are subsequently transferred onto a clean Si ++ /SiO2 substrate where the flakes are contacted in Hall bar geometry. Low temperature magneto-resistance measurements are presented which show a linear magneto-resistance for high magnetic fields and weak anti-localization (WAL) at low fields. Despite an overall strong charge carrier tunability for thinner devices, we find that electron transport is dominated by bulk contributions for all devices. Phase coherence lengths l φ as extracted from WAL measurements increase linearly with increasing electron density exceeding 1 µm at 1.7 K. While l φ is in qualitative agreement with electron electron interaction-induced dephasing, we find that spin flip scattering processes limit l φ at low temperatures. arXiv:1506.04097v1 [cond-mat.mes-hall]

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]