High Fermi-level spin polarization in the(Bi1−xSbx)2Te3family of topological insulators: A point contact Andreev reflection study

Abstract

Point contact Andreev reflection spectroscopy is employed to extract the effective Fermi-level spin polarization of three distinct compositions from the (${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$)${}_{2}{\mathrm{Te}}_{3}$ topological insulator family. The end members, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, exhibit high polarization of 70(4)% and 57(3)%, respectively. High-field $({\ensuremath{\mu}}_{0}H=14\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ point contact spectroscopy shows carrier depletion close to the Fermi level for these two compositions with small activation gaps of 0.40(4) and 0.28(2) meV, respectively. The almost fully suppressed bulk conductivity in the (${\mathrm{Bi}}_{0.18}{\mathrm{Sb}}_{0.82}$)${}_{2}{\mathrm{Te}}_{3}$ results in an even higher spin polarization of 83(9)%. Further, it is demonstrated that magnetic doping with Cr and V tends to reduce the spin-polarization values with respect to the ones of the pure compositions. ${\mathrm{Bi}}_{1.97}{\mathrm{Cr}}_{0.03}{\mathrm{Te}}_{3}, {\mathrm{Sb}}_{1.975}{\mathrm{Cr}}_{0.025}{\mathrm{Te}}_{3}, {\mathrm{Bi}}_{1.975}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}_{0.025}{\mathrm{Te}}_{3}$, and ${\mathrm{Sb}}_{1.97}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}_{0.03}{\mathrm{Te}}_{3}$ exhibit spin polarization of 52%, 52%, 58%, and 50%, respectively. In view of the rather high effective polarization, nonmagnetic topological insulators close to (${\mathrm{Bi}}_{0.18}{\mathrm{Sb}}_{0.82}$)${}_{2}{\mathrm{Te}}_{3}$ may provide a path towards the characterization of pair-breaking mechanisms in spin-triplet superconductors.