Non-local electrical detection of spin-polarized surface currents in the 3D topological insulator BiSbTeSe$_{2}$

Abstract

The spin-polarized surface states in topological insulators offer unique transport characteristics that make them distinguishable from trivial conductors. Here, we detect the impact of these surface states in the topological insulator BiSbTeSe$_{2}$ by electrical means using a non-local transport configuration with ferromagnetic Co/Al$_2$O$_3$ electrodes. We show that the non-local measurement allows to probe the surface currents flowing along the whole surface, i.e.~from the top along the side to the bottom surface and back to the top surface along the opposite side. Increasing the temperature increases the interaction between bulk and surface states, which shortens this non-local current path along the surface and hence leads to a complete disappearance of the non-local signal at around 20K. Interestingly, we observe that the ratio between spin signal to background signal is much larger in the non-local geometry compared to the local one. Given that the observed ratio in the non-local geometry aligns well with expectations for spin-polarized surface states, our findings suggest that an as-yet unresolved mechanism diminishes the spin signal in the local geometry.

Figures (4)

• Figure 1: (a) Distribution of the current inside the bulk of a material with trivial band structure (black arrows). Only a negligible amount of current due to the effect of current spreading is expected under the non-local detection electrodes. (b) Distribution of the current inside a 3D topological insulator. Depicted are the potentially present bulk channel (black arrows), the local surface channel directly between source and drain (blue), and a second surface channel (red), which connects source and drain in a non-local manner. The spin polarization directions are indicated by green arrows and correspond to the case where spin-momentum–locked transport carried by the topological surface states (TSS) dominates over transport via the Rashba-split states (RSS). (c) Optical image of the 40 nm thick exfoliated BSTS flake contacted with Co/Al$_2$O$_3$ electrodes. (d) AFM scan of the flake shown in panel (c) before electrode fabrication. (e) Four-probe resistance as a function of temperature. (f) Weak antilocalization measurement at a base temperature of $T$ = 3 K as a function of an out-of-plane magnetic field $B_z$. The red line is a fit using the Hikami-Larkin-Nagaoka model.
• Figure 2: Schematic illustration of (a) local and (d) non-local measurement configurations. Potentiometric detection of the spin-polarization in the topological surface states using (b,c) local and (e,f) non-local geometries at $T$ = 3 K. The flow direction of electrons ($\vec{k}$) and their spin-polarization direction ($\vec{S}$) assuming the spin-texture of the topological surface states are shown by black and green arrows, respectively. In-plane magnetization direction of the electrodes ($\vec{M}$) is indicated using the brown arrows. All measurements show a hysteretic switching of the spin signal when reversing the magnetization direction of the ferromagnetic detectors by an in-plane external magnetic field $B_y$.
• Figure 3: Non-magnetization-dependent background signals in both local (blue) and non-local (red) configurations as a function of the applied current between source and drain contacts.
• Figure 4: Temperature dependence of the (a) non-local and (d) local spin signals depicted as waterfall diagrams. Amplitudes of the spin signals in the (b) non-local and (e) local geometries as a function of temperature. The background signals on which the spin signal is superimposed in (a) and (d) are depicted in (c) for the non-local and in (f) for the local geometry by extracting the voltage values at B = 50 mT from (a) and (d) and normalizing these values by the applied current between the source and drain electrodes. The black arrows indicate the temperature at which the local voltage in panel (f) shows a minimum and the non-local spin signal in panel (b) disappears.