Imaging current distribution in a topological insulator Bi2Se3 in the presence of competing surface and bulk contributions to conductivity

Abstract

Two-dimensional (2D) topological surface states in a three-dimensional topological insulator (TI) should produce uniform 2D surface current distribution. However, our transport current imaging studies on Bi2Se3 thin film reveal non-uniform current sheet flow at 15 K with strong edge current flow. This is consistent with other imaging studies on thin films of Bi2Se3. In contrast to strong edge current flow in thin films, in single crystal of Bi2Se3 at 15 K our current imaging studies show the presence of 3.6 nm thick uniform 2D sheet current flow. Above 70 K, this uniform 2D sheet current sheet begins to disintegrate into a spatially non-uniform flow. The flow becomes patchy with regions having high and low current density. The area fraction of the patches with high current density rapidly decreases at temperatures above 70 K, with a temperature dependence of the form $$1/\left| {T - 70} \right|^{0.35}$$ 1 / T - 70 0.35 . The temperature scale of 70 K coincides with the onset of bulk conductivity in the crystal due to electron doping by selenium vacancy clusters in Bi2Se3. Thus our results show a temperature dependent competition between surface and bulk conductivity produces a temperature dependent variation in uniformity of current flow in the topological insulator.

Current-induced spin polarizations in the bulk of topological insulators with inhomogeneous magnetic textures

“…This model roughly emulates what would occur near the TI/FM interface in a SOT device and we can use it to make some numerical estimates for the bulk contribution to the SOT. Estimations based on experimental results of the 2D surface state electron mobility [65][66][67][68] and the bulk conductivity 48 indicate that Bi 2 Se 3 can have a range of scattering times from 0.1 to 2 ps. With number densities that can vary between 0.5 − 50 × 10 18 cm −336, 65, 66 depending on doping, indicating that Bi 2 Se 3 systems could be in either the weak scattering limit or DP limit.…”

“…45 Studies have shown that the chemical potential lies in the TI bulk conduction band for most TI/FM SOT devices 46,47 and that bulk transport dominates in a certain parameter regime. 48 Hence it is believed that the bulk makes a significant contribution to the spin torque, and this is customarily attributed to the spin-Hall effect. 22,43 In light of this, there has been surprisingly little theoretical work on spin torques stemming from the bulk states of the TI.…”

Spin transfer torques due to the bulk states of topological insulators

“…51 Studies have shown that the chemical potential lies in the TI bulk conduction band for most TI/FM SOT devices, 52,53 while bulk transport dominates in a certain parameter regime. 54 Hence it is believed that the bulk makes a strong contribution to the SOT, and this is customarily attributed to the spin-Hall effect, 32,50 although this has never been proven. In light of this, there has been surprisingly little theoretical work on spin torques stemming from the bulk states of the TI.…”

Electrically-induced spin torques due to the bulk states of topological insulators

“…The origins of the large spin torques appear to differ between experiments [7,31,39,48], which are not able to distinguish between surface and bulk contributions [49]. Studies have shown that the chemical potential lies in the TI bulk conduction band for most TI/FM SOT devices [50,51], while bulk transport has been shown to dominate in a certain parameter regime [52]. Hence it is believed that the bulk makes a strong contribution to the spin-orbit torque, and this is customarily attributed to the spin-Hall effect [31,48], although this has never been proven qualitatively or quantitatively.…”

“…The inhomogeneous kinetic equation was solved in both the weak scattering Ω k τ 1 and DP limits Ω k τ 1. Estimations based on experimental results of the 2D surface state electron mobility [71][72][73][74] and the bulk conductivity [52] indicate that Bi 2 Se 3 can have a range of scattering times…”