Disorder-induced decoupled surface transport channels in thin films of doped topological insulators

Semi-metallic bulk generated spin-orbit torques in disordered topological insulator

“…In contrast to the reported huge charge to spin conversion efficiencies that are widely attributed to the spin-momentum locking of the TSS, we observe sizable spin Hall conductivities σ s originating from the bulk SHE in the Ni 81 Fe 19 /Bi 0.1 Sb 0.9 heterostructures with negligible contribution from the TSS although the WAL measurements of Bi 0.1 Sb 0.9 single layers at low temperature suggest an intermediate coupling between the surface and bulk, a hallmark of surface channels. Considering the inherently strong SOC, the comparable σ s to the literature data observed in our Ni 81 Fe 19 /Bi 0.1 Sb 0.9 bilayers implies that the unavoidable self-doping in the bulk of TIs may give rise to an efficient SOT generation from the bulk, allowing us to pursue the plethora of the SHE following routes such as the modulation of structural disorder by annealing [25], and the compensation of the doping to control the carrier transport in the bulk [56,57]. Moreover, we found that the complementary data of the cavity FMR and ST-FMR measurements can be well understood by a spin tunneling contact existing at the FM/TI interface that generates a FL torque with the same order of magnitude as a DL torque, which captures the characteristics of early reported SOTs in FM/Bi 2 Se 3 systems [8].…”

“…A recent experimental study demonstrates that a sputtered TI film can be an efficient SOT generator with a critical magnetization switching current density of the order of 10 5 A/cm 2 in FM/TI heterostructures at room temperature (RT), even lower than its single crystal counterpart [24]. In fact, the polycrystalline feature of sputtered TI films will introduce the structural disorder, which may localize the bulk electrons and decouple the top and bottom TSS [25,26].…”

In-depth Analysis of Anisotropic Magnetoconductance in Bi$_2$Se$_3$ thin films with electron-electron interaction corrections

“…Comparably smaller value of l φ,i in SiO 2 /Bi 2 Se 3 films may suggest the presence of higher disorder in these films which is also observed in the RHEED images and the value of µ obtained from the Hall measurements. The presence of disorder can effectively decouple the top and bottom surface states [29,60] resulting in a higher value of Ã1 as observed in our SiO 2 /Bi 2 Se 3 samples. Previous reports have achieved similar decoupling between the top and bottom TSSs in TI thin films by tuning the Fermi level [20,60], modulating the film thickness [49] and controlling the disorder by varying the annealing conditions [29].…”

“…The presence of disorder can effectively decouple the top and bottom surface states [29,60] resulting in a higher value of Ã1 as observed in our SiO 2 /Bi 2 Se 3 samples. Previous reports have achieved similar decoupling between the top and bottom TSSs in TI thin films by tuning the Fermi level [20,60], modulating the film thickness [49] and controlling the disorder by varying the annealing conditions [29].…”

“…WAL signal in the MC allows us to probe the electronic modifications in TSS due to finite film thickness [17][18][19], surface-bulk coupling [20,21], inter-surface coupling [17,22,23], magnetic doping [14,15] and magnetic proximity effect [16,[24][25][26]. Recent theoretical [27] and experimental [28,29] investigations show that a WAL signal appears from the TSSs even when magnetic field is applied parallel to the film plane due to the finite penetration depth of the surface states. These observations suggest that the WAL signal from the TSSs, in both out-of-plane (OOP) and in-plane (IP) magnetic field, can contribute to the anisotropic magnetoconductance (AMC) of a TI.…”