Electrical detection of current generated spin in topological insulator surface states: Role of interface resistance

TL;DR

This paper introduces a Mott two-spin current resistor model that incorporates interface resistances to explain the sign variability in spin voltage measurements in topological insulator surface states, resolving conflicting experimental reports.

Contribution

The study presents a novel model accounting for interface resistances, explaining the sign reversal in spin voltage measurements in topological insulators, which previous electrochemical potential models overlooked.

Findings

Inclusion of interface resistances can cause crossing of spin-up and spin-down voltage profiles.

The model explains the sign variability of measured spin voltages.

Realistic parameters are crucial for accurate interpretation of spin signals.

Abstract

Current generated spin polarization in topological insulator (TI) surface states due to spin-momentum locking has been detected recently using ferromagnet/tunnel barrier contacts, where the projection of the TI spin onto the magnetization of the ferromagnet is measured as a voltage. However, opposing signs of the spin voltage have been reported, which had been tentatively attributed to the coexistence of trivial two-dimensional electron gas states on the TI surface which may exhibit opposite current-induced polarization than that of the TI Dirac surface states. Models based on electrochemical potential have been presented to determine the sign of the spin voltage expected for the TI surface states. However, these models neglect critical experimental parameters which also affect the sign measured. Here we present a Mott two-spin current resistor model which takes into account these parameters such as spin-dependent interface resistances, and show that such inclusion can lead to a crossing of the voltage potential profiles for the spin-up and spin-down electrons within the channel, which can lead to measured spin voltages of either sign. These findings offer a resolution of the ongoing controversy regarding opposite signs of spin signal reported in the literature, and highlight the importance of including realistic experimental parameters in the model.