A rechargeable spin battery
With an increased interest in spin logic devices compatible with the existing semiconductor technology, manipulating electron spins in nonmagnetic semiconductors has been one of the most active research directions in spintronics, promising smaller, faster, less power-consuming information-processing and communication devices. However, several technical challenges have to be overcome in semiconductor spintronics, such as efficient spin injection, long spin lifetime, and efficient spin transport, manipulation, and detection. A particularly important goal is to generate a controllable electronic spin polarization that can have a long lifetime, using all-electric approaches that are free from magnetic material
Three-dimensional (3D) topological insulators (TIs) represent a new class of electronic quantum phases with strong spin-orbit coupling, hosting spin-helical topological surface states (TSSs) protected by time-reversal symmetry. One of the most fundamental and marked properties of TSS is spin-momentum locking (SML), where the electron spin is "locked" in plane and perpendicular to its momentum, making 3D TIs highly promising for applications in nanoelectronics and spintronics. Several recent experiments in 3D TIs have demonstrated current-induced helical spin polarization attributed to the SML. The observed spin polarization reverses upon reversing the current, requires a DC current to maintain it, and vanishes as soon as the current is removed. However, for many applications, a persistent and long-lived spin polarization (sometimes referred to as a spin "battery" or memory) may be desired.
Here, we report spin potentiometric measurements in Bi2Te2Se (BTS221) TI thin flakes that have revealed a new phenomenon not observed previously in TIs: a current-induced persistent electron spin polarization (ESP). A voltage between a ferromagnetic (FM) and one of the nonmagnetic contacts is monitored as a function of an in-plane magnetic (B) field applied to magnetize the FM contact. We observe a hysteretic step-like voltage change when the B field is swept between the opposite directions, resulting in a clear difference in the voltage detected between the opposite FM magnetizations. Such a voltage difference measured by spin potentiometry is a measure of spin chemical potential, representing an out-of-equilibrium ESP. However, in stark contrast to previous experiments, such a spin signal both its sign and amplitude-shows little dependence on the sign and magnitude of a relatively small DC detection current.
Topological insulators (TIs), with their helically spin-momentum-locked topological surface states (TSSs), are considered promising for spintronics applications. Several recent experiments in TIs have demonstrated a current-induced electronic spin polarization that may be used for all-electrical spin generation and injection. We report spin potentiometric measurements in TIs that have revealed a long-lived persistent electron spin polarization even at zero current. Unaffected by a small bias current and persisting for several days at low temperature, the spin polarization can be induced and reversed by a large "writing" current applied for an extended time. Although the exact mechanism responsible for the observed long-lived persistent spin polarization remains to be better understood, we speculate on possible roles played by nuclear spins hyperfine-coupled to TSS electrons and dynamically polarized by the spin-helical writing current. Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information.
