Spin-pumping into surface states of topological insulator {\alpha}-Sn,
  spin to charge conversion at room temperature

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Abstract

We present experimental results on the conversion of a spin current into a charge current by spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) {\alpha}-Sn[1-3]. By angle-resolved photoelectron spectroscopy (ARPES) we first confirm that the Dirac cone at the surface of {\alpha}-Sn (0 0 1) layers subsists after covering with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into {\alpha}-Sn layers induces a lateral charge current that can be ascribed to the Inverse Edelstein Effect[4-5]. Our observation of an Inverse Edelstein Effect length[5-6] much longer than for Rashba interfaces[5-10] demonstrates the potential of the TI for conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of TI for spin to charge conversion and the conditions to reach it.

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Spin Transfer Torque Generated by the Topological Insulator Bi_2Se_3

N Samarth, D C Ralph, A. Mellnik … (2014)

Magnetic devices are a leading contender for implementing memory and logic technologies that are nonvolatile, that can scale to high density and high speed, and that do not suffer wear-out. However, widespread applications of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort, in particular discoveries that spin-orbit interactions in heavy metal/ferromagnet bilayers can yield strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba-Edelstein effect in the ferromagnet. As part of the search for materials to provide even more efficient spin-orbit-induced torques, some proposals have suggested topological insulators (TIs), which possess a surface state in which the effects of spin-orbit coupling are maximal in the sense that an electron's spin orientation is locked relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the TI Bi_2Se_3 at room temperature can indeed apply a strong spin-transfer torque to an adjacent ferromagnetic permalloy (Py = Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in the Bi_2Se_3 is greater than for any other spin-torque source material measured to date, even for non-ideal TI films wherein the surface states coexist with bulk conduction. Our data suggest that TIs have potential to enable very efficient electrical manipulation of magnetic materials at room temperature for memory and logic applications.

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Large-gap quantum spin Hall insulators in tin films

Binghai Yan, Yong Xu, Hai-Jun Zhang … (2013)

The search of large-gap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on first-principles calculations we find two-dimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the \Gamma point, similar to the case of HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized.

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Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

T Durakiewicz, M. Hasan, H. Ji … (2013)

The advent of Dirac materials has made it possible to realize two dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity - the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.