Detection of the Spin-Chemical Potential in Topological Insulators Using Spin-Polarized Four-Probe STM

Abstract

We demonstrate a new method for the detection of the spin-chemical potential in topological insulators using spin-polarized four-probe scanning tunneling microscopy on in situ cleaved Bi_{2}Te_{2}Se surfaces. Two-dimensional (2D) surface and 3D bulk conductions are separated quantitatively via variable probe-spacing measurements, enabling the isolation of the nonvanishing spin-dependent electrochemical potential from the Ohmic contribution. This component is identified as the spin-chemical potential arising from the 2D charge current through the spin momentum locked topological surface states (TSS). This method provides a direct measurement of spin current generation efficiency and opens a new avenue to access the intrinsic spin transport associated with pristine TSS.

Room temperature in-situ measurement of the spin voltage of a BiSbTe3 thin film

“…Since the current is partially carried by the bottom TSS of the film, only the fraction of the current that propagates through the top TSS is relevant for determining p. Assuming that the current divides equally between the top and the bottom TSS and using typical values for the effective spin polarisation of the ferromagnetic tip [33][34][35] of P FM ~ 0.25-0.5 (without the geometric factor 1/ 2), a spin polarisation in the TSS of p ~ 0.3-0.6 results. This range is similar to values observed in other transport investigations 14,16,18,20,36 . The main source of variation of p in our experiment is the value of the tip polarisation.…”

“…The helical spin texture of the topological surface states of 3D TIs has been studied extensively by means of angle-resolved photoemission spectroscopy (ARPES) [8][9][10][11][12] . Recently, also the electrical detection of the intrinsic spin polarisation in TSS has been reported in transport experiments [13][14][15][16][17][18][19][20][21] , employing a multi-terminal potentiometric approach with ferromagnetic (FM) contacts. However, most transport investigations suffer from intrinsic difficulties, such as the parasitic contribution to charge transport through electronic states from the bulk and contaminations introduced during the ex-situ fabrication of devices, which can modify the electronic structure of the TI and the TSS and consequently reduce its spin polarisation.…”

Accessing the Intrinsic Spin Transport in a Topological Insulator by Controlling the Crossover of Bulk-to-Surface Conductance

“…Topological insulators possess non-trivial topology which results in the topological surface states with massless Dirac fermions and peculiar spin texture, while their bulk states have band gap and behave as insulator [1,2]. The transport through the surface is expected to exhibit superior mobility from prohibited backscattering and spin-polarized current from spinmomentum locking, which makes them a promising material for spintronic applications [3][4][5][6][7][8][9][10][11].…”

Rectification in Spin-Orbit Materials Using Low-Energy-Barrier Magnets

“…(1) has been observed on diverse spinorbit materials e.g. topological insulator (TI) [3][4][5][6], Kondo insulators [23], transition metals [7], semimetals [24], and semiconductors [8].…”

“…In this paper, we predict that the spinmomentum locking (SML) observed in spin-orbit materials when coupled to a nano-magnet with low-energy barrier, will rectify the channel current in the form of a voltage in a multi-terminal structure. We start our arguments with the spin-potentiometric measurements well-established in diverse classes of spin-orbit materials (see, for example, [3][4][5][6][7][8]) where a high-energy-barrier stable ferromagnet (FM) is used to measure the charge current induced spin potential in the SML channel. We show that such spin-potential measurement on a metallic SML channel using a low-energy barrier magnet (LBM) will result in a rectified voltage, even for arbitrarily small channel current.…”