Electrical Detection of Spin-Polarized Surface States Conduction in (Bi_0.53Sb_0.47)₂Te₃ Topological Insulator

Abstract

Strong spin−orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/ detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al 2 O 3 ferromagnetic tunneling contact in which the compound topological insulator (Bi 0.53 Sb 0.47 ) 2 Te 3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi 0.53 Sb 0.47 ) 2 Te 3 . Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators. KEYWORDS: Topological insulator, spin polarization, surface states, spin-momentum locking, spin detection S ince the discovery of two-dimensional (2D) and threedimensional (3D) topological insulators (TIs), 1−5 they have attracted extensive research interest for their exotic physical properties that could lead to dissipationless transport in the quantum spin Hall state. 6−9 Recent studies have shown a giant spin−orbit torque in TI originating from the strong spin− orbit interaction, 10,11 which enabled the current-induced magnetization switching through spin-transfer torque with a low current density. The unique feature of 3D TI, for instance, is that it has both insulating bulk and gapless Dirac surface states. 8,9 Ternary TI compounds, such as (Bi x Sb 1−x ) 2 Te 3 , have been widely investigated for their tunability to achieve low bulk carrier density and manifest topological surface states conduction. 12,13 The presence of surface states is supported by extensive angle-resolved photoemission spectroscopy (ARPES) measurements and transport studies, 14−20 such as Shubnikov-de Haas (SdH) and Aharonov Bohm (AB) quantum oscillations. Because of the strong spin−orbital interaction in TI, direct back scatterings from nonmagnetic impurities are prohibited by the time-reversal symmetry. 8,9 More importantly, the spin-momentum locking naturally leads to a currentinduced spin polarization in surface states; 21 the surface states conduction is spin-polarized once an electric current is passed through a TI film, and this spin polarization can be...

Anomalous Hall Coulomb drag of massive Dirac fermions

“…Dirac fermions in 2D are described by the Hamiltonian H D = A σ · (k ×ẑ) + M σ z , with σ = (σ x , σ y , σ z ) the usual Pauli matrices, k = (k x , k y ) the 2D wave vector, A stems from the Fermi velocity and M a generic mass term. In the limit M → 0 the quasi-particle dispersion is linear, a feature that has aroused intense interest experimentally [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and theoretically [22][23][24][25][26][27][28][29][30][31][32][33][34]. These studies have illuminated the considerable potential of Dirac fermions for spintronics, thermoelectricity, magnetoelectronics and topological quantum computing [35].…”

Detection of current induced spin polarization in epitaxial Bi2Te3 thin film

“…This current induced spin polarization of the SS, controllable by the magnitude and the direction of the current, can be used to torque a ferromagnet (FM) [4,5]. In recent experiments [7][8][9][10][11][12][13][14][15][16][17][18][19], the spin accumulation on the surface of 3D TIs Bi 2 Se 3 , (Bi x Sb 1−x ) 2 Te 3 , Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 , BiSbTeSe 2 , Bi 2 Te 2 Se and Sb 2 Te 3 , mostly grown by molecular beam epitaxy (MBE) or exfoliated, were electrically measured by the voltage probed with FM contact, where the voltage depends on the projection of SS spin polarization onto the FM magnetization direction.In this work, we detect the current induced spin polarization on the surface of an MBE grown Bi 2 Te 3 thin film using Fe contact deposited on the surface and separated by a thin MgO barrier. We also provide a theoretical estimate of the detected spin signal, i.e., the voltage probed with the FM contact.…”

Work function of bulk-insulating topological insulator Bi2–xSbxTe3–ySey

“…Conversely, the injection of spins into the Dirac-cone state induces electromotive force to drive charge currents. Such a spin-electricity conversion has been recently investigated by spin-transfer torque [18][19][20] , spin pumping [21][22][23] , and allelectrical measurements of the charge-current-induced spin polarization [24][25][26][27][28][29][30][31] .While such studies have opened a pathway toward application of TIs in spintronics, the performance of devices so far achieved is not as high as one would expect from theory 21 . This is largely related to the TI-metal contact necessary for operating the electrical circuits and devices; for example, electrodes are always required for electrical measurements, and ferromagnetic metals are used as a spin detector in the measurements of current-induced spin polarizations [24][25][26][27][28][29][30][31] .…”

“…This is largely related to the TI-metal contact necessary for operating the electrical circuits and devices; for example, electrodes are always required for electrical measurements, and ferromagnetic metals are used as a spin detector in the measurements of current-induced spin polarizations [24][25][26][27][28][29][30][31] . It is thus essential to well characterize the TI-metal interface to integrate bulk insulating TIs into efficient devices.…”

“…Conversely, the injection of spins into the Dirac-cone state induces electromotive force to drive charge currents. Such a spin-electricity conversion has been recently investigated by spin-transfer torque [18][19][20] , spin pumping [21][22][23] , and allelectrical measurements of the charge-current-induced spin polarization [24][25][26][27][28][29][30][31] .…”