Spin‐Momentum Locking in the Gate Tunable Topological Insulator BiSbTeSe₂ in Non‐Local Transport Measurements

Abstract

The helical spin‐momentum locking of an electron in a topological surface state is a feature excellently suited for the use in spintronic applications. Devices are fabricated that allow to generate, transport, and detect the spin‐polarization coming from an electronic current in the topological surface state of BiSbTeSe2; a topological insulator reported to have a negligible bulk contribution to its conduction. The successful creation of such a device is described, as is a study of the generated spin‐polarized current as the BiSbTeSe2 surface state is gated through its Dirac point. A non‐local voltage difference across separated ferromagnetic leads is observed, larger than previously reported in literature. The spin‐polarization has a maximum when the Fermi level crosses the Dirac point.

Gate-tunable spin-galvanic effect in graphene-topological insulator van der Waals heterostructures at room temperature

“…This allows to take advantage of their topologically protected Dirac surface states with a distinct spin-momentum locking (SML) feature which gives rise to a spontaneous electron spin polarization by application of an electric field. Recently, electric detection of SML states [15][16][17][18][19] and large charge-spin conversion with an efficient SOT [20][21][22] have been demonstrated up to room temperature using TIs in contact with ferromagnetic materials. In particular, stronger charge-spin conversion efficiency of highly-conducting p-type TIs has been revealed 1,23 .…”

Unconventional Charge–Spin Conversion in Weyl‐Semimetal WTe2

“…[ 18 ] Additionally, in the search for spin‐polarized current sources in topological quantum materials, various experiments have been reported on TIs. [ 19 ] However, a reliable nonlocal measurement for spin polarization in TIs and its utilization for spin injection into non‐magnetic materials are so far limited to cryogenic temperatures (below 20 K), [ 20,21 ] because of the interference from nontrivial bulk bands. [ 19 ] Therefore, finding a highly efficient spin‐polarized topological material at room temperature is indispensable for practical applications in spintronics and quantum technologies.…”

“…[ 24,28–30 ] These observations of the unconventional CSC are fundamentally different from the conventional REE, which is an interface phenomenon where the spins and current density are confined in the 2D plane [ 48,49 ] as measured in the heterostructures of metals [ 48 ] and oxides, [ 50 ] topological insulator, [ 51,52 ] Transition metal dichalcogenides (TMDCs) [ 53 ] and in graphene heterostructures with MoS 2 , [ 28 ] WS 2 , [ 23,30 ] TaS 2 , [ 25 ] MoTe 2 [ 24 ] and topological insulator (Bi 0.15 Sb 0.85 ) 2 Te 3 . [ 54 ] This unconventional charge–spin conversion phenomenon in WTe 2 , however, is shown to be useful for injection and detection of spin polarization in graphene at room temperature, avoiding problems existing in topological insulators, [ 20,21 ] and open ways for spintronic devices without the use of traditional ferromagnets. The WTe 2 based van der Waals heterostructure devices also provide the advantage that their operating temperature is not restricted by a Curie temperature ( T c ), such as recently discovered 2D ferromagnets have T c much below the room temperature.…”

“…The unconventional spin conductivities in WTe 2 are allowed by considering strain as well as the WTe 2 /graphene interface to break the crystal symmetry. The spin polarization created in WTe 2 is shown to be utilized for spin injection and detection in a graphene channel in an all‐electrical van der Waals heterostructure spintronic device at room temperature, which circumvents the problem existing in topological insulators for spin injection into graphene below 20 K. [ 20,21 ] Such unique spin‐polarized electronic states in Weyl semimetal candidates with novel spin topologies can be further tuned by tailoring their electronic band structure through enhancing their spin–orbit interaction strength, increasing the separation between the Weyl nodes through Berry curvature design, and controlling strain to break the crystal symmetry. These findings in Weyl semimetal WTe 2 for efficiently transforming the electric current into a spin polarization at room temperature is highly desirable for energy‐efficient spintronic memory and information processing technologies.…”

Charge-spin conversion in layered semimetal TaTe2 and spin injection in van der Waals heterostructures

“…Topological insulators (TIs) were shown to possess SML characteristics in the Dirac surface states, which could be detected using potentiometric measurements [19][20][21] and the charge-spin conversion process [22][23][24]. However, the spin injection from TIs into graphene is so far restricted to very low temperatures due to contributions from trivial bulk states [25,26]. Utilizing the SHE in Pt metal thin films, spin injection and detection has been realized in a graphene channel [10,11].…”