Date Published: April 14, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Qisheng Wang, Jie Li, Jean Besbas, Chuang‐Han Hsu, Kaiming Cai, Li Yang, Shuai Cheng, Yang Wu, Wenfeng Zhang, Kaiyou Wang, Tay‐Rong Chang, Hsin Lin, Haixin Chang, Hyunsoo Yang.
The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin‐polarized states and can carry a very large current density. In addition, the intrinsic non‐centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry‐controlled spin–orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room‐temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time‐resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long‐lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron–hole recombination rate, and suppression of backscattering required by time‐reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out‐of‐plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long‐lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.
Synthesis and Characterization: The few‐layer WTe2 and MoTe2 thin films were synthesized in a three‐zone CVD system (Figure S5, Supporting Information).For WTe2 thin film (sample 1) as a representative example, 0.3 g tungsten chloride (WCl6) powders (99.99%, Alfar Aesar) and 0.4 g tellurium (Te) powder (99.99%, Alfar Aesar) were placed at the first and second zone, respectively. Silicon (Si) substrates with 300 nm silicon dioxide (SiO2) were placed at the third zone. The furnace flowed by a mixture of 160 sccm N2 and 40 sccm H2 under ambient condition was heated to 500 °C for 20 min. Synthesis details of sample 2 is provided in Note S1 in the Supporting Information. The CVD furnace was cooled down to room temperature naturally after the growth. To prevent the samples from oxidation, all thin films were protected by a vacuum‐evaporated 1 nm Al capping which was oxidized to Al2O3 naturally in air. The samples were characterized by the HR800 Raman system (JY Horiba), AFM (SPM9700, Shimadzu Co.), XPS (AXIS‐ULTRA DLD system with an Mg KαX‐ray source), reflective OM (MV6100, Jiangnan Yongxin Co.), XRD (Empyrean, PANalytical B.V.), and TEM (Tecnai G2 F30, FEI Co.).
The authors declare no conflict of interest.