Picosecond-long spin current generation along the low-symmetry axis of WTe₂

Spin current generation and detection underlie modern spintronics. In particular, the spin current with an out-of-plane spin polarization can efficiently manipulate magnetic devices with perpendicular magnetic anisotropy. Weyl semimetal WTe₂ is of great potential in generating spin currents with an out-of-plane spin polarization due to intrinsic inversion symmetry breaking. Here we use an optical-driven terahertz (THz) emission spectroscopy and show a picosecond-long spin current generation along the rotational asymmetry a-axis of WTe₂ with the out-of-plane spin orientation. The underlying mechanism of the THz emission is the circular photogalvanic effect with a circularly polarized laser excitation, which is incident to the sample surface normal. The inversion symmetry breaking induces a spin-splitting band, enabling the manipulation of the spin current direction with opposite spin orientations depending on the helicity of the laser excitation. The temperature dependent study reveals that the CPGE vanishes at 175 K due to the carrier compensation. Our finding shows a great potential for the future development of THz spintronic devices with Weyl semimetals.