Room-temperature long-lived spin polarization in Weyl semimetal thin film

The Weyl semimetals WTe₂ and MoTe₂ show a 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 noncentrosymmetry 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. In this work, we show a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in CVD-grown WTe₂ (MoTe₂) thin films using the time-resolved Kerr rotation (TRKR) . Based on ab initio calculation, we identify a mechanism of long-lived spin polarization resulting from a large spin splitting (~40 meV) around the bottom of the conduction band, low electron-hole recombination rate and suppression of backscattering required by time-reversal and lattice symmetry operation. In addition, we find the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. Our 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.