Hole spin relaxation in bilayerWSe2

Abstract

We investigate the hole spin relaxation due to the Rashba spin-orbit coupling induced by an external perpendicular electric field in bilayer WSe2. The Rashba spin-orbit coupling coefficients in bilayer WSe2 are constructed from the corresponding monolayer ones. In contrast to monolayer WSe2, the out-of-plane component of the bilayer Rashba spin-orbit coupling acts as a Zeemanlike field with opposite directions but identical values in the two valleys. For in-plane spins, this Zeeman-like field, together with the intervalley hole-phonon scattering, opens an intervalley spin relaxation channel, which is found to dominate the in-plane spin relaxation in bilayer WSe2 even at low temperature. For out-of-plane spins, this Zeeman-like field is superimposed by the identical Hartree-Fock effective magnetic fields in the two valleys, and hence different total effective magnetic fields between two valleys are obtained. Owing to the large difference of the total fields at large spin polarization, different out-of-plane spin relaxation times in the two valleys are obtained when the intervalley hole-phonon scattering is weak at low temperature and low hole density. This difference in the spin relaxation times can be suppressed by enhancing the intervalley hole-phonon scattering through increasing temperature or hole density. Moreover, at large spin polarization and low temperature, due to the weak intravalley hole-phonon scattering but relatively strong hole-hole Coulomb scattering, the fast spin precessions are found to result in a quasi hot-hole Fermi distribution characterized by an effective hot-hole temperature larger than the temperature, which also enhances the intervalley scattering. During this process, it is interesting to discover that the initially equal hole densities in the two valleys are broken in the temporal evolution, and a valley polarization is built up. It is further revealed that this comes from the different spin relaxation processes at large spin polarization and different spin-conserving intervalley scattering rates between spin-up and -down holes due to the different effective hot-hole temperatures.

Spin diffusion inp-type bilayerWSe2

“…Furthermore, in the presence of the spin spatial precessions, the quasi hot-hole Fermi distributions with different effective hot-hole temperatures between spin-up and -down holes are expected at large spin injection and low temperature. Hence, similar to the induced valley polarization in the time domain as mentioned above, 36 one may also expect a steady-state valley polarization in the spatial domain. In the present work, by the kinetic spin Bloch equation (KSBE) approach, 43 we investigate the steady-state outof-plane spin diffusion in p-type BL WSe 2 with all the relevant scatterings included.…”

“…The impurity density is taken to be N i = 0.02N h according to Ref. 36. The spin diffusion lengths as function of temperature at different hole densities are plotted in Figs.…”

“…This indicates that the above mentioned suppression on the hole spin relaxation in ML TMDs is absent in BL TMDs. In the presence of an external out-of-plane electric field E z , the experimentally realized Rashba SOC in BL TMDs can be written as 9,10,36 Ω µ R (k) = − νk y , νk x , µη E z ,…”

“…[16][17][18][19][20][21][22][23][24][25][26][27][28][29] To realize the spintronic device, a great deal of efforts have been devoted to the study on the carrier spin dynamics in this material, including the spin relaxation [30][31][32][33][34][35][36][37][38] and spin diffusion.…”

“…All the material parameters used in our calculation are given in Ref. 36. The Fermi energy in the calculation is chosen to be larger than the effective Rashba SOC energy.…”