Anisotropic spin transport in GaAs quantum wells in the presence of competing Dresselhaus and Rashba spin-orbit coupling

Aiming at the optimization of the spin-diffusion length in (001) GaAs quantum wells, we explore the effect of the anisotropy of the spin-orbit coupling on the competition between the Rashba and the Dresselhaus spin-orbit couplings by solving the kinetic spin Bloch equations with the electron-phonon and the electron-electron scattering explicitly included. For identical strengths of the Rashba and the Dresselhaus spin-orbit couplings, the spin-diffusion length shows strong anisotropy not only for the spin-polarization direction but also for the spin-diffusion direction. Two special directions are used seeking for the large diffusion length: (110) and (110). Without the cubic term of the Dresselhaus spin-orbit coupling and with the identical Dresselhaus and Rashba strengths, infinite diffusion lengths can be obtained either for the spin-diffusion/injection direction along (110), regardless of the direction of spin polarization, or for the spin polarization along (110), regardless of the direction of the spin diffusion/injection. However, the cubic Dresselhaus term cannot be neglected, resulting in a finite spin-diffusion length which decreases with the temperature and the electron density. The anisotropy for the spin-diffusion direction and spin-polarization direction is maintained. For the spin-diffusion/injection direction along (110), the spin-diffusion length increases first with the increase of the Rashba strength (from 0) which can be tuned by the external gate voltage; when the Rashba strength is slightly smaller than (instead of equal to) the Dresselhaus strength, the diffusion length reaches its maximum, followed by a decrease with further increase of the Rashba strength.