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Spin diffusion in ultracold spin-orbit coupled $^{40}$K gas

We investigate the steady-state spin diffusion for ultracold spin-orbit coupled $^{40}$K gas by the kinetic spin Bloch equation approach both analytically and numerically. Four configurations, i.e., the spin diffusions along two specific directions with the spin polarization perpendicular (transverse configuration) and parallel (longitudinal configuration) to the effective Zeeman field are studied. It is found that the behaviors of the steady-state spin diffusion for the four configurations are very different, which are determined by three characteristic lengths: the mean free path $l_τ$, the Zeeman oscillation length $l_Ω$ and the spin-orbit coupling oscillation length $l_α$. It is analytically revealed and numerically confirmed that by tuning the scattering strength, the system can be divided into {\it five} regimes: I, weak scattering regime ($l_τ\gtrsim l_Ω, l_α$); II, Zeeman field-dominated moderate scattering regime ($l_Ω\ll l_τ\ll l_α$); III, spin-orbit coupling-dominated moderate scattering regime ($l_α\ll l_τ\ll l_Ω$); IV, relatively strong scattering regime ($l_τ^c\ll l_τ\ll l_Ω, l_α$); V, strong scattering regime ($l_τ\ll l_Ω, l_α,l_τ^c$), with $l_τ^c$ representing the crossover length between the relatively strong and strong scattering regimes. In different regimes, the behaviors of the spacial evolution of the steady-state spin polarization are very rich, showing different dependencies on the scattering strength, Zeeman field and spin-orbit coupling strength. The rich behaviors of the spin diffusions in different regimes are hard to be understood in the framework of the simple drift-diffusion model or the direct inhomogeneous broadening picture in the literature. ...