Universal Scaling of the Spin Hall Effect

We study the spin Hall (SH) effect for the Dirac electrons in terms of the spin and magnetic-moment accumulation coefficients $-\Gamma^{00} g_{s(m)z}^{\phantom{s(m)z} xy}$. We take short-range nonmagnetic impurities into account within the self-consistent $T$-matrix approximation. Similarly to the universal scaling for the anomalous Hall (AH) effect, we find three disctinct regimes by changing the electric conductivity $\sigma^{yy}$; the superclean regime with $-\Gamma^{00} g_{s(m)z}^{\phantom{s(m)z} xy} \propto \sigma^{yy}$ owing to the skew scattering, moderately dirty regime with almost constant $-\Gamma^{00} g_{s(m)z}^{\phantom{s(m)z} xy}$, and dirty regime with a new scaling relation $-\Gamma^{00} g_{s(m)z}^{\phantom{s(m)z} xy} \propto (\sigma^{yy})^{0.6}$ whose exponent differs from that of the AH conductivity. Our results construct a unified theory of the SH effect without any ambiguity of spin current.