Enhancement of Spin-Charge Conversion in Dilute Magnetic Alloys by Kondo Screening

We derive a kinetic theory capable of dealing both with large spin-orbit coupling and Kondo screening in dilute magnetic alloys. We obtain the collision integral nonperturbatively and uncover a contribution proportional to the momentum derivative of the impurity scattering $S$ matrix. The latter yields an important correction to the spin diffusion and spin-charge conversion coefficients, and fully captures the so-called side-jump process without resorting to the Born approximation (which fails for resonant scattering), or to otherwise heuristic derivations. We apply our kinetic theory to a quantum impurity model with strong spin-orbit, which captures the most important features of Kondo-screened Cerium impurities in alloys such as ${\text{Ce}}_x{\mathrm{La}}_{1-x}{\mathrm{Cu}}_6$. We find (1) a large zero-temperature spin-Hall conductivity that depends solely on the Fermi wave number and (2) a transverse spin diffusion mechanism that modifies the standard Fick’s diffusion law. Our predictions can be readily verified by standard spin-transport measurements in metal alloys with Kondo impurities.

Figure 1

Sketch of the minimal quantum impurity model to which we have applied our kinetic theory. The impurity contains a single electron in an $l=1$ orbital that, by virtue of strong spin-orbit coupling, splits into a $j=1/2$ doublet and a $j=3/2$ quartet. Strong electron correlation leads to the formation of a local moment. Kondo screening of the latter by the conduction electrons induces a scattering phase shift ${\eta }_1=\pi /2$ at the Fermi energy. See the Supplemental Material [35] for a detailed explanation of how this model captures some essential features of Ce impurities in alloys like ${\mathrm{Ce}}_x{\mathrm{La}}_{1-x}{\mathrm{Cu}}_6$ for $x<0.7$.