Correlations of spin currents through a quantum dot induced by the Kondo effect

We study correlations of spin currents flowing through a Coulomb blockaded quantum dot. While they vanish for generic parameter values in the regime of elastic cotunneling, these correlations develop as the quantum dot enters the Kondo regime. We analyze this effect for a small effective Kondo coupling such that fluctuations of the spin of the quantum dot can be captured by the perturbative renormalization group. Even in this regime we find that—unlike the mean current—the spin current correlator is not accessible by perturbation theory. It is nonperturbative in the effective coupling of the QD to the leads.

Figure 1

Illustration of the mechanism that introduces spin-current correlations in the Kondo regime. Only the relevant spin-flip scattering processes are depicted. The spin of the QD is flipped by scattering electrons and changes from one scattering time $t$ to the following $t+\Delta t$. Consequently, the outgoing electrons carry alternating spin.

Figure 2

Voltage dependence of the Fano factor $F_{\uparrow \downarrow }$ for $T=eV∕30$, ${ϵ}_d=1\mathrm{meV}$, and $T_K={10}^{-7}{ϵ}_d$ (solid line, ${\gamma }_S=0$; dashed line, ${\gamma }_S={10}^5{\mathrm{s}}^{-1}$). $F_{\uparrow \downarrow }$ develops in parallel with Kondo correlations upon lowering the voltage/temperature.