Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads: A Numerical Renormalization Group Analysis

We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes.

Figure 1

Left: Local DOS of the QD for the symmetric Anderson model. (a) $A_{\uparrow }(\omega )$, (b) $A_{\downarrow }(\omega )$, and (c) $A(\omega )$ for ${\epsilon }_d=-U/2=-0.1D$ ($D$ is the bandwidth). Right: DOS for the asymmetric case. (d) $A_{\uparrow }(\omega )$, (e) $A_{\downarrow }(\omega )$, and (f) $A(\omega )$ for ${\epsilon }_d=-0.1D$ and $U=0.45D$. In all cases ${\Gamma }_0=0.02D$.

Figure 2

(a) Splitting $\delta$ of the Kondo peak as a function of ${\epsilon }_d$ for $p=0.25$ and $U=0.4D$. (b) $\delta$ versus $p$ for ${\epsilon }_d=-0.1D$ and $U=0.45D$. (c) TMR versus $p$ for ${\epsilon }_d=-0.1D$ and $U=0.4D$. (d) TMR as a function of ${\epsilon }_d$ for $p=0.25$ and $U=0.4D$. In all cases ${\Gamma }_0=0.02D$.

Figure 3

The effects of spin-flip processes $R$ for the symmetric (left panels with ${\epsilon }_d=-U/2=-0.1D$) and asymmetric (right panels with ${\epsilon }_d=-0.1D$ and $U=0.4D$) Anderson model. Local DOS for (a),(d) $p=0$ and (b),(e) $p=0.25$. (c),(f) TMR versus $R$ for $p=0.25$. In all cases ${\Gamma }_0=0.02D$.