Zero-bias anomaly in cotunneling transport through quantum-dot spin valves

We predict a zero-bias anomaly in the differential conductance through a quantum dot coupled to two ferromagnetic leads with antiparallel magnetization. The anomaly differs in origin and properties from other anomalies in transport through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade valleys with an unpaired dot electron. It is a consequence of the interplay of single- and double-barrier cotunneling processes and their effect on the spin accumulation in the dot. The anomaly becomes significantly modified when a magnetic field is applied.

Figure 1

(a) Differential conductance in the parallel and antiparallel configurations as a function of bias voltage for different values of spin polarization $p$ at $\epsilon =-U∕2$, $U=30\Gamma$, $k_{B}T=0.5\Gamma$, and (b) for different temperatures and $p=0.5$.

Figure 2

Single-barrier (a) and double-barrier (b) cotunneling processes, and the occupation probabilities for spin-up and spin-down electrons in the antiparallel configuration (c). The parameters are $k_{B}T=0.5\Gamma$, $U=30\Gamma$, $\epsilon =-U∕2$, and $p=0.5$.

Figure 3

(Color online) Differential conductance in presence of a Zeeman splitting with $p=0.5$, $k_{B}T=0.2\Gamma$, and $U=40\Gamma$ for the symmetric (a),(b) and asymmetric (c),(d) Anderson model with parallel (a),(c),(d) and antiparallel (b) relative magnetization of the leads. In (c), the Zeeman splitting favors a dot polarization parallel, in (d) antiparallel to the leads.