Spin-induced charge correlations in transport through interacting quantum dots with ferromagnetic leads

We study the full counting statistics of electronic transport through a single-level quantum dot weakly coupled to two leads, with either one or both of them being ferromagnetic. The interplay of Coulomb interaction and finite spin polarization implies spin-correlation induced charge correlations that give rise to super-Poissonian transport behavior and positive cross correlations of the currents of the two spin species. In the case of two ferromagnetic leads, we analyze the nontrivial dependence of the cumulants on the angle between the polarization directions of the leads. We find diverging second and higher cumulants for spin polarizations approaching unity.

Figure 1

(Color online) A quantum-dot spin valve consists of a QD connected to two ferromagnetic leads (F-D-F), whose polarizations ${\mathbf{p}}_r$ enclose a tunable angle $\varphi$. If the polarization of one of the leads vanishes, the structure defines a F-D-N system.

Figure 2

(Color online) Cumulants of a symmetric $\left({\Gamma }_{\text{F}}={\Gamma }_{\text{N}}=\Gamma /2\right)$ noninteracting F-D-N system.

Figure 3

(Color online) Cross correlations between the current fluctuations of the two spin species in the strongly interacting system with one ferromagnetic lead for both transport directions $\left({\Gamma }_{\text{F}}={\Gamma }_{\text{N}}=\Gamma /2\right)$.

Figure 4

(Color online) Cumulants of the N-D-F-system in which the electrons are injected from the normal electrode. The total current [solid (black)] splits up into two equal spin currents [dashed (green) and dotted (red)] and the noise is enhanced due to bunching. For higher cumulants the enhancement increases $\left({\Gamma }_{\text{F}}={\Gamma }_{\text{N}}=\Gamma /2\right)$.

Figure 5

Cumulants of the symmetric $\left(p_L=p_R=p,{\Gamma }_L={\Gamma }_R=\Gamma /2\right)$ noninteracting quantum-dot spin valve as a function of the angle $\varphi$. For $\varphi =0$ the statistics of noninteracting particles in a double barrier system is assumed while for $\varphi =\pi$ transport is Poissonian due to spin blockade.

Figure 6

Cumulants of the strongly interacting quantum-dot spin valve with symmetric parameters: ${\Gamma }_L={\Gamma }_R=\Gamma /2$, $p_L=p_R=p$. In the parallel case $\left(\varphi =0\right)$ Coulomb blockade of minority spins leads to bunching. In the antiparallel situation $\left(\varphi =\pi \right)$ the statistics becomes Poissonian for $p\to 1$ due to spin blockade.

Figure 7

(Color online) The exchange field increases the current of a strongly interacting quantum-dot spin valve and smoothens sharp characteristics of the cumulants. Parameters: $p_L=p_R=0.8$, ${\Gamma }_L={\Gamma }_R=\Gamma /2$, ${\mu }_{L/R}=\pm 15k_{B}T$, and $\epsilon =0$.