Pauli Spin Blockade and the Ultrasmall Magnetic Field Effect

Based on the spin-blockade model for organic magnetoresistance, we present an analytic expression for the polaron-bipolaron transition rate, taking into account the effective nuclear fields on the two sites. We reveal the physics behind the qualitatively different magnetoconductance line shapes observed in experiment, as well as the ultrasmall magnetic field effect (USFE). Since our findings agree in detail with recent experiments, they also indirectly provide support for the spin-blockade interpretation of organic magnetoresistance. In addition, we predict the existence of a similar USFE in semiconductor double quantum dots tuned to the spin-blockade regime.

Figure 1

When a site already contains a polaron (the right site in the pictures), charge transport through this site relies on the formation of a bipolaron (thick blue arrows). This bipolaron must be a spin singlet, which leads to spin blockade.

Figure 2

(a) The current given by Eq. (3) as a function of $x=B_s/B_a$ for fixed $B_a$ and $\varphi$, evaluated for different parameters $a$. (b),(c) The averaged MC. (b) The blue trace represents ${\Gamma }_s/K\ll 1$. The field $B_{\mathrm{ext}}$ is plotted in units of $K=⟨K_{L,R}^2{⟩}^{1/2}$. The peak of this curve is flat [21]. The green trace represents ${\Gamma }_s/K=3/2$. (c) ${\Gamma }_s/K=50$. Now, $B_{\mathrm{ext}}$ is plotted in units of ${\Gamma }_s$. Inset: The range where $B_{\mathrm{ext}}\sim K$.

Figure 3

When $B_{\mathrm{ext}}$ is swept for a given realization of ${\mathbf{K}}_{L,R}$, the field ${\mathbf{B}}_s$ (blue arrow) follows a trace like the dashed blue line. We indicated with green arrows ${\mathbf{K}}_s$ as well as $K_s^{\perp }$, which equals the minimum value of $B_s$.