Nuclear-Spin-Induced Oscillatory Current in Spin-Blockaded Quantum Dots

We show experimentally that electron transport through GaAs-based double quantum dots can be affected by ambient nuclear spin states in a certain regime where transport is blocked in the absence of electron spin flip. Current through the dots oscillates in time with a period up to 200 s depending on magnetic field. Oscillation is quenched by application of a continuous wave ac magnetic field which can induce nuclear magnetic resonance in ${}^{71}\mathrm{Ga}$ or ${}^{69}\mathrm{Ga}$. A possible mechanism for dynamically polarizing the nuclear spins is proposed.

Figure 1

(a) Accessible electric configuration A–D in spin-blockade region. (b) Current (I) voltage ($V_S$) characteristic measured at $T=1.8\mathrm{K}$, zero magnetic field. Black and gray lines at $V_G$’s are indicated by black and gray arrows in the right inset. In the spin blockade region for $2<V_S<6\mathrm{mV}$ the system is in a spin triplet state B. Left inset: schematic of vertical double dot devices. Directions of dc and ac magnetic fields are indicated. Note that, for the black line, for small $V_S$, transitions between A–D become reversible, hence the system does not show spin blockade. For the gray line, the two-electron Coulomb blockade is present for $|V_S|<1\mathrm{mV}$, and spin blockade for $V_S>1\mathrm{mV}$.

Figure 2

(a) Magnetic field ($B_{\mathrm{dc}}$) dependence of the leakage current at $V_S=3.0\mathrm{mV}$, in the middle of the spin-blockade region, as a function of in-plane magnetic field for sweep up (black) and sweep down (gray). Detailed positions of the step and largest fluctuations depend on the $B_{\mathrm{dc}}$ sweep rate and values of $V_S$ and $V_G$. (b) Leakage current evolving with time measured for fixed magnetic fields of $B_{\mathrm{dc}}=0.70$ to 0.85 T with 0.01 T step for the curves from bottom to top. Each curve is vertically offset by 0.5 pA for clarity. (c) Transient behavior of the oscillatory current. $V_G$ set at the gray arrow in Fig. 1b, right inset. $V_S$ switched from 3.0 to $-1\mathrm{mV}$, where the Coulomb blockade is almost lifted and a small current of $\sim -1\mathrm{pA}$ flows. After dwelling for 300, 150, 75, 36, and 18 s outside the SB region we switch $V_S$ back to 3.0 mV. The peak spacing $\Delta t$ (right axis) is plotted.

Figure 3

(a) Current oscillations under ac magnetic field in the frequency range of $f=11.00$ to 11.10 MHz with 0.01 MHz step for the curves from bottom to top. Each curve is vertically offset by 0.75 pA for clarity. Amplitude of the ac voltage applied to the coil is adjusted so that the effects of neither the heating nor “pumping current” due to the stray electrical coupling to the coil are reasonably small. (b) Time period of the current oscillations measured for various frequencies of the ac magnetic field ($f$). Inset: dc magnetic field dependence of the resonance frequency observed in the oscillation period.