Voltage-controlled nuclear polarization switching in a single ${\text{In}}_x{\text{Ga}}_{1-x}\text{As}$ quantum dot

Sharp thresholdlike transitions between two stable nuclear-spin polarizations are observed in optically pumped individual ${\text{In}}_x{\text{Ga}}_{1-x}\text{As}$ self-assembled quantum dots embedded in a Schottky diode when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the Overhauser field in the dot by up to 3 T. The bias-dependent photoluminescence measurements reveal the importance of the electron-tunneling-assisted nuclear-spin pumping. We also find evidence for the resonant LO-phonon-mediated electron cotunneling, the effect controlled by the applied bias and leading to the reduction of the nuclear-spin pumping rate.

Figure 1

(Color online) (a) Bias dependence of the QD PL recorded at $B_z=1.5\text{T}$ under ${\sigma }^{-}$ excitation. (b) Power dependence of the $X^{+}$ Zeeman splitting measured at $B_z=2\text{T}$ and bias $-0.45\text{V}$ for ${\sigma }^{-}$ polarized excitation. $P_{\text{thr}}$ denotes the power where the nuclear-spin ”switch” is observed in the scan where the power is increased. Arrows at the switching thresholds indicate the directions of the power scans.

Figure 2

(Color online) (a) Voltage scans at $B_z=2.1\text{T}$ and power 0.4 mW under ${\sigma }^{-}$ excitation. $V_{\text{thr}1}$ $\left(V_{\text{thr}2}\right)$ denotes the bias where the nuclear-spin switching occurs when reducing (increasing) the bias. Directions of the bias scans are shown with arrows. (b) The bias dependence of the switching threshold power $P_{\text{thr}}$. [(c) and (d)] Bias-dependence of the integrated PL intensity measured in both circular polarizations at $B_z=2.1\text{T}$ under ${\sigma }^{-}$ excitation with powers (c) 0.2 mW and (d) 0.5 mW for $X^0$ (electron and hole pairs or $eh$ state), $X{X}^0$ $\left(eehh\right)$, $X^{+}$ $\left(ehh\right)$, and $X^{-}$ $\left(eeh\right)$ peaks. Horizontal dashed lines show the level of noise in PL, below which the line intensities cannot be measured.