Exchange-Induced Spin Blockade in a Two-Electron Double Quantum Dot

We have experimentally identified the exchange-induced spin blockade in a GaAs double quantum dot. The transport is suppressed only when the eigenstates are well-defined singlet and triplet states, and thus sensitive to dynamic nuclear-spin polarization that causes singlet-triplet mixing. This gives rise to unusual current spectra, such as a sharp current dip and an asymmetric current profile near the triplet resonance of a double quantum dot. Numerical simulations suggest that the current dip is a signature of identical nuclear-spin polarization in the two dots, which is attractive for coherent spin manipulations in a material with nuclear spins.

Figure 1

(a) An energy diagram for the EXSB, which is attributed to the large exchange energy $J_1$. The transport is blocked by $(1,1)S$ if the relaxation to $(0,2)S$ can be neglected. (b) A scanning electron micrograph with a false color and the measurement setup. A quantum point contact (PC) charge detector was used to identify the electron numbers $(n_L,n_R)$ in the two dots. (c) Current $I_D$ as a function of the two gate voltages $V_{\mathrm{PL}}$ and $V_{\mathrm{PR}}$, at $V_D=800\mu \mathrm{V}$, $B=200\mathrm{mT}$, and $V_C=-0.66\mathrm{V}+\mathrm{\Delta }{V}_{\mathrm{C}}$ with $\mathrm{\Delta }{V}_C=10\mathrm{mV}$. The charge stability diagram is attached. A PSB region is seen in the two triangular conductive regions marked by E (an electronlike transport) and H (a holelike one) [2]. A sharp current dip in a peak marked by ES is the signature of the EXSB found in the study. (d) Line traces of $I_D$ as a function of $\mathrm{\Delta }{V}_{\mathrm{PL}}$ around the ES resonance. A very sharp dip at $B=100\mathrm{mT}$ is magnified in the inset.

Figure 2

(a) $B$ dependence and (b) $\mathrm{\Delta }{V}_{\mathrm{\Gamma }}$ dependence of current traces $I_D$. The parameter $\mathrm{\Delta }{V}_{\mathrm{\Gamma }}$ controls $V_L\equiv \mathrm{\Delta }{V}_{\mathrm{\Gamma }}-0.46\mathrm{V}$ and $V_R\equiv \mathrm{\Delta }{V}_{\mathrm{\Gamma }}-0.45\mathrm{V}$.

Figure 3

(a) An energy diagram of the system at $\mathrm{\Delta }p=0$. (b),(e) Energy diagrams, (c),(f) current, and (d),(g) polarization rate $\mathrm{\Delta }\dot{p}$ at (b),(c),(d) $ϵ=-50\mu \mathrm{eV}$ and (e),(f),(g) $ϵ=+50\mu \mathrm{eV}$. Dashed and solid lines in (d) and (g) are obtained for a symmetric ($N_L=N_R$) and asymmetric ($N_L=2N_R$) DQD, respectively. SFPs and UFPs are marked by solid and open circles, respectively. $t_s=1\mu \mathrm{eV}$, $t_p=10\mu \mathrm{eV}$, ${\mathrm{\Delta }}_{\mathrm{ST}}=400\mu \mathrm{eV}$, $J_1=400\mu \mathrm{eV}$, $E_Z=2.5\mu \mathrm{eV}$, $\hslash {\mathrm{\Gamma }}_L=3\mu \mathrm{eV}$, $\hslash {\mathrm{\Gamma }}_R=30\mu \mathrm{eV}$, $N_R={10}^6$, and ${\gamma }_{\mathrm{nuc}}=0.01{\mathrm{s}}^{-1}$.

Figure 4

(a) The calculated stable fixed point $\mathrm{\Delta }{p}^{(\mathrm{SFP})}$ and (b) corresponding current level $I_D^{(\mathrm{SFP})}$ reached from $\mathrm{\Delta }p=0$ for $N_L=2N_R$. (c) The current profile $I_D^{(\mathrm{SFP})}$ for various $\hslash {\mathrm{\Gamma }}_R$ and $\hslash {\mathrm{\Gamma }}_L=0.1\hslash {\mathrm{\Gamma }}_R$. Other parameters are the same as those for Fig. 3. Rich current spectra in Fig. 2 are reproduced.