Selective dynamic nuclear spin polarization in a spin-blocked double dot

We study the mechanism of dynamical nuclear spin polarization by hyperfine interaction in a spin-blocked double quantum dot system. We incorporate the effects of the nuclear quadrupole coupling due to the doping-induced local lattice distortion and strain, and propose a new spin depolarization channel due to quadrupolar state mixing and hyperfine coupling. We calculate the hyperfine transition rates and solve the master equations for the nuclear spins. Our results show that nuclear quadrupole coupling induced by the 5% indium doping can lead to significant nuclear spin relaxation, and thus can be used to explain the recent experimental observation of missing arsenic NMR signal in the spin-blocked double dots.

Figure 1

Schematic of hyperfine induced transitions among the energy eigenstates of Hamiltonian $H_n$. The downward arrows represent simultaneous flip-flop scattering of nuclei and the electron in a triplet state. $F$ and $G$ are the nuclear spin relaxation rates induced by the effective hyperfine magnetic field of the electron $\left((A∕N)I_z{S}_z\right)$.

Figure 2

Ratio of Ga and As nuclei polarization as a function of the relative quadrupole interaction strength $\alpha (={\nu }_{\mathrm{Ga}}∕{\nu }_{\mathrm{As}})$ for $\Gamma =10\mu \mathrm{eV}$ (a) and $\Gamma =30\mu \mathrm{eV}$ (b). $\Gamma$ is the width of level broadening due to electron tunneling between the lead and the dot. The three curves in each graph represent three different As quadrupole coupling strengths ${\nu }_{\mathrm{As}}∕{\omega }_{\mathrm{As}}=0.1$, 0.3, and 0.5. $\theta$ has chosen to be 45° for both As and Ga nuclei.

Figure 3

Ga nuclear spin polarization as a function of the relative quadrupole interaction strength $\alpha$ for $\Gamma =10\mu \mathrm{eV}$ (a) and $\Gamma =30\mu \mathrm{eV}$ (b). All the parameters are the same as those used in Fig. 2.