Observation of Hysteretic Transport due to Dynamic Nuclear Spin Polarization in a GaAs Lateral Double Quantum Dot

　

Takashi Kobayashi^1,2, Kenichi Hitachi¹, Satoshi Sasaki^1,2, and Koji Muraki^1
1Physical Science Laboratory, ²Tohoku University

　Electron spin dynamics in a semiconductor quantum dot (QD) is affected by the nuclear spin ensemble of the host material. This makes nuclear spin (NS) manipulation important in view of quantum information processing using physical qubits based on electron spins in QDs. Recently, we have observed a hysteretic transport signaling dynamic nuclear spin polarization (DNP) in a GaAs lateral double QD (DQD) regarded as a promising candidate for the physical implementation of the spin qubit because of its high controllability [1]. Magneticfield dependence of this signal suggests strongly unbalanced DNP between two dots, which is applicable to a rapid manipulation of a spin qubit.
　Figure 1 shows the DQD sample [(a)] and the dot current I_dot measured in the few-electron regime [(b)]. I_dot is clearly suppressed in the trapezoid region owing to the spin blockade (SB), where transport is blocked whenever parallel-spin electrons singly occupy both two dots.
　In the SB region, electron spin relaxation results in leak current reflecting the spin relaxation mechanism [2]. To investigate this mechanism, we measured I_dot as a function of ε defined by the arrow in Fig. 1(b) and in-plane magnetic field B_||. While the I_dot spectrum obtained by ε sweeps at each value of B_|| [Fig. 2(a)] does not show any feature at ε < 0, clear I_dot enhancement appears over a wide range of ε and B_|| in that obtained by up sweeps of B_|| at each value of ε [Fig. 2(b)]. Figure 2(c), which displays I_dot measured by round sweeps of B_||, reveals that this I_dot enhancement is hysteretic with respect to the sweep direction of B_||. This result indicates DNP occurring for up sweeps of B_||, similarly to previous reports on DQDs of different materials and/or shapes [3]. Furthermore, the gradually escalating current level of this DNP signal with B_|| suggests strongly unbalanced DNP between the two dots, where NSs are polarized preferentially in one of the two dots.

[1] T. Kobayashi et al., Phys. Rev. Lett. 107 (2011) 216802.
[2] F. H. L. Koppens et al., Science 309 (2005) 1346.
[3] K. Ono and S. Tarucha, Phys. Rev. Lett. 92 (2004) 256803; A. Pfund et al., Phys. Rev. Lett. 99 (2007) 036801; H. O. H. Churchill et al., Nature Phys. 5 (2009) 321.
　

Fig. 1. (a) SEM image of the DQD sample. (b)Idot spectrum in the few-electron regime. (n,m) denotes the occupation of the left and right dots.		Fig. 2. ε and B|| dependence of Idot around the SB regime. (a) is obtained by ε sweeps at each value of B||. (b) is obtained by up sweeps of B|| at each value of ε . (c) Idot obtained by round sweeps of B||.

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