Coherent manipulation of nuclear spins in the breakdown regime of integer quantum Hall states

We demonstrate a new method for electrical manipulation of nuclear spins utilizing dynamic nuclear polarization induced by quantum Hall effect breakdown. Nuclear spins are polarized and detected through the hyperfine interaction between a nuclear spin system and a two-dimensional electron system located at an interface of GaAs/AlGaAs single heterostructure. Coherent oscillations between the nuclear-spin quantum states are observed by measuring the longitudinal voltage of the conductor. Manipulation and detection of nuclear spins have attracted much interests in research field of low-dimensional semiconductor systems ever since the proposed use of nuclear spins as solidstate quantum bits [1]. Dynamic nuclear polarization (DNP) is one of the essential techniques to initialize nuclear-spin qubits. So far, coherent manipulation of solid-state nuclear spins has been demonstrated in several low-dimensional semiconductor systems using different kinds of DNP techniques[2, 3, 4]. They utilize spin-flip process of electrons associated with electron scatterings between spin-resolved quantum Hall edge channels[2], inter-domain scatterings between spinpolarized and -unpolarized domain structures in fractional quantum Hall systems[3], and spinselective excitation of electrons by circularly polarized laser beam irradiation[4]. In these DNP techniques, nuclear spins are polarized by the flip-flop process of electron spins S and nuclear spins I in the hyperfine interaction, Hhyp = AI · S = A(I S + IS)/2 + AIzSz, where A is the hyperfine constant. In this paper, we report our novel method for coherent manipulation of nuclear spins. We initialize nuclear spins located at the interface of GaAs/AlGaAs heterostructure by a DNP technique using breakdown of integer quantum Hall effect (QHE)[5]. Then, the nuclearspin states are manipulated by irradiation of radio-frequency (rf) pulse magnetic field. The manipulated nuclear-spin states are read out by standard dc voltage measurements of the quantum Hall conductor. By changing the rf-pulse width, we observe coherent evolution of the nuclear-spin states, i.e. the Rabi oscillation. We study a Hall-bar device fabricated from a wafer of GaAs/AlGaAs single heterostructue with two-dimensional electron gas (2DEG) at the interface. The electron mobility and the carrier density are 220 m/Vs and 1.6 × 10 m, respectively. Figure 1(a) shows an optical micrograph of the Hall-bar device. The central part of the Hall bar is covered with a 10-μm-wide