Hyperfine Interaction-Dominated Dynamics of Nuclear Spins in Self-Assembled InGaAs Quantum Dots

Abstract

We measure the dynamics of nuclear spins in a self-assembled quantum dot at a magnetic field of 5 Tesla and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay which lasts ∼ 100 seconds to intra-dot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. In addition, we observe a gate-voltage and temperature dependent decay stemming from co-tunneling mediated nuclear spin flip processes. By adjusting the gate-voltage and lowering the electron temperature to ∼ 200 milliKelvin, we prolong the corresponding decay time to ∼ 30 hours. Our measurements indicate possibilities for exploring quantum dynamics of the central spin model using a single self-assembled quantum dot.Hyperfine interaction between a single quantum dot (QD) electron and the nuclear spin ensemble defined by the nano-scale confinement provides a realization of the central spin problem [1][2][3][4][5][6]. This model has attracted considerable attention recently since the correlations between the confined electron and the nuclear spin ensemble induced by hyperfine coupling constitute the principal decoherence mechanism for the electron spin [7]. It has been recognized in this context that an enhancement of electron coherence time could be achieved by preparing nuclear spins in eigenstates of the Overhauser (OH) field operator [8,9]: for this approach to be effective, it is essential to understand and characterize the dynamics of prepared (polarized) nuclear spin states.In this Letter, we present measurements of nuclear spin dynamics in a single electron charged self-assembled QD. In contrast to prior work in self-assembled and gateconfined QDs [10-13], we probe nuclear spin dynamics when both the exchange coupling to a Fermionic reservoir (FR) and the dipolar interaction between nuclear spins are vanishingly small. Our observations reveal a spatially limited, temperature-independent, nuclear spin diffusion originating from electron mediated nuclear spin interactions in addition to co-tunneling mediated, temperature dependent, decay of the OH field approaching 10 5 s. Remarkably, the diffusion induced reduction in the OH field taking place on ∼100 s timescale can be strongly suppressed by repeating the preparation cycle consisting of polarization (pump) and free-evolution (wait).The QDs in this sample are separated by a 35 nm GaAs tunnel barrier from a doped n++-GaAs layer. A bias voltage applied between a top semi-transparent Ti/Au Schottky gate and back contacts allows to control the charging state of the QD and the relative alignment of its electronic levels with the Fermi energy of the FR [14]. High-resolution resonance-scattering spectroscopy [15] was performed on a single QD in a fiberbased confocal microscope incorporated in a dilution refrigerator [16]. The electron temperature was varied between 200 mK and 4 K while the applied magnetic field in the Faraday geometry was kept constant at 5 Tesla. We adopted a modulation-free measurement to keep the en...

Nuclear spin effects in semiconductor quantum dots

“…The presence of strain and hence strong quadrupole effects is one of the main reasons that DNP is possible in zero external magnetic field 21,33,36,61,67 , where nuclear polarization life-times in the dark of up to tens of seconds have been observed 36,61 . Remarkably, in magnetic fields of a few T, nuclear polarization in strained self-assembled dots survives as long as 30 hours 36,60,61 . On the contrary, if a QD is brought in contact with an unpolarized electron reservoir, as is often the case in charge-tunable devices, nuclear spins are depolarized in a few milliseconds 36 .…”

“…Nuclear spin depolarization can occur via flip-flops between interacting (neighboring) nuclei, giving rise to nuclear spin diffusion. However, in most QDs the nuclear Zeeman splitting matching required for such nuclear-nuclear flip-flops is not fulfilled owing to quadrupole interactions occurring mainly as a result of strain 13,19,60,[64][65][66] . Thus nuclear spin diffusion is typically suppressed in QDs 36,60,61,63 .…”

“…However, in most QDs the nuclear Zeeman splitting matching required for such nuclear-nuclear flip-flops is not fulfilled owing to quadrupole interactions occurring mainly as a result of strain 13,19,60,[64][65][66] . Thus nuclear spin diffusion is typically suppressed in QDs 36,60,61,63 . The presence of strain and hence strong quadrupole effects is one of the main reasons that DNP is possible in zero external magnetic field 21,33,36,61,67 , where nuclear polarization life-times in the dark of up to tens of seconds have been observed 36,61 .…”

Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots

“…The minimum NCSHFI-mediated depolarization is seen to be on the order of an hour. 67,68 Just as in the NMR spectra, above B Q level where Zeeman regime takes over, the NCSFHI gradually becomes weaker. …”

Nuclear spin depolarization via slow spin diffusion in single InAlAs quantum dots observed by using erase-pump-probe technique

“…24,29 In the case that only an electron exists in the QD region, which can be realized after the X À emission, the contact term contributes dominantly to the nuclear spin depolarization, because it is much stronger than the other depolarization terms. Latta et al revealed that the resident electron causes the nuclear spin depolarization via the hyperfine-mediated indirect nuclear spin interaction and the cotunneling mediated nuclear spin flips.…”