Nuclear Spin Switch in Semiconductor Quantum Dots

Abstract

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a threshold-like enhancement or reduction of the local nuclear field by up to 3 Tesla can be generated by varying the intensity of light. The excitation power threshold for such a nuclear spin "switch" is found to depend on both external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of spin transfer from electrons to the nuclei of the dot.The hyperfine interaction in solids [1] arises from the coupling between the magnetic dipole moments of nuclear and electron spins. It produces two dynamical effects: (i) inelastic relaxation of electron spin via the "flip-flop" process ( Fig.1a) and (ii) the Overhauser shift of the electron energy [2]. Recently, the hyperfine interaction in semiconductor quantum dots (QDs) has attracted close attention [3,4,5,6,7,8,9,10,11,12,13,14] fuelled by proposals for QD implementation in quantum information applications [15]. The full quantization of the electron states in QDs is beneficial for removing decoherence mechanisms present in extended systems [16,17]. However, the electron localization results in a stronger (than in a bulk material) overlap of its wave-function with a large number of nuclei (N ∼ 10 4 in small selfassembled InGaAs/GaAs dots and up to 10 5 ÷ 10 6 in electrostatically-defined GaAs QDs), and the resulting hyperfine interaction with nuclear spins has been found to dominate the decoherence [3,4,5,12,13,14] and life-time [9] of the electron spin at low temperatures.In this Letter, we report the observation of a pronounced bistable behaviour of nuclear spin polarisation, S, in optically pumped self-assembled InGaAs/GaAs dots. In our experiments, spin-polarized electrons are introduced one-by-one into an individual InGaAs dot at a rate w x (see Fig.1b) by the circularly polarized optical excitation of electron-hole pairs 120 meV above the lowest QD energy states. Due to hole spin-flip during its energy relaxation, both bright and dark excitons can form in the dot ground state. The former will quickly recombine radiatively with a rate w rec ≈ 10 9 sec −1 , whereas the dark exciton can recombine with simultaneous spin transfer to a nucleus via a spin "flip-flop" process (as in Fig.1a) at the rate w rec N p hf [12,18]. Here N is the number of nuclei interacting with the electron and p hf is the probability of a "flip-flop" process, which from our perturbation theory treatment is given by:(1) Here γ is the exciton life-time broadening, h hf is the strength of the hyperfine interaction of the electron with a single nucleus and E eZ is the electron Zeeman splitting. E eZ is strongly dependent on the effective nuclear magnetic field B N generated by the nuclei. This provides a feedback mechanism between the spin transfer rate and the degree of nuclear polarization (B N ∝ S) in the dot [19]. Th...

Nuclear spin effects in semiconductor quantum dots

“…Electron Overhauser shifts in excess of 100 μeV can be obtained using optical pumping [22][23][24][25]33,34 , and B nuc up to 3 T have been reported [23][24][25] . Using rough estimates of the dot composition, degrees of nuclear polarization up to 60% are now routinely obtained.…”

“…However, there are a number of ways to overcome the energy mismatch and to induce a preferential pumping direction, both by optical and electrical means. There have been many attempts to find an efficient way of achieving DNP [21][22][23][24][25]33,34,49,52,53 , with the main motivation to achieve a 100% polarization of nuclear spins, which would prevent nuclear-nuclear flip-flops, strongly suppressing the randomness in the nuclear field and concomitant electron spin decoherence 10 .…”

“…In the limit of large N , where N is the effective number of nuclei, this can be described by a Gaussian distribution 10,11 with the standard deviation σ Bnuc = B max nuc / √ N , where B max nuc is the maximum Overhauser field of the order of a few Tesla [22][23][24][25] . For an electron confined in a GaAs quantum dot and interacting with a typical number of 10 6 spin-3/2 nuclei this results in σ Bnuc ∼ 6 mT.…”

“…DNP is readily observed under excitation of QDs with circularly polarized light [21][22][23][24][25][33][34][35][36][37][38]54,55 : when σ + or σ − polarized photons are absorbed by the sample, electrons with well-defined spin orientation may be created 14,56 . This still holds for so-called 'non-resonant' excitation when the laser is tuned up to 100-200 meV above the QD lowest energy levels [21][22][23][24][25]34,54 .…”

Electron spin-flip correlations due to nuclear dynamics in driven GaAs double dots

“…The coupling of the electrons to the underlying nuclear environment plays an important role in spintronics [1] and utilization of electron spins for quantum computation [2][3][4]. More generally, the interaction between a driven electron-spin qubit and its many-body environment leads to complex dynamical phenomena which are absent if the system is assumed to be at equilibrium with its environment [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Thus, a qubit can be utilized as a probe for studying out-of-equilibrium physics in interacting quantum systems.…”