Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble

Abstract

We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots under the influence of weak external magnetic fields (up to 2 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. Quantum dots, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest-neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron spin dynamics, and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system, thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine-tune the angle which the Overhauser field makes with the optical axis.

Erratum: Quantum model for mode locking in pulsed semiconductor quantum dots [Phys. Rev. B 94 , 245308 (2016)]

“…In particular, it has been suggested that the nuclear Zeeman effect, absent in our model, leads to a significant decrease in the mode-locking rate [53,57]. Further interactions that affect the nuclear dynamics are the quadrupolar interaction of the nuclei (in case they are considered as spin-3 2 particles) [39][40][41][42], the dipoledipole interaction between nuclei [43], and anisotropy of the dipolar hyperfine interaction or of the g factors (in case of a hole central spin rather than an electron) [40,47,[61][62][63][64][65]. The present framework of perturbation theory could be extended with these additional interactions with relatively small effort.…”

“…The coupling strengths are encoded through the energies A j . For the sake of simplicity, we neglect the effect of the external magnetic field on the nuclear spins, and omit any additional couplings that are relevant only at time scales much longer than the pulse repetition period, such as the quadrupolar coupling term between the electron and the nuclei [39][40][41][42] or the hyperfine interaction among the nuclear spins [43].…”

Measurements of a fast nuclear-spin dynamics in a single InAs quantum dot with positively charged exciton

“…The applied BB ext will ensure that the depolarization of DNSP via dipolar interaction between nuclear spins is suppressed [9,10] However, the B ext B field is still too small to have any significant influence on the electron spin polarization. Thus the measured PL circular polarization, i.e., electron spin polarization, will follow the variation of nuclear field, i.e.…”

“…-Hyperfine interaction between the electron in a single quantum dot (QD) and the ensemble nuclear spins recently has attracted considerable attention due to its potential applications in spin-based quantum information processing [1][2][3][4][5][6][7][8]. Dynamical nuclear-spin polarization (DNSP) can be achieved by optically polarized electron in QD even in the absence of external magnetic field provided that the Knight field BB e (~ 1mT) is larger than the local nuclear dipolar field of ~ 0.1mT [9,10]. It is reported that the effective depolarization of DNSP via electron-mediated hyperfine interaction in InGaAs QDs has a decay time of ~ 10 s, being longer than the fast depolarization via nuclear dipole-dipole interaction which is on a time scale of ~ 10 s [1,11].…”

“…From this curve, the obtained buildup and decay times are 270 μs and 500 μs, respectively, by a single exponential fit to the data Now we apply a small magnetic field BB ext (~8 mT) in parallel to the sample growth direction, the strength of B ext B is larger than the dipolar field of ~ 0.1mT, but is still much smaller than the dispersion of the nuclear hyperfine field of ~ 26 mT in single QD [5]. The applied BB ext will ensure that the depolarization of DNSP via dipolar interaction between nuclear spins is suppressed [9,10] However, the B ext B field is still too small to have any significant influence on the electron spin polarization. Thus the measured PL circular polarization, i.e., electron spin polarization, will follow the variation of nuclear field, i.e.…”