  The dynamics of the coupled electron-nuclear spin system is studied in an
ensemble of singly-charged (In,Ga)As/GaAs quantum dots (QDs) using periodic
optical excitation at 1 GHz repetition rate. In combination with the
electron-nuclei interaction, the highly repetitive excitation allows us to lock
the electron spins into magnetic resonance in a transverse external magnetic
field. Sweeping the field to higher values, the locking leads to an effective
"diamagnetic" response of significant strength due to dynamic nuclear
polarization, which shields the QD electrons at least partly from the external
field and can even keep the internal magnetic field constant up to 1.3 T field
variation. We model the effect through a magnetic field-dependent polarization
rate of the nuclei, from which we suggest a strategy for adjusting the nuclear
polarization through the detuning between optical excitation and electronic
transition, in addition to tuning the magnetic field.
