  Early experiments on spin-blockaded double quantum dots revealed surprising
robust, large-amplitude current oscillations in the presence of a static (dc)
source-drain bias [see e.g. K. Ono, S. Tarucha, Phys. Rev. Lett. 92, 256803
(2004)]. Experimental evidence strongly indicates that dynamical nuclear
polarization plays a central role, but the mechanism has remained a mystery.
Here we introduce a minimal albeit realistic model of coupled electron and
nuclear spin dynamics which supports robust self-sustained oscillations. Our
mechanism relies on a nuclear-spin analog of the tunneling magnetoresistance
phenomenon (spin-dependent tunneling rates in the presence of an inhomogeneous
Overhauser field) and nuclear spin diffusion, which governs dynamics of the
spatial profile of nuclear polarization. The extremely long oscillation periods
(up to hundreds of seconds) observed in experiments as well as the differences
in phenomenology between vertical and lateral quantum dot structures are
naturally explained in the proposed framework.
