  We consider a system of two strongly coupled electron spins in zero magnetic
field, each of which is interacting with an individual bath of nuclear spins
via the hyperfine interaction. Applying the long spin approximation (LSA)
introduced in Europhys. Lett. 95, 47009 (here each bath is replaced by a single
long spin), we numerically study the electron spin and entanglement dynamics.
We demonstrate that the decoherence time is scaling with the bath size
according to a power law. As expected, the decaying part of the dynamics
decreases with increasing bath polarization. However, surprisingly it turns out
that, under certain circumstances, combining quantum dots of different geometry
to the double dot setup has a very similar effect on the magnitude of the spin
decay. Finally, we show that even for a comparatively weak exchange coupling
the electron spins can be fully entangled.
