  We study theoretically the spin relaxation rate in quasi-one-dimensional
coupled double semiconductor quantum dots. We consider InSb and GaAs-based
systems in the presence of the Rashba spin-orbit interaction, which causes
mixing of opposite-spin states, and allows phonon-mediated transitions between
energy eigenstates. Contributions from all phonon modes and coupling mechanisms
in zincblende semiconductors are taken into account. The spin relaxation rate
is shown to display a sharp, cusp-like maximum as function of the
interdot-barrier width, at a value of the width which can be controlled by an
external magnetic field. This remarkable behavior is associated with the
symmetric-antisymmetric level splitting in the structure.
