We estimate the triplet-singlet relaxation rate due to spin-orbit coupling
assisted by phonon emission in weakly-confined quantum dots. Our results for
two and four electrons show that the different triplet-singlet relaxation
trends observed in recent experiments under magnetic fields can be understood
within a unified theoretical description, as the result of the competition
between spin-orbit coupling and phonon emission efficiency. Moreover, we show
that both effects are greatly affected by the strength of the confinement and
the external magnetic field, which may give access to very long-lived triplet
states as well as to selective population of the triplet Zeeman sublevels.Comment: 5 pages, 3 figures. Closely related to recent experiments in
cond-mat/060972